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RE: Code Cracking in Java
From: "Oleg Dubovskoy" <omd () interstron ru>
Date: Wed, 12 May 2004 15:48:51 +0400

Hi Chitresh!

As far as I can see, you've gone too deep...

Firstly, IDA (Interactive Disassembler) can be used to disassemble the
class file, necessary instructions then can be patched and a difference
file produced. =)

Secondly, Can we recompile the whole application without that patching?

Thirdly, the main thing... Client somehow communicates with the server,
right? =) So if all the checks are done on client side, and if SQL is
used, can't we just communicate with server ourself using some dirty
home-grown client, and run exactly the SQL we need? With no application


Best regards,

Oleg Dubovskoy.

-----Original Message-----
From: Chitresh Sen [mailto:chitresh_sen () yahoo com]
Sent: Wednesday, May 12, 2004 10:35 AM
To: webappsec () securityfocus com
Subject: Code Cracking in Java

Code Cracking in Java


This is not a pure technical paper but I would like to share
one of my discovery in Java Security which I had done during
one of my assignments and it was greatly appreciated by the
client because my finding help them to develop a strong
business case to convince top management for discarding the
product. The following document provides some loopholes in Java
and how they can be rectified. It also provides a guideline to
the software designers while developing an application
especially in Java.

Who can read this paper?

Software developers, designers, security professional,
auditors, managers responsible for evaluating products.

About Me

Hi! I love programming in Java but unfortunately I didn’t get a
chance to work in software development firm but I am very much
happy with my profile, basically I am an ethical hacker
currently working in Wipro and previously in PwC.

Let me tell you about my experience in Java. During my third
year of engineering I learnt Java. When I was about to join a
fast track course at Concourse for Java my senior told me that
socket programming in Java is hard to understand, that time I
don’t know anything about Java so its OK for me. After the
completion of course within one month I wrote a chat software
in Java consisting of Socket programming and multithreading
without reading any book. The teacher who taught me asked,
could you share your code with me? Later on I had written some
more applications for which I was rewarded. During my PG I was
awarded for a project at national level, the project title was
“Managing Network using Mobile Phone”. I had designed and
developed the application in J2ME (Java 2 Micro Edition), the
application allows system administrator to manage his network
using mobile phone to some extent. Mr. Ashutosh Bhosle,
Director of Try Catch Solutions, Pune gave the idea for the
above project.

After my PG I was keen to join a software development firm but
unfortunately or fortunately I had been selected in PwC in IT
security and there were no software development. Initially I
was frustrated because my core interest was programming and
here I will never get a chance to work in Java. After some time
I got a chance to work on cryptanalysis project in which I had
written compromising code for checking encryption strength.
Also I had done some research work in Java Security and
consulted security features of Java, it was a great experience
and this project gave me a new vision to work in Java security
other then development. Now I start looking it in different
manner that is how to exploit it loopholes. But for long time I
didn’t get much chance to work in Java Security.

Code Cracking

After a year I got a chance to work in Java Security meanwhile
I was doing assignments on Penetration Testing (PT),
Vulnerability Assessment (VA), formulation of Information
System Security Policy and some few small assignments. The
Assignment, which we got, was basically a PT/VA but the client
also wants a PT for their Internet Banking Application (IBA).

Note: For security reasons I will not mention client name,
application vendor and detail about the application
architecture. Also a person having basic knowledge of Java can
best understand the following text.

The IBA was developed in Java and it is downloadable from
Internet. The real code cracking starts from here. Client has
provided us some test user ids for testing the application. Our
objective was to find the loopholes in IBA and try to explore
that how an authorised user can misuse the application.

As we commonly know that the basic loophole in Java is that it
can be reengineered from class file to source code, the class
file consist of byte codes which is interpreted by Java Virtual
Machine (JVM) to make Java platform independent.

