CVE-2024-29943: Mozilla Firefox Buffer Overflow Flaw

CVE-2024-29943 Overview

CVE-2024-29943 is a critical out-of-bounds read/write vulnerability in Mozilla Firefox that allows attackers to bypass range-based bounds check elimination in the JavaScript engine. By crafting malicious JavaScript code that fools the JIT compiler's optimization logic, an attacker can achieve arbitrary memory access on JavaScript objects, potentially leading to remote code execution.

Critical Impact

This vulnerability enables attackers to perform arbitrary out-of-bounds memory operations through malicious web content, potentially allowing full browser compromise without user interaction beyond visiting a malicious webpage.

Affected Products

• Mozilla Firefox versions prior to 124.0.1

Discovery Timeline

• 2024-03-22 - CVE-2024-29943 published to NVD
• 2025-04-01 - Last updated in NVD database

Technical Details for CVE-2024-29943

Vulnerability Analysis

This vulnerability exists in Firefox's JavaScript JIT (Just-In-Time) compiler, specifically in the range-based bounds check elimination optimization. Modern JavaScript engines use bounds check elimination (BCE) to improve performance by removing redundant array bounds checks when the compiler can statically prove that an index will always be within valid bounds.

The flaw allows an attacker to craft JavaScript code that deceives the range analysis component into believing certain array accesses are within bounds when they are not. Once the bounds checks are incorrectly eliminated during JIT compilation, subsequent array operations can read from or write to memory locations outside the intended JavaScript object boundaries.

This type of vulnerability is particularly dangerous because it operates at the JIT compilation level, making it difficult to detect through traditional runtime protections. The out-of-bounds access can be leveraged to leak sensitive memory contents (information disclosure) or corrupt adjacent memory structures (potentially achieving code execution).

Root Cause

The root cause lies in incorrect range analysis during the JIT compilation phase. The bounds check elimination pass fails to properly account for certain edge cases or maliciously crafted code patterns, leading to the erroneous removal of necessary bounds checks. This is classified as CWE-125 (Out-of-bounds Read), though the vulnerability also enables out-of-bounds writes.

Attack Vector

The attack can be executed remotely over the network without requiring any privileges or user interaction beyond visiting a malicious webpage. An attacker would:

1. Craft malicious JavaScript code designed to trigger incorrect range analysis in Firefox's JIT compiler
2. Host this code on a malicious webpage or inject it into a compromised legitimate site
3. When a victim visits the page, the JIT compiler processes the malicious JavaScript
4. The compiler incorrectly eliminates bounds checks for certain array operations
5. Subsequent array accesses operate outside valid memory boundaries
6. The attacker can leverage this primitive for information disclosure or code execution

The vulnerability mechanism involves manipulating the JIT compiler's understanding of value ranges through carefully constructed JavaScript operations. Technical details regarding the specific triggering conditions are documented in the Mozilla Bug Report #1886849 and discussed in the OpenWall OSS-Security thread.

Detection Methods for CVE-2024-29943

Indicators of Compromise

• Unusual JavaScript execution patterns involving array operations with complex index calculations
• Firefox crashes or unexpected behavior when processing specific web content
• Memory corruption signatures in crash dumps related to JavaScript object arrays
• Anomalous memory access patterns detected by endpoint protection during Firefox execution

Detection Strategies

• Monitor for Firefox versions below 124.0.1 across the enterprise using asset inventory tools
• Deploy web content filtering to block known malicious domains attempting exploitation
• Enable enhanced browser telemetry to detect JIT-related anomalies
• Implement network-level JavaScript analysis for suspicious array manipulation patterns

Monitoring Recommendations

• Track Firefox version deployments and flag systems running vulnerable versions
• Monitor security feeds for active exploitation campaigns targeting CVE-2024-29943
• Enable crash reporting and analyze dumps for patterns indicative of OOB exploitation
• Review endpoint detection logs for memory corruption events associated with browser processes

How to Mitigate CVE-2024-29943

Immediate Actions Required

• Update Mozilla Firefox to version 124.0.1 or later immediately
• Enable automatic updates for Firefox to receive future security patches promptly
• Consider temporarily using an alternative browser if immediate patching is not feasible
• Block untrusted web content at the network perimeter while updates are deployed

Patch Information

Mozilla has released Firefox version 124.0.1 which addresses this vulnerability. The security fix corrects the range-based bounds check elimination logic to properly account for the edge cases that allowed the bypass. Full details are available in Mozilla Security Advisory MFSA-2024-15.

Organizations should prioritize deployment of this update given the network attack vector and critical severity rating. The patch should be deployed through standard software distribution mechanisms or by enabling Firefox's built-in automatic update functionality.

Workarounds

• Disable JavaScript execution in Firefox via about:config by setting javascript.enabled to false (note: this will break most modern websites)
• Use browser isolation or containerization to limit the impact of potential exploitation
• Implement network-level filtering to block access to untrusted or suspicious websites
• Consider deploying browser extensions that restrict JavaScript execution to trusted domains only

# Verify Firefox version via command line
firefox --version
# Expected output for patched version: Mozilla Firefox 124.0.1 or higher

# On enterprise systems, query installed Firefox version
# Windows PowerShell example:
Get-ItemProperty "HKLM:\SOFTWARE\Mozilla\Mozilla Firefox" | Select-Object -ExpandProperty CurrentVersion