CVE-2026-6766: Mozilla Firefox Buffer Overflow Vulnerability

CVE-2026-6766 Overview

CVE-2026-6766 is a boundary condition vulnerability affecting the Network Security Services (NSS) Libraries component used by Mozilla Firefox and Thunderbird. This improper check for unusual or exceptional conditions (CWE-754) allows attackers to exploit incorrect boundary validation, potentially leading to information disclosure through network-based attacks without requiring user interaction or elevated privileges.

Critical Impact

This vulnerability enables unauthorized access to sensitive information through network-based exploitation of the NSS Libraries component, affecting multiple Mozilla products including Firefox and Thunderbird across standard and ESR releases.

Affected Products

• Mozilla Firefox versions prior to 150
• Mozilla Firefox ESR versions prior to 140.10
• Mozilla Thunderbird versions prior to 150
• Mozilla Thunderbird ESR versions prior to 140.10

Discovery Timeline

• April 21, 2026 - CVE-2026-6766 published to NVD
• April 22, 2026 - Last updated in NVD database

Technical Details for CVE-2026-6766

Vulnerability Analysis

The vulnerability resides in the NSS (Network Security Services) Libraries component, which provides a critical foundation for cryptographic operations in Mozilla products. The flaw stems from incorrect boundary conditions that fail to properly validate input parameters during cryptographic processing operations.

When the NSS library processes specially crafted data, the incorrect boundary checks allow an attacker to read memory contents beyond intended limits. This can result in exposure of sensitive information that should remain protected within the application's memory space. The vulnerability is particularly concerning because it affects the security-critical cryptographic infrastructure that underlies secure communications in affected browsers and email clients.

Root Cause

The root cause is classified as CWE-754 (Improper Check for Unusual or Exceptional Conditions). The NSS Libraries component fails to properly validate boundary conditions when processing certain inputs, allowing operations to proceed with parameters that should be rejected. This inadequate validation creates an exploitable condition where memory can be read beyond expected boundaries.

Attack Vector

The vulnerability is exploitable over the network without requiring authentication or user interaction. An attacker can craft malicious content (such as a web page or email) that triggers the vulnerable code path when processed by an affected Mozilla product. Successful exploitation allows the attacker to extract confidential data from the application's memory, potentially including cryptographic keys, session tokens, or other sensitive information.

The attack requires no special privileges and can be triggered by simply loading malicious content in a vulnerable browser or email client. The network-based attack vector combined with no user interaction requirements makes this vulnerability particularly dangerous for widespread exploitation.

Detection Methods for CVE-2026-6766

Indicators of Compromise

• Unexpected memory access patterns in Firefox or Thunderbird processes related to NSS library operations
• Anomalous network traffic patterns involving malformed TLS/SSL handshakes or certificate processing
• Crash reports or stability issues related to cryptographic operations in affected applications
• Unusual data exfiltration attempts following visits to suspicious websites or processing of malicious emails

Detection Strategies

• Monitor for exploitation attempts by analyzing network traffic for malformed requests targeting NSS library functions
• Deploy endpoint detection rules to identify abnormal memory access patterns in nss3.dll or libnss3.so library operations
• Implement browser extension policies that can detect and block known exploitation patterns
• Review application logs for cryptographic operation failures that may indicate exploitation attempts

Monitoring Recommendations

• Enable detailed logging for Mozilla applications to capture potential exploitation attempts
• Configure network security tools to inspect TLS traffic for anomalous patterns that may indicate boundary condition attacks
• Deploy SentinelOne Singularity agents across endpoints to detect memory-based exploitation techniques
• Establish baseline behavior for cryptographic operations to identify deviations that may signal active exploitation

How to Mitigate CVE-2026-6766

Immediate Actions Required

• Update Mozilla Firefox to version 150 or later immediately
• Update Mozilla Firefox ESR to version 140.10 or later
• Update Mozilla Thunderbird to version 150 or later
• Update Mozilla Thunderbird ESR to version 140.10 or later
• Deploy updates through centralized software management systems for enterprise environments

Patch Information

Mozilla has released security patches addressing this vulnerability in the following versions:

• Firefox 150 - Mozilla Security Advisory MFSA-2026-30
• Firefox ESR 140.10 - Mozilla Security Advisory MFSA-2026-32
• Thunderbird 150 - Mozilla Security Advisory MFSA-2026-33
• Thunderbird ESR 140.10 - Mozilla Security Advisory MFSA-2026-34

Additional technical details are available in Mozilla Bug Report #2023207.

Workarounds

• Restrict access to untrusted websites and email content until patches can be applied
• Implement network-level filtering to block known malicious content targeting this vulnerability
• Consider using alternative browsers for sensitive operations if immediate patching is not possible
• Enable enhanced security policies in enterprise environments to limit exposure to potential exploitation

# Verify Firefox version (ensure 150 or later)
firefox --version

# Verify Thunderbird version (ensure 150 or later)
thunderbird --version

# For enterprise deployments, use package managers to update
# Debian/Ubuntu
sudo apt update && sudo apt upgrade firefox thunderbird

# RHEL/Fedora
sudo dnf update firefox thunderbird