CVE-2026-4718: Mozilla Firefox WebRTC Vulnerability

CVE-2026-4718 Overview

CVE-2026-4718 is an undefined behavior vulnerability in the WebRTC Signaling component of Mozilla Firefox and Thunderbird. The flaw is classified under [CWE-758] (Reliance on Undefined, Unspecified, or Implementation-Defined Behavior). Mozilla addressed the issue in Firefox 149, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9.

The vulnerability is network-reachable and requires user interaction, such as visiting a malicious webpage that initiates a WebRTC session. Successful exploitation can compromise both confidentiality and integrity of the affected browser process.

Critical Impact
A remote attacker can trigger undefined behavior in the WebRTC signaling path through a crafted page, potentially leading to memory corruption and execution of attacker-controlled logic within the browser context.

Affected Products

Mozilla Firefox (versions prior to 149)
Mozilla Firefox ESR (versions prior to 140.9)
Mozilla Thunderbird (versions prior to 149 and 140.9)

Discovery Timeline

2026-03-24 - CVE-2026-4718 published to NVD
2026-04-13 - Last updated in NVD database

Technical Details for CVE-2026-4718

Vulnerability Analysis

The issue resides in the WebRTC Signaling component, the subsystem responsible for negotiating peer connections, exchanging Session Description Protocol (SDP) offers and answers, and managing Interactive Connectivity Establishment (ICE) candidates. The component invokes operations whose results are not defined by the C++ language specification, producing implementation-dependent behavior at runtime.

When an attacker influences the inputs that flow into the signaling path, the resulting undefined behavior can manifest as memory corruption, incorrect control flow, or compiler-elided safety checks. The CVSS vector indicates user interaction is required, consistent with a victim loading a malicious page or attachment that triggers WebRTC negotiation.

Root Cause

The root cause is reliance on undefined, unspecified, or implementation-defined behavior [CWE-758] within the WebRTC signaling code. Modern optimizing compilers may remove or rewrite code paths that depend on undefined behavior, creating exploitable conditions that differ from the developer's intent. Mozilla's advisories MFSA-2026-20, MFSA-2026-22, MFSA-2026-23, and MFSA-2026-24 document the fix across release channels.

Attack Vector

The attack vector is network-based. An attacker hosts content that initiates a WebRTC RTCPeerConnection with malformed signaling data. When the victim's browser processes the offer or answer, the undefined behavior is triggered. Thunderbird users can be affected through remote content rendered in HTML messages where scripting or WebRTC features are exposed.

No public proof-of-concept exploit code is available. Technical details are tracked in Mozilla Bug Report #2014864, which remains the authoritative reference for the underlying defect.

Detection Methods for CVE-2026-4718

Indicators of Compromise

Unexpected Firefox or Thunderbird process crashes correlated with WebRTC activity or webrtc.org log entries.
Outbound connections from browser processes to untrusted Session Traversal Utilities for NAT (STUN) or Traversal Using Relays around NAT (TURN) servers.
Browser child process spawning unexpected shells, scripting engines, or persistence mechanisms.

Detection Strategies

Inventory Firefox and Thunderbird installations and flag any build below 149 (or ESR 140.9) as vulnerable.
Monitor endpoint telemetry for anomalous behavior originating from firefox.exe, firefox, or thunderbird processes immediately after WebRTC sessions.
Correlate web proxy logs with browser crash reports to identify pages that consistently precede signaling failures.

Monitoring Recommendations

Enable crash reporting and forward Firefox and Thunderbird crash telemetry to a central log store for analysis.
Track Mozilla advisory feeds MFSA-2026-20 through MFSA-2026-24 for related signaling fixes.
Alert on browser binaries loading unsigned modules or making unusual child process calls following WebRTC negotiation.

How to Mitigate CVE-2026-4718

Immediate Actions Required

Upgrade Firefox to version 149 or later on all managed endpoints.
Upgrade Firefox ESR to 140.9 for environments standardized on the extended support release.
Upgrade Thunderbird to 149 or 140.9 across user mailboxes and shared workstations.
Restart browser and mail client processes after patching to ensure the updated binaries are loaded.

Patch Information

Mozilla released fixed builds across all supported channels. Refer to Mozilla Security Advisory MFSA-2026-20, MFSA-2026-22, MFSA-2026-23, and MFSA-2026-24 for version-specific guidance and changelog details.

Workarounds

Disable WebRTC by setting media.peerconnection.enabled to false in about:config until updates are deployed.
Block outbound STUN and TURN traffic at the network edge for hosts that do not require WebRTC functionality.
Restrict permissions for camera and microphone access through enterprise policy to limit attack surface.

bash

# Configuration example: enforce policy via Firefox enterprise policies.json
{
  "policies": {
    "Preferences": {
      "media.peerconnection.enabled": {
        "Value": false,
        "Status": "locked"
      }
    }
  }
}