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jarsurgeon

jarsurgeon makes it easy to instrument and rebuild JAR files without source code. It uses the Krakatau assembler and disassembler to convert binary .class files to and from ".j" bytecode files. The bytecode files can be modified in any standard text editor, adding debug prints or modifying the program's behavior.

Unlike most Java decompiler output, the human-readable bytecode files used by Krakatau/jarsurgeon can usually be converted back into fully functional Java binaries, even for complex applications. Displaying intermediate values, function arguments, and other internal data can significantly speed up the analysis of undocumented code.

Prerequisites

  • Ruby 1.9.3 or above. Instructions for upgrading ruby on old Ubuntu systems can be found here.
  • The rubyzip gem.
  • Krakatau requires Python 2.x (will not run with >= 3.x).
  • Krakatau also requires the ply parser package.

Sample usage

First, clone the necessary source repos, and set up some environment variables to tell jarsurgeon where to find them:

git clone git://github.com/Storyyeller/Krakatau
export KRAKATAU_HOME=`pwd`/Krakatau
git clone git://github.com/cernekee/jarsurgeon
export JARSURGEON_INST=`pwd`/jarsurgeon/Inst.java

Now look at the bundled test program. It prints "hello world" and exits:

$ cd jarsurgeon
$ java -jar examples/app.jar
hello world

We can unpack the JAR and add a debug print:

$ ./jarsurgeon.rb --git examples/app.jar
  COPY      META-INF/MANIFEST.MF
  DISASM    com/example/Test.class
  COPY      inst/Inst.java
  GIT       app
$ cd app
$ git am -3 ../examples/num-args.patch 
Applying: main: Add 'number of args' instrumentation
$ make
  ASM    obj/com/example/Test.class
  JAR    new.jar

(The --git flag initializes app/ as its own git repository, making it easy to track and revert changes to the bytecode.)

Executing the new JAR, we see the result:

$ java -jar new.jar
;Number of args passed to main: 0
hello world

The supplied Inst.java contains a few example methods that have proven useful in the past, but adding more instrumentation is as simple as editing the source file and running "make". To see the type signatures for calling each Inst.java function, use:

javap -public -s obj/inst/Inst

By default, Inst.java prepends ';' to each output line, making it easy to cut and paste them as comments back into the .j files.

jarsurgeon.rb can be copied to any directory in your PATH, if desired.

Security notes

It is assumed that the input files do not contain malicious content, such as:

  • Filenames containing "../../../", shell/make metacharacters, spaces, etc.
  • Malformed .class files intended to break Krakatau
  • Zip bombs

If this tool is being used to reverse-engineer malware, it should probably be run in a VM.

Signed JAR files can be unpacked, but executing rebuilt versions may involve reconfiguring or modifying your Java runtime environment. The .SF and .DSA files are automatically omitted from new.jar so that "jarsigner -verify" doesn't report a bad signature.

Licensing

jarsurgeon is distributed under the MIT License.