CVE-2026-6754: Mozilla Firefox Use-After-Free Flaw

CVE-2026-6754 Overview

CVE-2026-6754 is a use-after-free vulnerability [CWE-416] in the JavaScript engine component shared by Mozilla Firefox and Thunderbird. The flaw allows a remote attacker to trigger memory corruption by serving crafted JavaScript content to a vulnerable browser or mail client. No user interaction beyond visiting a page or rendering message content is required, and no authentication is needed.

Mozilla addressed the issue in Firefox 150, Firefox ESR 115.35, Firefox ESR 140.10, Thunderbird 150, and Thunderbird 140.10. The advisories tracking the fix are MFSA-2026-30 through MFSA-2026-34.

Critical Impact

Remote attackers can crash the browser process and potentially corrupt heap memory in the JavaScript engine, leading to denial of service on any unpatched Firefox or Thunderbird client.

Affected Products

• Mozilla Firefox (versions prior to 150)
• Mozilla Firefox ESR (versions prior to 115.35 and 140.10)
• Mozilla Thunderbird (versions prior to 150 and 140.10)

Discovery Timeline

• 2026-04-21 - CVE-2026-6754 published to NVD
• 2026-04-22 - Last updated in NVD database

Technical Details for CVE-2026-6754

Vulnerability Analysis

The vulnerability is a use-after-free condition in the JavaScript engine that powers Firefox and Thunderbird. A use-after-free occurs when code continues to reference a memory region after it has been freed, allowing an attacker to influence the contents of that region before the dangling pointer is dereferenced.

In this case, the dangling reference resides within the JavaScript engine's object lifecycle handling. Crafted script execution can cause the engine to operate on a stale pointer, producing memory corruption. The CVSS vector indicates impact is limited to availability, meaning Mozilla has observed reliable crashes but has not confirmed code execution paths.

The shared JavaScript engine codebase means Thunderbird is also affected. Email clients render HTML and script content during message display, expanding the attack surface beyond traditional web browsing.

Root Cause

The root cause is improper lifetime management of an object inside the SpiderMonkey JavaScript engine. Memory was freed while another execution path retained a pointer to it, violating the safety invariant tracked under [CWE-416].

Attack Vector

Exploitation requires only network access. An attacker hosts malicious JavaScript on a web page or embeds it in HTML email content. When the victim visits the page or previews the message, the JavaScript engine processes the crafted input and triggers the use-after-free. The vulnerability mechanism is documented in Mozilla Bug Report #2027541.

No proof-of-concept exploit code is publicly available, and the EPSS probability of exploitation is low.

Detection Methods for CVE-2026-6754

Indicators of Compromise

• Unexpected crashes of firefox.exe, firefox-bin, or thunderbird.exe processes accompanied by access violation exceptions in JavaScript engine modules.
• Crash reports referencing SpiderMonkey symbols or js:: namespace stack frames following JavaScript execution.
• Outbound connections from Firefox or Thunderbird child processes to unfamiliar domains shortly before a crash event.

Detection Strategies

• Inventory installed versions of Firefox and Thunderbird across managed endpoints and flag any builds older than the fixed releases listed in MFSA-2026-30 through MFSA-2026-34.
• Monitor endpoint telemetry for repeated browser or mail client crashes correlated with web page visits or email previews.
• Hunt for content-process termination events in Windows Application logs and macOS crash reporter logs that name JavaScript engine modules.

Monitoring Recommendations

• Forward browser and mail client process crash events into a centralized log platform for correlation with browsing and email telemetry.
• Track patch deployment status of Firefox and Thunderbird through software inventory tooling until all endpoints report fixed versions.
• Subscribe to Mozilla security advisory updates to receive notification of related JavaScript engine fixes.

How to Mitigate CVE-2026-6754

Immediate Actions Required

• Upgrade Firefox to version 150 or later, Firefox ESR to 115.35 or 140.10, and Thunderbird to 150 or 140.10 as listed in the Mozilla Security Advisory MFSA-2026-30.
• Push the update through enterprise software distribution to eliminate unmanaged installations.
• Restart browser and mail client sessions after patch deployment to ensure the new binaries are loaded.

Patch Information

Mozilla released coordinated fixes across five advisories: MFSA-2026-30, MFSA-2026-31, MFSA-2026-32, MFSA-2026-33, and MFSA-2026-34. The fixed versions are Firefox 150, Firefox ESR 115.35, Firefox ESR 140.10, Thunderbird 150, and Thunderbird 140.10. Apply the release that matches your support channel.

Workarounds

• Disable JavaScript in Thunderbird message display through mail.display_glyph and remote content controls until patching is complete.
• Restrict JavaScript execution in Firefox using enterprise policies or extensions that block scripts on untrusted domains.
• Route web traffic through a secure web gateway that can strip or sandbox JavaScript from unknown sources during the remediation window.

# Configuration example - Verify Firefox version on Linux endpoints
firefox --version
# Expected output for patched systems:
# Mozilla Firefox 150.0  (or 115.35esr / 140.10esr)

# Verify Thunderbird version
thunderbird --version
# Expected output for patched systems:
# Thunderbird 150.0  (or 140.10)