CVE-2026-8391: Mozilla Firefox JavaScript Engine Vulnerability

CVE-2026-8391 Overview

CVE-2026-8391 is a medium-severity vulnerability in the JavaScript Engine component of Mozilla Firefox. Mozilla resolved the issue in Firefox 150.0.3 as part of security advisory MFSA-2026-45. The flaw is classified under [CWE-20] (Improper Input Validation) and can be triggered remotely without authentication or user interaction. Successful exploitation results in limited confidentiality impact, with no effect on integrity or availability of the affected system. No public proof-of-concept code, exploit kits, or in-the-wild abuse have been reported as of the latest update, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog.

Critical Impact

Remote attackers can leverage crafted web content processed by the Firefox JavaScript engine to access limited confidential information without privileges or user interaction.

Affected Products

• Mozilla Firefox versions prior to 150.0.3
• CPE: cpe:2.3:a:mozilla:firefox:*:*:*:*:-:*:*:*
• Component: mozilla:firefox JavaScript Engine

Discovery Timeline

• 2026-05-12 - CVE-2026-8391 published to the National Vulnerability Database
• 2026-05-13 - Last updated in NVD database

Technical Details for CVE-2026-8391

Vulnerability Analysis

The vulnerability resides in the JavaScript Engine component of Mozilla Firefox. The issue is categorized as [CWE-20] Improper Input Validation, indicating that the engine does not adequately validate or sanitize specific inputs processed during script execution. An attacker can deliver crafted JavaScript through any web page rendered by a vulnerable browser. Because the attack vector is network-based and requires neither authentication nor user interaction beyond visiting a malicious or compromised page, the exploitation surface is broad.

The impact is scoped to limited information disclosure. The vulnerability does not permit code execution, memory corruption, or denial of service based on the published metrics. Mozilla's advisory MFSA-2026-45 describes the issue as an "other" class defect in the JavaScript engine, fixed in Firefox 150.0.3.

Root Cause

The root cause is improper input validation within the JavaScript engine's handling of untrusted script content. Insufficient validation allows attacker-controlled input to influence engine behavior in a way that exposes limited information. Refer to Mozilla Bug Report #2038575 for the upstream technical discussion.

Attack Vector

An attacker hosts or injects malicious JavaScript on a web page. When a user visits the page with a vulnerable Firefox build, the engine processes the crafted content and leaks limited data accessible within the browser context. The attack does not require credentials, prior compromise, or user interaction beyond normal browsing.

// No verified proof-of-concept code is available for CVE-2026-8391.
// See Mozilla Bug Report #2038575 for upstream technical details.

Detection Methods for CVE-2026-8391

Indicators of Compromise

• Outbound web traffic from Firefox clients running versions earlier than 150.0.3 to untrusted or newly registered domains hosting unusual JavaScript payloads.
• Browser telemetry showing repeated visits to pages serving heavily obfuscated or anomalous script content prior to patching.
• Endpoint logs indicating Firefox processes accessing or transmitting unexpected data shortly after page loads.

Detection Strategies

• Inventory Firefox installations across the environment and flag any version below 150.0.3.
• Monitor browser process behavior for anomalous network egress patterns originating from firefox.exe or platform equivalents.
• Correlate web proxy logs with browser version data to identify users on vulnerable builds visiting low-reputation domains.

Monitoring Recommendations

• Enable web proxy or secure web gateway inspection to identify suspicious JavaScript payloads delivered to client browsers.
• Track endpoint software inventory changes to confirm timely deployment of Firefox 150.0.3 or later.
• Review DNS and HTTP/HTTPS logs for connections to domains associated with malicious script delivery campaigns.

How to Mitigate CVE-2026-8391

Immediate Actions Required

• Update Mozilla Firefox to version 150.0.3 or later on all managed endpoints.
• Verify automatic update channels are enabled and reaching all Firefox installations, including portable and unmanaged copies.
• Restrict access to untrusted websites through web filtering policies until patching completes.

Patch Information

Mozilla released the fix in Firefox 150.0.3. Full remediation details are documented in the Mozilla Security Advisory MFSA-2026-45. Administrators should deploy the updated build through their standard software distribution mechanism and validate version compliance after rollout.

Workarounds

• Use enterprise policy to disable JavaScript on high-risk or untrusted sites until the patch is deployed.
• Enforce strict Content Security Policy headers on internal web applications to limit third-party script execution.
• Route browser traffic through a secure web gateway that blocks known malicious script sources.

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

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