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Mozilla Firefox 3.5 TraceMonkey JavaScript engine uninitialized memory vulnerability

Vulnerability Note VU#443060

Original Release Date: 2009-07-14 | Last Revised: 2009-07-17


Mozilla Firefox's javascript engine contains a vulnerability that may allow an attacker to execute code.


Mozilla Firefox version 3.5 contains a vulnerability in the TraceMonkey components of Firefox's JavaScript engine.

Per Mozilla Bug Bug 503286:
"This is a JS engine bug dealing with deep bailing not properly restoring the return value from the result of the (fast native) escape function. We then try to do something with the uninitialized memory and crash in the interpreter."

Note that proof of concept code that demonstrates issue this is publicly available.


A remote, unauthenticated attacker may be able to execute arbitrary code or cause Firefox to crash.


Firefox 3.5.1 has been released to address this issue. See Mozilla Foundation Security Advisory 2009-41 for more information. Until updates can be applied, the below workarounds may mitigate this issue.

Disable TraceMonkey

To disable the vulnerable components, use the about:config interface to set javascript.options.jit.content and to false. This will still allow JavaScript to run, but it will disable the TraceMonkey performance enhancements.

Use NoScript

Using the Mozilla Firefox NoScript extension to whitelist web sites that can run scripts will help to mitigate this vulnerability. Further details for configuring NoScript are available in the Securing Your Web Browser document.

Disable JavaScript

For instructions on how to disable JavaScript in Firefox, please refer to the Firefox section of the Securing Your Web Browser document.

Vendor Information


Mozilla Affected

Updated:  July 14, 2009



Vendor Statement

We have not received a statement from the vendor.

Vendor Information

The vendor has not provided us with any further information regarding this vulnerability.

CVSS Metrics

Group Score Vector
Base 0 AV:--/AC:--/Au:--/C:--/I:--/A:--
Temporal 0 E:Not Defined (ND)/RL:Not Defined (ND)/RC:Not Defined (ND)
Environmental 0 CDP:Not Defined (ND)/TD:Not Defined (ND)/CR:Not Defined (ND)/IR:Not Defined (ND)/AR:Not Defined (ND)



Information from zbyte, Mozilla, and other sources was used in this report.

This document was written by Ryan Giobbi.

Other Information

CVE IDs: None
Severity Metric: 40.50
Date Public: 2009-07-09
Date First Published: 2009-07-14
Date Last Updated: 2009-07-17 12:05 UTC
Document Revision: 22

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