The IBA was vulnerable and I can convert any class file of
application into source code and view its logic, but the
application consists of almost 1400-1500 class files and it is
required to understand the logical flow of application and find
out how it behaves. I find out the first class file, which
start the application and decompiled it and tried to understand
its flow. Frankly speaking its very hard to understand others
code and it requires lot of patience and dedication. Anyway
while browsing through a bunch of class files I found some
interesting class files where input validation checks for
password min/max length, special characters etc were
implemented. Only thing I have to change the reengineered class
file as per my requirement and recompiled and repackaged it so
that the application works as per my requirement. I did the
same but the application didn’t start normally, I again tried
but it didn’t work. Later on I found that they had implemented
Secure Class Loader
 which doesn’t allow repackaging a modified class file.

Now what to do, initially I was thinking I have achieved a lot
and I can screw the application very easily. Later on I got an
idea from my previous experience, during one of my assignment I
had changed the FTP banner by modifying its .dll file using HEX
editor. The idea was vague to change the class file directly
into HEX Editor but what to change in class file because I
don’t know anything about class file. Anyway I accept the
challenge because I don’t have any option. First of all to
change class file I have to understood the class file
structure, its very much logical if you want to play with low
level language you have to understand its format, somehow I
manage to understand it. Also I had gone through the JVM
specification. It was interesting and I was enjoying it but I
had a time constraint so I have to be serous.

After having an understanding of Java class format, the next
challenge is to find out which byte to change. The byte codes
are nothing but the assembly language instructions, which are
interpreted by JVM at run time. In order to find the exact byte
it is required to know the opcodes of JVM instructions. Then I
found a list of JVM Instructions opcodes with their mnemonics.
Now the next challenge was to search for exact byte in the
class file for manipulation.

Let me explain with an example. Let’s say a programmer written
a following if condition for checking minimum length of
password in one of the source file.

 If (pwdTextField.length() <= 6 )
       Error – Password Length should be greater then 6 Characters

Our aim is to change 6 to our requirement so that we can bypass
the minimum password length check. For this we required to see
the byte code in readable assembly language form. Byte Code
Engineering Library helps us to see the byte code into the
assembly language instructions i.e. functions in stack form. I
used this library to see the skeleton of class file in assembly
language instructions and I can easily see the stack of
functions and their calls. I found the instruction for the
above if statement which is kind of bipush 6 instruction, and
its equivalent opcode in hexadecimal format. The understanding
of Java class file format helps to search the hex value in a
specific part of class file. Now open a class file using a HEX
Editor and search for the opcode, which we had found above.
Once we had found the hex value change it and save the modified file.

The following snapshot reflects the above activities in technical form.

Following is an assembly language instructions of class file
generated using BCEL.


protected boolean checkPwd(String arg1)
Code(max_stack = 2, max_locals = 2, code_length = 57)
0:    aload_1
1:    invokevirtual     java.lang.String.length ()I (113)
4:    bipush            6
6:    if_icmplt         #18
9:    aload_1


The opcode of bipush mnemonic is 0x10 and 6 is represented as
0x06 in hex format. Now open the class file using Hex editor
and search for 0x1006 in a specific part of class file and
change it to 0x1000. Now the if condition will change to; If
(pwdTextField.length() <= 0 ) {
           Error – Password Length should be greater then 6 Characters

After modification I started the application and it was running
fine. I changed my test user ids password as 0 character since
there were no server side validation checks were implemented
hence after the client side security check was manipulated the
application allows me to keep password of any length. This way
the minimum password length check is overcomed and similar
process can be used to manipulate any checks implemented at
client side. Now I have the key with me only thing I have to
investigate the proper class files and understand its logic and
manipulate it. Later on I had overcomed the special character
checks which makes application vulnerable for SQL injection,
further exploitation of which leads to the compromisation of database.

I hope you have understood that what I had done more with the

In the above section I mentioned the vulnerabilities related to
Java but these vulnerabilities can be taken care. Obfuscation
can be used to scramble class files so that it becomes hard to
understand the decompiled source code; there are tools
available for obfuscation.

The solution for byte code manipulation can be taken care by
implementing hashing for a package and before starting an
application the hash should be calculated and compared with the
server side precalculated hash, if both of them match then only
allow further execution. Other way to solve the problem is to
implement server side checks no doubt it will affect the
performance of server.

Suggestions and Comments are Welcome!


Chitresh Sen
Email: chitesh_sen () yahoo com

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