CVE-2026-2767: Mozilla Firefox Use-After-Free Vulnerability

CVE-2026-2767 Overview

CVE-2026-2767 is a critical use-after-free vulnerability affecting the JavaScript WebAssembly component in Mozilla Firefox and Thunderbird. This memory corruption flaw occurs when the browser or email client improperly handles memory in the WebAssembly engine, potentially allowing attackers to execute arbitrary code through specially crafted web content or email messages.

Critical Impact

This use-after-free vulnerability in the WebAssembly component could allow remote attackers to execute arbitrary code, potentially leading to complete system compromise through malicious web pages or email content.

Affected Products

• Mozilla Firefox versions prior to 148
• Mozilla Firefox ESR versions prior to 140.8
• Mozilla Thunderbird versions prior to 148
• Mozilla Thunderbird ESR versions prior to 140.8

Discovery Timeline

• 2026-02-24 - CVE-2026-2767 published to NVD
• 2026-02-26 - Last updated in NVD database

Technical Details for CVE-2026-2767

Vulnerability Analysis

This vulnerability is classified as CWE-416 (Use After Free), a dangerous memory corruption issue that occurs when an application continues to use a pointer after the memory it references has been freed. In the context of Mozilla's WebAssembly implementation, this flaw exists within the JavaScript engine's handling of WebAssembly objects.

Use-after-free vulnerabilities in browser engines are particularly dangerous because they can be triggered through standard web browsing activity. An attacker could craft a malicious webpage that exploits this flaw to corrupt memory in a controlled manner, potentially achieving arbitrary code execution within the context of the browser process.

The network-accessible nature of this vulnerability means that exploitation requires no user privileges and no user interaction beyond visiting a malicious website or opening a malicious email in Thunderbird. The potential impact includes complete compromise of confidentiality, integrity, and availability of the affected system.

Root Cause

The root cause of CVE-2026-2767 lies in improper memory management within Mozilla's WebAssembly engine. Specifically, the vulnerability occurs when WebAssembly objects are freed but references to the freed memory are retained and subsequently accessed. This can happen due to race conditions, improper reference counting, or missing validation checks during WebAssembly module compilation or execution.

When JavaScript code interacts with WebAssembly modules, the browser must carefully manage memory allocations and deallocations. If an object is freed while still being referenced by other parts of the code, subsequent access to that memory location results in undefined behavior that attackers can exploit.

Attack Vector

The attack vector for CVE-2026-2767 is network-based, requiring an attacker to deliver malicious content to the victim's browser or email client. Potential attack scenarios include:

1. Malicious Website: An attacker hosts a webpage containing JavaScript code that triggers the use-after-free condition through carefully crafted WebAssembly interactions
2. Malvertising: Embedding exploit code in online advertisements that get displayed on legitimate websites
3. Email-based Attacks: Sending HTML emails with embedded JavaScript/WebAssembly that triggers the vulnerability when viewed in Thunderbird
4. Watering Hole Attacks: Compromising legitimate websites frequented by targeted users to serve exploit code

The vulnerability mechanism involves manipulating the WebAssembly engine's memory management through JavaScript API calls. Technical details are available in the Mozilla Bug Report #2013741.

Detection Methods for CVE-2026-2767

Indicators of Compromise

• Unusual browser crashes or hangs particularly when loading WebAssembly content
• Unexpected child processes spawned by Firefox or Thunderbird
• Memory access violations or segmentation faults in browser logs
• Anomalous network connections originating from browser processes

Detection Strategies

• Monitor for abnormal WebAssembly module loading patterns in browser telemetry
• Implement endpoint detection rules for suspicious memory operations in firefox.exe or thunderbird.exe processes
• Deploy network-based detection for known exploitation patterns targeting this vulnerability
• Utilize browser crash reports to identify potential exploitation attempts

Monitoring Recommendations

• Enable and centralize collection of Mozilla crash reports for security analysis
• Monitor browser process behavior for signs of heap spray or memory corruption exploitation techniques
• Implement application whitelisting to detect unauthorized code execution from browser processes
• Review DNS and network logs for connections to known malicious infrastructure following browser activity

How to Mitigate CVE-2026-2767

Immediate Actions Required

• Update Mozilla Firefox to version 148 or later immediately
• Update Mozilla Firefox ESR to version 140.8 or later
• Update Mozilla Thunderbird to version 148 or later
• Update Mozilla Thunderbird ESR to version 140.8 or later
• Enable automatic updates for all Mozilla products to receive security patches promptly

Patch Information

Mozilla has released security updates addressing this vulnerability. Detailed patch information is available in the official security advisories:

• Mozilla Security Advisory MFSA-2026-13
• Mozilla Security Advisory MFSA-2026-15
• Mozilla Security Advisory MFSA-2026-16
• Mozilla Security Advisory MFSA-2026-17

Organizations should prioritize deployment of these patches given the critical severity of this vulnerability.

Workarounds

• Disable WebAssembly in Firefox by navigating to about:config and setting javascript.options.wasm to false (may break some web applications)
• Use browser isolation solutions to contain potential exploitation attempts
• Implement content security policies to restrict WebAssembly execution on trusted domains only
• Consider using an alternative browser until patches can be applied in environments where immediate patching is not feasible

# Firefox WebAssembly mitigation via user.js
# Add to Firefox profile directory user.js file
echo 'user_pref("javascript.options.wasm", false);' >> ~/path/to/firefox/profile/user.js

# Verify Firefox version for patch status
firefox --version
# Should return: Mozilla Firefox 148.0 or higher

# For enterprise deployments, push policy via policies.json
# Location: /distribution/policies.json (Linux) or installation directory (Windows)