CVE-2026-6776: Mozilla Firefox Buffer Overflow Vulnerability

CVE-2026-6776 Overview

CVE-2026-6776 is a memory safety vulnerability in the WebRTC Networking component shared by Mozilla Firefox and Thunderbird. The flaw stems from incorrect boundary conditions [CWE-119] that can corrupt memory when processing WebRTC network traffic. Mozilla fixed the issue in Firefox 150, Firefox ESR 140.10, Thunderbird 150, and Thunderbird 140.10. Exploitation requires local access and user interaction, such as opening a crafted page or message that triggers the vulnerable WebRTC code path. Successful exploitation can lead to confidentiality, integrity, and availability impact on the affected host.

Critical Impact

An attacker can leverage the boundary condition flaw to corrupt memory in the browser or mail client process, potentially leading to code execution within the application's security context.

Affected Products

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

Discovery Timeline

• 2026-04-21 - CVE-2026-6776 published to NVD
• 2026-04-22 - Last updated in NVD database

Technical Details for CVE-2026-6776

Vulnerability Analysis

The vulnerability resides in the WebRTC Networking component, which handles real-time communication packets, transport, and signaling for browser-based audio, video, and data channels. Incorrect boundary conditions in this component allow input sizes or offsets to bypass validation. When the affected code path runs, it can read or write memory outside the intended buffer limits. The result is memory corruption that an attacker may shape into arbitrary code execution within the Firefox or Thunderbird process. Because WebRTC is enabled by default in Firefox and embedded inside Thunderbird, the attack surface extends to both browsing and message rendering contexts.

Root Cause

The root cause is a CWE-119 improper restriction of operations within the bounds of a memory buffer. The WebRTC networking code fails to enforce correct length or index checks before operating on packet or buffer data. This allows malformed network input to drive the process past expected memory boundaries.

Attack Vector

The attack vector is local with required user interaction. A target user must load attacker-controlled content, such as a malicious webpage that initiates a WebRTC session or an HTML email that processes embedded media. Once the vulnerable code path executes, the attacker can influence buffer state to trigger the boundary violation. No additional privileges are required prior to exploitation. Detailed exploitation specifics are restricted; refer to Mozilla Bugzilla Report #2021770 for technical details.

Detection Methods for CVE-2026-6776

Indicators of Compromise

• Unexpected crashes or hangs in firefox.exe, firefox, or thunderbird processes with stack traces referencing WebRTC modules.
• Child content processes spawning unusual sub-processes or initiating outbound network connections inconsistent with normal browsing.
• Creation of suspicious files in user profile directories shortly after WebRTC sessions begin.

Detection Strategies

• Inventory installed Firefox, Firefox ESR, and Thunderbird versions across the fleet and flag instances below the fixed releases.
• Monitor endpoint telemetry for browser process memory faults correlated with WebRTC or media network activity.
• Correlate web proxy or DNS logs with endpoint events to identify users visiting low-reputation sites that establish WebRTC peer connections.

Monitoring Recommendations

• Track Mozilla advisories MFSA-2026-30, MFSA-2026-32, MFSA-2026-33, and MFSA-2026-34 for follow-up guidance.
• Alert on anomalous Thunderbird child process behavior during message preview or HTML rendering.
• Capture and review crash reports from browser endpoints to spot WebRTC stack signatures.

How to Mitigate CVE-2026-6776

Immediate Actions Required

• Upgrade Firefox to version 150, Firefox ESR to 140.10, Thunderbird to 150, or Thunderbird to 140.10 as appropriate.
• Prioritize patching on systems where users browse untrusted content or process external email with HTML rendering enabled.
• Restart affected applications after patching to ensure the vulnerable code is unloaded from memory.

Patch Information

Mozilla addressed CVE-2026-6776 in Firefox 150, Firefox ESR 140.10, Thunderbird 150, and Thunderbird 140.10. Patch details are published in the Mozilla Security Advisory MFSA-2026-30 and related advisories MFSA-2026-32, MFSA-2026-33, and MFSA-2026-34.

Workarounds

• Disable WebRTC in Firefox by setting media.peerconnection.enabled to false in about:config until patching is complete.
• Configure Thunderbird to disable remote content and HTML rendering for untrusted senders.
• Restrict outbound UDP traffic associated with WebRTC peer connections at the network egress where feasible.

# Configuration example: disable WebRTC in Firefox via policies.json
{
  "policies": {
    "Preferences": {
      "media.peerconnection.enabled": {
        "Value": false,
        "Status": "locked"
      }
    }
  }
}