CVE-2026-2799: Mozilla Firefox Use-After-Free Vulnerability

CVE-2026-2799 Overview

CVE-2026-2799 is a use-after-free vulnerability affecting Mozilla Firefox and Thunderbird's DOM: Core & HTML component. This memory corruption flaw occurs when the application continues to reference memory after it has been freed, potentially allowing attackers to execute arbitrary code or cause application crashes through specially crafted web content.

Critical Impact

This use-after-free vulnerability in the DOM engine could allow remote attackers to achieve arbitrary code execution by enticing users to visit a malicious webpage or open a crafted email, potentially leading to complete system compromise.

Affected Products

• Mozilla Firefox versions prior to 148
• Mozilla Thunderbird versions prior to 148

Discovery Timeline

• 2026-02-24 - CVE-2026-2799 published to NVD
• 2026-02-26 - Last updated in NVD database

Technical Details for CVE-2026-2799

Vulnerability Analysis

This use-after-free vulnerability resides in the DOM: Core & HTML component of Mozilla's browser engines. The flaw is classified under CWE-416 (Use After Free), a memory safety issue that occurs when an application references memory after it has been deallocated.

In the context of browser DOM operations, use-after-free conditions typically manifest when event handlers, callbacks, or asynchronous operations retain stale references to DOM nodes or associated objects that have been freed during garbage collection or explicit destruction. When these dangling pointers are subsequently dereferenced, the attacker-controlled data that may have been placed in the freed memory region can be interpreted as object metadata, potentially hijacking program control flow.

The network-based attack vector means exploitation requires no authentication and can be triggered simply by visiting a malicious webpage in Firefox or previewing a crafted HTML email in Thunderbird.

Root Cause

The vulnerability stems from improper memory lifecycle management within the DOM: Core & HTML component. When DOM objects are manipulated during certain operations, the application fails to properly invalidate or update references to memory that has been freed. This creates a temporal memory safety violation where subsequent access to these references operates on potentially reallocated and attacker-controlled memory.

Attack Vector

Exploitation occurs through network-delivered content. An attacker can craft a malicious webpage or HTML email that triggers the specific DOM operations leading to the use-after-free condition. When a victim visits the attacker-controlled page in Firefox or views the crafted content in Thunderbird, the vulnerability can be exploited to achieve arbitrary code execution within the browser's security context.

The exploitation chain typically involves:

1. Triggering the memory deallocation through specific DOM manipulation sequences
2. Reallocating the freed memory with attacker-controlled data through heap spray techniques
3. Triggering the use of the stale pointer to achieve code execution or information disclosure

Technical details and proof-of-concept information can be found in the Mozilla Bug Report #2014551.

Detection Methods for CVE-2026-2799

Indicators of Compromise

• Unexpected browser crashes or hangs when rendering specific web content
• Anomalous memory access patterns in Firefox or Thunderbird processes
• Exploitation attempts may be preceded by unusual JavaScript execution patterns in browser logs
• Crash reports referencing DOM: Core & HTML component memory violations

Detection Strategies

• Monitor for abnormal Firefox or Thunderbird process behavior including unexpected child process spawning
• Implement browser version auditing to identify unpatched Firefox and Thunderbird installations across the enterprise
• Deploy endpoint detection rules to identify heap spray patterns and shellcode execution attempts within browser contexts
• Review system crash dumps for signatures consistent with use-after-free exploitation in browser processes

Monitoring Recommendations

• Enable crash reporting and centrally collect browser crash telemetry to identify potential exploitation attempts
• Monitor network traffic for delivery of suspicious HTML/JavaScript content targeting known DOM vulnerabilities
• Implement SentinelOne's behavioral AI to detect post-exploitation activities that may follow successful browser compromise
• Configure browser security telemetry forwarding to your SIEM for correlation with threat intelligence feeds

How to Mitigate CVE-2026-2799

Immediate Actions Required

• Update Mozilla Firefox to version 148 or later immediately
• Update Mozilla Thunderbird to version 148 or later immediately
• Prioritize patching on systems with internet-facing browser usage or email access
• Consider temporarily restricting access to untrusted websites until patches are deployed

Patch Information

Mozilla has released security patches addressing this vulnerability:

• Mozilla Security Advisory MFSA-2026-13 - Firefox security update
• Mozilla Security Advisory MFSA-2026-16 - Thunderbird security update

Organizations should update to Firefox 148 and Thunderbird 148 through their standard software deployment mechanisms. Enterprise deployments using Mozilla's Extended Support Release (ESR) channel should consult the security advisories for applicable ESR version information.

Workarounds

• Disable JavaScript execution in Firefox and Thunderbird as a temporary measure (note: this significantly impacts functionality)
• Configure Thunderbird to display emails as plain text only to reduce HTML rendering attack surface
• Implement network-level filtering to block access to known malicious domains
• Use browser isolation technologies to contain potential exploitation attempts

# Firefox configuration (user.js or about:config)
# Disable JavaScript as temporary mitigation (impacts functionality)
user_pref("javascript.enabled", false);

# Thunderbird configuration - Display emails as plain text
user_pref("mailnews.display.prefer_plaintext", true);
user_pref("mailnews.display.html_as", 1);