CVE-2026-2775: Mozilla Firefox Auth Bypass Vulnerability

CVE-2026-2775 Overview

CVE-2026-2775 is a critical mitigation bypass vulnerability affecting the DOM: HTML Parser component in Mozilla Firefox and Thunderbird. This security flaw allows attackers to circumvent built-in security mitigations through specially crafted HTML content, potentially enabling remote code execution or other malicious actions when users visit compromised web pages or interact with malicious email content in Thunderbird.

Critical Impact

This vulnerability allows attackers to bypass security mitigations in the HTML parser, potentially leading to arbitrary code execution with user-level privileges across multiple Mozilla products.

Affected Products

• Mozilla Firefox < 148
• Mozilla Firefox ESR < 115.33
• Mozilla Firefox ESR < 140.8
• Mozilla Thunderbird < 148
• Mozilla Thunderbird < 140.8

Discovery Timeline

• 2026-02-24 - CVE-2026-2775 published to NVD
• 2026-02-25 - Last updated in NVD database

Technical Details for CVE-2026-2775

Vulnerability Analysis

CVE-2026-2775 resides in the DOM HTML Parser component, a core part of Mozilla's rendering engine responsible for parsing and interpreting HTML content. The vulnerability enables attackers to bypass security mitigations that are designed to prevent malicious code execution during HTML parsing operations.

When the HTML parser processes specially crafted content, it fails to properly enforce security boundaries, allowing attackers to circumvent protections that would normally prevent exploitation of memory corruption or other low-level vulnerabilities. This mitigation bypass is particularly dangerous because it can be chained with other vulnerabilities to achieve full remote code execution.

The attack requires no privileges or user interaction beyond visiting a malicious webpage or viewing a compromised email in Thunderbird, making it highly exploitable via drive-by attacks or phishing campaigns.

Root Cause

The vulnerability stems from improper enforcement of security mitigations within the DOM HTML Parser component. The parser fails to maintain security invariants when processing certain HTML constructs, creating conditions where protective measures can be bypassed. This allows attackers to manipulate parser state in ways that should be prevented by the security architecture.

Attack Vector

The vulnerability is exploitable over the network without authentication. An attacker can exploit this flaw by:

1. Hosting malicious HTML content on a controlled web server
2. Convincing a victim to visit the malicious page (Firefox) or view a crafted email (Thunderbird)
3. The malicious HTML triggers the mitigation bypass during parsing
4. The attacker leverages the bypass to execute further attacks, potentially achieving code execution

The vulnerability mechanism involves crafted HTML content that exploits weaknesses in the parser's security enforcement. When the DOM HTML Parser processes this content, the security mitigations that normally prevent exploitation are bypassed, allowing the attacker to proceed with secondary exploitation techniques. For detailed technical information, refer to the Mozilla Bug Report #2015199 and the official security advisories.

Detection Methods for CVE-2026-2775

Indicators of Compromise

• Unusual process behavior from firefox.exe or thunderbird.exe including unexpected child processes or network connections
• Crash dumps containing references to DOM parser components with suspicious memory patterns
• Browser or email client processes accessing sensitive system files or registry keys unexpectedly
• Network traffic to known malicious domains immediately following HTML parsing operations

Detection Strategies

• Monitor for anomalous browser process behavior using endpoint detection and response (EDR) tools
• Implement content security policies and web filtering to block access to known malicious domains
• Deploy network intrusion detection signatures for known exploitation patterns targeting Mozilla products
• Use application whitelisting to detect unauthorized code execution from browser processes

Monitoring Recommendations

• Enable enhanced logging for browser process activity and monitor for suspicious parent-child process relationships
• Configure SIEM rules to correlate browser crashes with subsequent suspicious network activity
• Monitor for unexpected DLL loads or memory allocations within Firefox and Thunderbird processes
• Review web proxy logs for access to newly registered domains or known malware distribution sites

How to Mitigate CVE-2026-2775

Immediate Actions Required

• Update Mozilla Firefox to version 148 or later immediately
• Update Mozilla Firefox ESR to version 115.33 or 140.8 or later
• Update Mozilla Thunderbird to version 148 or 140.8 or later
• Consider temporarily using alternative browsers until updates can be applied in managed environments

Patch Information

Mozilla has released security patches addressing this vulnerability across all affected product versions. Organizations should prioritize deployment of these updates given the critical severity rating and network-exploitable nature of the vulnerability.

Official security advisories with patch details:

• Mozilla Security Advisory MFSA-2026-13
• Mozilla Security Advisory MFSA-2026-14
• Mozilla Security Advisory MFSA-2026-15
• Mozilla Security Advisory MFSA-2026-16
• Mozilla Security Advisory MFSA-2026-17

Workarounds

• Disable JavaScript execution in Firefox and Thunderbird to reduce the attack surface until patches can be applied
• Implement strict Content Security Policies to limit the sources from which content can be loaded
• Configure email clients to display messages in plain text mode to avoid HTML parsing
• Use network-level controls to filter potentially malicious HTML content

# Check installed Firefox version
firefox --version

# Check installed Thunderbird version
thunderbird --version

# For enterprise deployments, verify update policies
# Windows: Check HKLM\SOFTWARE\Policies\Mozilla\Firefox
# Linux: Review /etc/firefox/policies/policies.json