CVE-2026-0884: Firefox Use-After-Free Vulnerability

CVE-2026-0884 Overview

CVE-2026-0884 is a critical use-after-free vulnerability in Mozilla Firefox's JavaScript Engine component. This memory corruption flaw occurs when the browser continues to reference memory after it has been freed, potentially allowing attackers to execute arbitrary code in the context of the browser process. The vulnerability affects both the standard Firefox browser and the Firefox Extended Support Release (ESR) versions.

Critical Impact
This use-after-free vulnerability in the JavaScript Engine could allow remote attackers to achieve arbitrary code execution by crafting malicious web content that triggers the memory corruption condition.

Affected Products

Firefox versions prior to 147
Firefox ESR versions prior to 140.7

Discovery Timeline

2026-01-13 - CVE-2026-0884 published to NVD
2026-01-13 - Last updated in NVD database

Technical Details for CVE-2026-0884

Vulnerability Analysis

This use-after-free vulnerability (CWE-416) exists within Firefox's JavaScript Engine component. Use-after-free conditions occur when a program continues to use a pointer after the memory it references has been deallocated. In the context of a browser's JavaScript engine, this type of flaw is particularly dangerous because JavaScript is executed automatically when users visit web pages.

When exploited, an attacker could manipulate the freed memory region to contain attacker-controlled data. If the vulnerable code path subsequently accesses this memory, it may interpret the malicious data as legitimate program structures, leading to control flow hijacking and arbitrary code execution.

Root Cause

The root cause is improper memory management within the JavaScript Engine component. Specifically, a memory object is being freed while references to that object still exist elsewhere in the code. When these dangling references are later dereferenced, the program accesses memory that may have been reallocated for a different purpose, leading to memory corruption.

Attack Vector

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

Creating a malicious webpage containing specially crafted JavaScript code
Luring a victim to visit the malicious page through phishing or other social engineering techniques
The malicious JavaScript triggers the use-after-free condition in the browser's JavaScript engine
The attacker gains code execution within the browser's process context

No user interaction beyond visiting the malicious page is required. The vulnerability can be triggered remotely through any network-accessible means that loads web content.

For detailed technical information about this vulnerability, refer to the Mozilla Bug Report #2003588 and the official Mozilla security advisories.

Detection Methods for CVE-2026-0884

Indicators of Compromise

Unusual browser crashes or instability when visiting specific websites
Unexpected child processes spawned by the Firefox browser process
Memory access violation errors in system logs associated with Firefox
Anomalous network connections originating from the browser process

Detection Strategies

Monitor for Firefox processes exhibiting abnormal behavior such as spawning shells or making unusual system calls
Implement network monitoring to detect connections to known malicious domains that may host exploit code
Deploy endpoint detection and response (EDR) solutions capable of detecting memory corruption exploitation techniques
Review browser crash reports for patterns consistent with use-after-free exploitation

Monitoring Recommendations

Enable Firefox crash reporting and analyze crash dumps for evidence of exploitation attempts
Monitor system event logs for suspicious activity following browser sessions
Implement application allowlisting to detect unauthorized code execution from browser processes
Deploy SentinelOne's Singularity platform to detect and prevent memory corruption exploitation in real-time

How to Mitigate CVE-2026-0884

Immediate Actions Required

Update Firefox to version 147 or later immediately
Update Firefox ESR to version 140.7 or later immediately
Restrict access to untrusted websites until patches can be applied
Consider temporarily using an alternative browser if immediate patching is not possible

Patch Information

Mozilla has released security patches addressing this vulnerability. Organizations should update to the following versions:

Firefox: Update to version 147 or later
Firefox ESR: Update to version 140.7 or later

For complete patch details, refer to:

Workarounds

Disable JavaScript in Firefox via about:config by setting javascript.enabled to false (note: this will significantly impact web functionality)
Use browser extensions that selectively block JavaScript execution on untrusted sites
Implement network-level filtering to block access to known malicious domains
Deploy web proxy solutions to inspect and filter potentially malicious content before it reaches end users

bash

# Firefox configuration to disable JavaScript (temporary workaround)
# Navigate to about:config and modify the following preference:
# javascript.enabled = false

# Alternatively, use enterprise policies to restrict JavaScript
# Create policies.json in Firefox installation directory:
# {
#   "policies": {
#     "DisableDefaultBrowserAgent": true,
#     "Preferences": {
#       "javascript.enabled": false
#     }
#   }
# }