CVE-2026-100805: Firefox Race Condition Vulnerability

CVE-2026-100805 Overview

CVE-2026-100805 is a race condition leading to a use-after-free in the Audio/Video component of Mozilla Firefox and Thunderbird. Mozilla fixed the flaw in Firefox 157 and Thunderbird 157. The issue is tracked in Mozilla Security Advisories MFSA-2026-97 and MFSA-2026-101, with root-cause discussion in Mozilla Bug #2058785. The vulnerability is classified under [CWE-362] (Concurrent Execution using Shared Resource with Improper Synchronization).

Critical Impact
A remote attacker who convinces a user to load a crafted page can trigger memory corruption in the Audio/Video pipeline, potentially leading to arbitrary code execution in the browser process.

Affected Products

Mozilla Firefox versions prior to 157
Mozilla Thunderbird versions prior to 157
Downstream distributions bundling the vulnerable Gecko Audio/Video component

Discovery Timeline

2026-09-29 - CVE-2026-100805 published to NVD
2026-09-30 - Last updated in NVD database

Technical Details for CVE-2026-100805

Vulnerability Analysis

The defect resides in the Audio/Video component of Gecko, the rendering engine shared by Firefox and Thunderbird. Concurrent threads operating on media-pipeline objects can access an object after one thread has freed it. This use-after-free window is the direct consequence of the underlying race condition. Successful exploitation can corrupt heap metadata or hijack function pointers reached during media playback, enabling arbitrary code execution within the content or media process.

In Thunderbird, remote content in email messages is disabled by default, which narrows exposure. Firefox users are exposed whenever a crafted media resource is loaded, including via advertisements, iframes, or attacker-controlled sites.

Root Cause

The root cause is improper synchronization between threads that share Audio/Video pipeline state. One thread releases a media object while another thread retains and dereferences a stale pointer. The bug maps to [CWE-362] and manifests only under specific interleavings, which explains the higher attack complexity reflected in the CVSS vector.

Attack Vector

An attacker hosts a malicious page or media resource and induces a victim to visit it. User interaction is required. When the browser processes the crafted stream, the race window opens and the freed object is reused, corrupting memory. See Mozilla Bug Report #2058785 for the underlying technical discussion.

// No verified proof-of-concept is available.
// Refer to MFSA-2026-97 and MFSA-2026-101 for vendor-supplied technical details.

Detection Methods for CVE-2026-100805

Indicators of Compromise

Firefox or Thunderbird content processes crashing during media playback with heap corruption signatures
Unexpected child-process termination referencing Audio/Video modules such as media decoder threads
Outbound connections initiated by browser processes to unknown hosts immediately after media rendering

Detection Strategies

Inventory Firefox and Thunderbird installations and flag any build earlier than 157 as vulnerable
Correlate browser crash telemetry with URLs visited in the preceding session to identify weaponized pages
Monitor for process-injection or shellcode-execution patterns spawned from Firefox or Thunderbird processes

Monitoring Recommendations

Ingest browser crash reports and endpoint telemetry into a central data lake for correlation
Alert on browser child processes launching command interpreters or writing to autorun locations
Track network egress from browser processes to newly registered or low-reputation domains

How to Mitigate CVE-2026-100805

Immediate Actions Required

Upgrade Firefox to version 157 or later on all managed endpoints
Upgrade Thunderbird to version 157 or later, including ESR channels once patches propagate
Validate that auto-update policies are enforced and not blocked by proxy or endpoint controls

Patch Information

Mozilla addressed the issue in Firefox 157 and Thunderbird 157. Refer to Mozilla Security Advisory MFSA-2026-97 and Mozilla Security Advisory MFSA-2026-101 for the full advisory list and downstream package guidance.

Workarounds

Keep remote content disabled in Thunderbird, which is the default configuration
Restrict browsing to trusted sites and block untrusted media sources via web filtering until patches are deployed
Enforce site isolation and reduce the browser attack surface by disabling non-essential media codecs where feasible

bash

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

# Verify installed Thunderbird version
thunderbird --version

# Example apt upgrade after distribution package refresh
sudo apt update && sudo apt install --only-upgrade firefox thunderbird