CVE-2026-0885: Firefox Use-After-Free Vulnerability

CVE-2026-0885 Overview

CVE-2026-0885 is a use-after-free vulnerability in the JavaScript Garbage Collection (GC) component of Mozilla Firefox. This memory corruption flaw occurs when the JavaScript engine improperly handles memory that has already been freed during garbage collection operations, potentially allowing attackers to manipulate memory and cause unintended behavior in the browser.

Critical Impact

Exploitation of this use-after-free vulnerability could allow remote attackers to cause information disclosure or denial of service conditions through specially crafted web content.

Affected Products

• Mozilla Firefox versions prior to 147
• Mozilla Firefox ESR versions prior to 140.7

Discovery Timeline

• 2026-01-13 - CVE-2026-0885 published to NVD
• 2026-01-13 - Last updated in NVD database

Technical Details for CVE-2026-0885

Vulnerability Analysis

This use-after-free vulnerability (CWE-416) resides in the JavaScript Garbage Collection component of Firefox. Use-after-free vulnerabilities occur when a program continues to use a pointer to memory after it has been freed, leading to undefined behavior. In browser contexts, this class of vulnerability is particularly dangerous as it can be triggered remotely through malicious web content.

The vulnerability is network-accessible, requiring no user authentication or special privileges to exploit. This makes it particularly concerning for drive-by attack scenarios where simply visiting a malicious webpage could trigger the vulnerability.

Root Cause

The root cause of CVE-2026-0885 lies in improper memory management within the JavaScript GC component. During garbage collection cycles, memory objects may be freed while references to them still exist elsewhere in the JavaScript engine. When these dangling pointers are subsequently dereferenced, the application accesses memory that may have been reallocated for other purposes, leading to memory corruption.

Attack Vector

The attack vector for this vulnerability is network-based. An attacker could exploit this vulnerability by:

1. Crafting malicious JavaScript code that triggers specific garbage collection patterns
2. Hosting the malicious code on a website or injecting it into legitimate websites through other means
3. Luring victims to visit the malicious content
4. The victim's browser executes the JavaScript, triggering the use-after-free condition

The vulnerability requires no authentication and can be exploited without user interaction beyond visiting the malicious page. For detailed technical information, refer to the Mozilla Bug Report #2003607 and the security advisories MFSA-2026-01 and MFSA-2026-03.

Detection Methods for CVE-2026-0885

Indicators of Compromise

• Unexpected Firefox browser crashes, particularly when visiting unfamiliar websites
• Memory corruption errors or segmentation faults in browser logs
• Anomalous JavaScript execution patterns detected by endpoint protection
• Browser process behavior indicating memory manipulation attempts

Detection Strategies

• Deploy browser version monitoring to identify installations running vulnerable Firefox versions (< 147 or ESR < 140.7)
• Implement web content filtering to block known malicious domains attempting exploitation
• Configure endpoint detection rules to monitor for suspicious JavaScript execution patterns
• Enable crash reporting and analysis to identify potential exploitation attempts

Monitoring Recommendations

• Monitor browser crash reports for patterns consistent with use-after-free exploitation
• Track network connections from Firefox processes to identify communication with suspicious domains
• Implement logging for JavaScript engine errors and memory-related exceptions
• Review endpoint telemetry for unusual browser memory consumption or behavior

How to Mitigate CVE-2026-0885

Immediate Actions Required

• Update Mozilla Firefox to version 147 or later immediately
• Update Mozilla Firefox ESR to version 140.7 or later
• Enable automatic updates to ensure timely patch deployment
• Consider restricting JavaScript execution on untrusted sites using browser extensions

Patch Information

Mozilla has addressed this vulnerability in Firefox 147 and Firefox ESR 140.7. Organizations should prioritize deploying these updates across all managed systems. For detailed patch information, refer to Mozilla Security Advisory MFSA-2026-01 and Mozilla Security Advisory MFSA-2026-03.

Workarounds

• Use browser security extensions to control JavaScript execution on untrusted websites
• Configure enterprise browsers to restrict access to known malicious domains
• Implement network-level filtering to block exploitation attempts
• Consider using alternative browsers temporarily if immediate patching is not feasible

# Firefox update verification (Linux)
firefox --version
# Expected output for patched version: Mozilla Firefox 147.0 or later

# For ESR installations
firefox-esr --version
# Expected output for patched version: Mozilla Firefox ESR 140.7 or later