iDefense Security Advisory 12.04.08: Sun Java JRE Pack200 Decompression Integer Overflow Vulnerability

Type securityvulns
Reporter Securityvulns
Modified 2008-12-09T00:00:00



iDefense Security Advisory 12.02.08 Dec 02, 2008


Pack200 is a compression method introduced by Sun in the 1.5 release of the JRE. It is used to compress Jar files, and is optimized for the compression of Java class files. A Java applet can be compressed using the pack200 tool, and if the browser plugin supports the pack200-gzip encoding it will pass the compressed Jar file to the JRE for unpacking. For more information, see the vendor's site at the following links.


Remote exploitation of an integer overflow vulnerability in Sun Microsystems Inc.'s Java JRE could allow an attacker to execute arbitrary code with the privileges of the current user.

The vulnerability occurs when reading the Pack200 compressed Jar file during decompression. In order to calculate the size of a heap buffer, the code multiplies and adds several integers. The bounds of these values are not checked, and the arithmetic operations can overflow. This results in an undersized buffer being allocated, which leads to a heap based buffer overflow.


Exploitation allows attackers to execute arbitrary code in the context of the currently logged-on user. To exploit this vulnerability, a targeted user must load a malicious Web page created by an attacker. An attacker typically accomplishes this via social engineering or injecting content into compromised, trusted sites.

Exploitation of heap overflow vulnerabilities on modern operating systems can at times be difficult due to various heap integrity protections. However, the Pack200 code uses a custom allocator that does not contain such integrity checks. Labs testing has demonstrated that code execution is possible on the Linux platform. A similar methodology is likely to be successful on the Windows platform.


iDefense has confirmed the existence of this vulnerability in Sun Microsystem Inc.'s Java JRE version 1.6.0_07 for Windows and Linux. According to Sun, Pack200 was first introduced in JRE 1.5.0. The latest version of JRE 1.5, 1.5.0_15, does contain the vulnerable code, but the browser plugin does not handle Pack200 encoding. As such, exploitation through the browser does not appear to be possible with JRE 1.5.


The library containing the vulnerability can be renamed, which will prevent it from being loaded. This workaround will prevent users from loading Pack200 format Jar files, and from using the pack/unpack tools that come with the JRE. However, normal applets and Java applications will continue to function correctly. The vulnerable library is called 'unpack', and can be found in:

"%SYSTEMDRIVE%\Program Files\Java\JAVA VERSION\bin\unpack.dll"

on Windows, and in differing locations dependent upon the distribution/platform on Unix systems.


Sun Microsystem Inc.'s has released a patch which addresses this issue. For more information, consult their advisory at the following URL.


A Mitre Corp. Common Vulnerabilities and Exposures (CVE) number has not been assigned yet.


10/02/2008 Initial Vendor Notification 11/25/2008 Initial Vendor Reply 12/02/2008 Coordinated Public Disclosure


This vulnerability was reported to iDefense by regenrecht.

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