CVE-2026-12302: Mozilla Firefox Auth Bypass Vulnerability

CVE-2026-12302 Overview

CVE-2026-12302 is a mitigation bypass vulnerability in the DOM: Security component of Mozilla Firefox and Mozilla Thunderbird. The flaw allows a remote attacker to circumvent browser security mitigations through specially crafted web content, resulting in limited confidentiality and integrity impact. The vulnerability is classified under [CWE-693] (Protection Mechanism Failure) and requires no privileges or user interaction to exploit over the network.

Mozilla addressed the issue in Firefox 152, Firefox ESR 140.12, Firefox ESR 115.37, Thunderbird 152, and Thunderbird 140.12.

Critical Impact
Remote attackers can bypass DOM security mitigations without authentication or user interaction, weakening browser defenses against malicious web content.

Affected Products

Mozilla Firefox versions prior to 152
Mozilla Firefox ESR versions prior to 140.12 and prior to 115.37
Mozilla Thunderbird versions prior to 152 and prior to 140.12

Discovery Timeline

2026-06-16 - CVE-2026-12302 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2026-12302

Vulnerability Analysis

The vulnerability resides in the DOM: Security component, which enforces browser-level protections against malicious or untrusted web content. An attacker who controls a web page can craft DOM interactions that bypass the intended security mitigations. Successful exploitation does not require authentication or user interaction, making drive-by attack scenarios feasible.

The impact is limited to partial confidentiality and integrity loss. Availability is not affected. The flaw weakens defense-in-depth protections rather than directly granting code execution.

Thunderbird is affected because it shares the Gecko rendering engine with Firefox. However, scripting is disabled by default in Thunderbird mail, reducing the practical attack surface for email-borne content.

Root Cause

The root cause is a protection mechanism failure [CWE-693] in the DOM Security subsystem. Specific implementation details are tracked in Mozilla Bug Report #2034489. The bypass condition allows DOM operations to proceed in a state where a security check should have blocked or constrained them.

Attack Vector

Exploitation occurs over the network. An attacker hosts malicious web content and lures a victim to visit the page using a vulnerable Firefox version. The attacker can then trigger DOM operations that bypass the intended mitigation, leading to information disclosure or limited data manipulation within the browser context.

The vulnerability manifests in DOM-level security policy enforcement. Refer to the Mozilla Security Advisory MFSA-2026-57 and related advisories for technical guidance.

Detection Methods for CVE-2026-12302

Indicators of Compromise

Firefox or Thunderbird process versions older than the patched releases (Firefox 152, ESR 140.12, ESR 115.37, Thunderbird 152, Thunderbird 140.12)
Browser telemetry showing access to suspicious or newly registered domains serving heavy DOM-manipulating JavaScript
Outbound connections from browser processes to attacker-controlled infrastructure following web navigation events

Detection Strategies

Inventory installed browser versions across the enterprise and flag hosts running unpatched Firefox or Thunderbird builds
Monitor proxy and DNS logs for browser-initiated requests to low-reputation domains hosting content with abnormal DOM activity
Correlate endpoint process telemetry for firefox.exe and thunderbird.exe against threat intelligence on malicious web hosts

Monitoring Recommendations

Track browser version compliance through software inventory and patch management systems
Enable browser security telemetry and forward logs to a central SIEM for analysis
Alert on anomalous child processes spawned by browser binaries following web navigation

How to Mitigate CVE-2026-12302

Immediate Actions Required

Upgrade Firefox to version 152 or later across all managed endpoints
Upgrade Firefox ESR deployments to 140.12 or 115.37 depending on the ESR channel in use
Upgrade Thunderbird to version 152 or Thunderbird ESR 140.12
Validate patch deployment through software inventory verification

Patch Information

Mozilla released fixes in Firefox 152, Firefox ESR 140.12, Firefox ESR 115.37, Thunderbird 152, and Thunderbird 140.12. See Mozilla Security Advisory MFSA-2026-57, MFSA-2026-58, MFSA-2026-59, MFSA-2026-60, and MFSA-2026-61 for vendor guidance.

Workarounds

No official vendor workaround is documented; patching is the only complete remediation
Restrict browsing to trusted sites through enterprise web filtering until patches are deployed
Ensure Thunderbird remote content loading remains disabled by default for email messages
Apply enterprise policies that block execution of untrusted JavaScript where business workflows allow

bash

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

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

# Example Windows registry check for installed Firefox version
reg query "HKLM\SOFTWARE\Mozilla\Mozilla Firefox" /v CurrentVersion