CVE-2026-2805: Mozilla Firefox Use-After-Free Vulnerability

CVE-2026-2805 Overview

CVE-2026-2805 is an invalid pointer vulnerability in the DOM: Core & HTML component of Mozilla Firefox and Mozilla Thunderbird. The flaw is tracked under CWE-824 (Access of Uninitialized Pointer) and affects how the browser engine references DOM objects. Mozilla addressed the issue in Firefox 148 and Thunderbird 148 through advisories MFSA-2026-13 and MFSA-2026-16. An attacker who hosts a crafted web page can trigger the condition remotely without authentication or user interaction beyond visiting the page.

Critical Impact

Remote attackers can leverage the invalid pointer condition through a malicious web page to corrupt memory, potentially leading to arbitrary code execution within the browser process.

Affected Products

• Mozilla Firefox versions prior to 148
• Mozilla Thunderbird versions prior to 148
• Systems running affected Mozilla DOM: Core & HTML component

Discovery Timeline

• 2026-02-24 - CVE-2026-2805 published to NVD
• 2026-04-13 - Last updated in NVD database

Technical Details for CVE-2026-2805

Vulnerability Analysis

The vulnerability resides in the DOM: Core & HTML component, which manages Document Object Model (DOM) parsing and manipulation in the Gecko engine. The issue is classified as an invalid pointer access, mapped to CWE-824. The defect allows code paths to dereference a pointer that does not refer to a valid, initialized object.

Because the DOM engine processes untrusted content from any visited site, a remote attacker can deliver a crafted HTML or scripting payload to drive the affected code path. Successful exploitation can compromise confidentiality, integrity, and availability of the browser process. The same engine ships within Thunderbird's message rendering, which extends the attack surface to email content where remote content loading is permitted.

Root Cause

The root cause is improper pointer initialization or premature reuse within DOM object handling. According to Mozilla Bug Report #2014549, the affected code dereferences a pointer that does not point to a valid object, producing undefined behavior that attackers can shape into a memory corruption primitive.

Attack Vector

Exploitation occurs over the network with low attack complexity and no privileges. An attacker hosts a malicious page or delivers crafted HTML content. When Firefox or Thunderbird renders the content, the DOM engine reaches the vulnerable code path and operates on the invalid pointer. The EPSS score is 0.023% (percentile 6.523), indicating no current observed exploitation activity, and the vulnerability is not listed on the CISA Known Exploited Vulnerabilities catalog.

No public proof-of-concept code has been published. Refer to MFSA-2026-13 for vendor technical details.

Detection Methods for CVE-2026-2805

Indicators of Compromise

• Firefox or Thunderbird processes crashing while parsing DOM content from specific URLs or email messages
• Unexpected child processes spawned by firefox.exe or thunderbird.exe following web content rendering
• Outbound connections from browser processes to previously unseen domains immediately after a page visit

Detection Strategies

• Inventory all endpoints running Firefox or Thunderbird and flag versions earlier than 148
• Correlate browser process crash telemetry with URL navigation logs from web proxies
• Monitor for anomalous memory-region permissions or mprotect calls within Mozilla processes

Monitoring Recommendations

• Track Firefox and Thunderbird version distribution through endpoint management tooling
• Forward browser crash reports and Windows Error Reporting events to a centralized log platform
• Alert on browser processes performing file writes outside expected profile directories

How to Mitigate CVE-2026-2805

Immediate Actions Required

• Upgrade Mozilla Firefox to version 148 or later on all managed endpoints
• Upgrade Mozilla Thunderbird to version 148 or later, including extended support channels where applicable
• Disable automatic remote content loading in Thunderbird until patching completes

Patch Information

Mozilla released fixes in Firefox 148 and Thunderbird 148. Apply the updates referenced in Mozilla Security Advisory MFSA-2026-13 and Mozilla Security Advisory MFSA-2026-16. Enable automatic updates where supported to reduce exposure windows for future browser vulnerabilities.

Workarounds

• Restrict outbound web browsing to vetted destinations through proxy allow-lists until patches are deployed
• Configure Thunderbird to render messages in plain text mode to limit DOM parsing of untrusted email content
• Apply application allow-listing to prevent browser processes from launching unauthorized child executables

# Verify installed Firefox version on Linux endpoints
firefox --version

# Verify installed Thunderbird version on Linux endpoints
thunderbird --version

# Example: Thunderbird plain-text rendering preference (user.js)
user_pref("mailnews.display.prefer_plaintext", true);
user_pref("mailnews.display.html_as", 1);