Decompilation is your best bet. Decompile the existing class using
JAD or FrontEndPlus, modify it to suit your needs (say, writing out
login credentials somewhere), recompile and re-JAR the classes. You
could also use decompilation to determine all of the method
signatures and then write your own class with those particular method
signatures and replace the original class with your malicious one.
The effect is the same.
You'll have to make sure the JAR isn't signed for this to work
directly. If it is signed, you'll need to resign the JAR with a new
private key and certificate. If the certs are validated against a CA
then you'll need the original private key that was used to sign the
JAR. There may be other tamper-proofing mechanisms in place you'll
discover while working on this. They can usually be circumvented
with a little creativity.
Good luck!
dhs-
Dean H. Saxe, CEH
dean@fullfrontalnerdity.com
"I have always strenuously supported the right of every man to his
own opinion, however different that opinion might be to mine. He who
denies another this right makes a slave of himself to his present
opinion, because he precludes himself the right of changing it."
-- Thomas Paine, 1783
On Dec 28, 2005, at 7:03 PM, funkyforumemail@hotmail.com wrote:
I have a java .jar, and would like to write the variables being
sent to a particular class into a seperate file, then continue to
execute the class as normal. An example would be a login.class, i
would like to intercept the username and password going into the
class file. The point is that I dont have the original source code,
and decompiling and recompiling the class is difficult. Replacing
the class with my own and somehow resume normal execution seems to
be the best way.
Please help.
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