CVE-2026-6779: Mozilla Firefox JavaScript Engine Vulnerability

CVE-2026-6779 Overview

CVE-2026-6779 is an input validation vulnerability affecting the JavaScript Engine component in Mozilla Firefox and Mozilla Thunderbird. This flaw allows remote attackers to potentially access limited confidential information through network-based attacks without requiring any user interaction or special privileges. The vulnerability was addressed in Firefox 150 and Thunderbird 150.

Critical Impact

Network-accessible information disclosure vulnerability in widely-deployed Mozilla browser and email client applications that could expose limited confidential data without authentication requirements.

Affected Products

• Mozilla Firefox (versions prior to 150)
• Mozilla Thunderbird (versions prior to 150)

Discovery Timeline

• 2026-04-21 - CVE-2026-6779 published to NVD
• 2026-04-22 - Last updated in NVD database

Technical Details for CVE-2026-6779

Vulnerability Analysis

This vulnerability stems from improper input validation (CWE-20) within the JavaScript Engine component of Mozilla products. The flaw allows attackers to craft malicious input that bypasses expected validation controls, potentially leading to unauthorized information disclosure. The attack can be executed remotely over the network with low complexity and requires no user interaction or authentication.

The vulnerability impacts the confidentiality of affected systems, allowing limited access to sensitive information. While the integrity and availability of the system remain unaffected, the ease of exploitation—requiring no privileges and no user interaction—increases the practical risk for organizations running unpatched versions of Firefox or Thunderbird.

Root Cause

The root cause is an input validation error (CWE-20) in the JavaScript Engine component. Improper validation of input data allows specially crafted content to be processed in unintended ways, leading to information leakage. This type of vulnerability typically occurs when the engine fails to properly sanitize or validate JavaScript input before processing, allowing malformed or malicious input to expose internal data.

Attack Vector

The attack vector is network-based, meaning an attacker can exploit this vulnerability remotely without physical access to the target system. The exploitation path involves:

1. An attacker crafts malicious JavaScript content or web page
2. The victim browses to the attacker-controlled page using an unpatched Firefox browser, or opens malicious content in Thunderbird
3. The JavaScript Engine processes the malicious input without proper validation
4. Limited confidential information is exposed to the attacker

Since no user interaction beyond visiting a malicious page is required, drive-by attacks are a viable exploitation scenario.

Detection Methods for CVE-2026-6779

Indicators of Compromise

• Anomalous JavaScript execution patterns in browser logs indicating unusual engine behavior
• Unexpected outbound network connections from browser processes following page loads
• Unusual memory access patterns in the JavaScript Engine component

Detection Strategies

• Monitor for exploitation attempts by analyzing web traffic for known malicious JavaScript patterns targeting the engine
• Deploy endpoint detection rules to identify suspicious JavaScript Engine behavior in Firefox and Thunderbird processes
• Implement network intrusion detection signatures to flag traffic indicative of information exfiltration attempts

Monitoring Recommendations

• Enable detailed logging for Firefox and Thunderbird applications to capture JavaScript execution anomalies
• Monitor for connections to suspicious domains from browser and email client processes
• Implement file integrity monitoring on Mozilla application directories to detect unauthorized modifications

How to Mitigate CVE-2026-6779

Immediate Actions Required

• Update Mozilla Firefox to version 150 or later immediately
• Update Mozilla Thunderbird to version 150 or later immediately
• Review systems for indicators of compromise prior to patching
• Consider temporarily disabling JavaScript in high-risk environments until patches are applied

Patch Information

Mozilla has released patched versions addressing this vulnerability. Organizations should update to Firefox 150 and Thunderbird 150 as documented in the official security advisories:

• Mozilla Security Advisory MFSA-2026-30 - Firefox security update
• Mozilla Security Advisory MFSA-2026-33 - Thunderbird security update

Additional technical details are available in the Mozilla Bug Report #2023343.

Workarounds

• Disable JavaScript execution in Firefox via about:config by setting javascript.enabled to false (impacts functionality)
• Configure content security policies to restrict JavaScript execution from untrusted sources
• Use network-level filtering to block access to known malicious domains
• Deploy browser isolation solutions to contain potential exploitation attempts

# Firefox configuration example - disable JavaScript (use as temporary mitigation only)
# Navigate to about:config and set:
# javascript.enabled = false

# Alternatively, enforce via enterprise policies (policies.json):
{
  "policies": {
    "DisableJavaScript": true
  }
}