CVE-2026-74961: Mozilla Firefox Information Disclosure

CVE-2026-74961 Overview

CVE-2026-74961 is a side-channel vulnerability in the Web Audio component of Mozilla Firefox and Thunderbird. The flaw is categorized under [CWE-203] (Observable Discrepancy) and allows a remote attacker to infer sensitive information through observable behavioral differences in the Web Audio subsystem. Mozilla addressed the issue in Firefox 154, Firefox ESR 153.1, Thunderbird 154, and Thunderbird 153.1. The vulnerability is exploitable over the network without authentication or user interaction.

Critical Impact
A remote attacker can exploit timing or behavioral discrepancies in the Web Audio component to leak confidential data across security boundaries in the browser or mail client.

Affected Products

Mozilla Firefox versions prior to 154
Mozilla Firefox ESR versions prior to 153.1
Mozilla Thunderbird versions prior to 154 and prior to 153.1

Discovery Timeline

2026-08-18 - CVE-2026-74961 published to NVD
2026-08-20 - Last updated in NVD database

Technical Details for CVE-2026-74961

Vulnerability Analysis

The vulnerability resides in the Web Audio component, which processes audio streams and applies signal-processing operations in the browser. A discrepancy in observable behavior, such as processing time or state changes, allows an attacker-controlled page to distinguish between values that should be indistinguishable to untrusted code. This class of flaw undermines the same-origin isolation guarantees enforced by the browser. Because Thunderbird shares Gecko rendering internals with Firefox, the mail client is also affected when rendering remote content.

Root Cause

The root cause is an observable discrepancy [CWE-203] in the Web Audio pipeline. Operations on privileged or cross-origin data produced measurable side effects, such as differing execution times or resource states, that an attacker script can observe. These differences form a side channel that leaks information the attacker is otherwise not authorized to access.

Attack Vector

Exploitation requires the victim to load attacker-controlled web content in Firefox, or to render remote content in Thunderbird. A malicious page uses AudioContext and related Web Audio APIs to submit crafted workloads and measures the responses. By statistically analyzing the observed variations, the attacker reconstructs cross-origin data, keying material, or other sensitive state. No local privileges or user interaction beyond visiting the page are required.

See the Mozilla Bug Report #2050380 and Mozilla Security Advisory MFSA-2026-74 for additional technical details.

Detection Methods for CVE-2026-74961

Indicators of Compromise

Browser telemetry showing sustained AudioContext and OfflineAudioContext instantiation from low-reputation domains.
Repeated high-resolution timing measurements originating from JavaScript on untrusted pages.
Firefox or Thunderbird process versions below the fixed builds still deployed in the environment.

Detection Strategies

Inventory Firefox, Firefox ESR, and Thunderbird versions across managed endpoints and flag any build predating the fixed releases.
Monitor endpoint process telemetry for outdated firefox.exe, firefox, and thunderbird binaries loading unknown web content.
Correlate DNS and web proxy logs with visits to newly registered domains that host heavy Web Audio workloads.

Monitoring Recommendations

Alert on browser or mail client versions that fall outside the organization's approved patch baseline.
Track outbound traffic patterns consistent with side-channel probing, such as repeated small measurement requests from a single tab.
Feed browser process and network telemetry into a centralized data lake to enable retrospective hunting once new indicators emerge.

How to Mitigate CVE-2026-74961

Immediate Actions Required

Upgrade Firefox to version 154 or later on all endpoints.
Upgrade Firefox ESR to 153.1 or later where extended support builds are deployed.
Upgrade Thunderbird to 154 or later, or to 153.1 on the ESR track.
Prioritize systems that browse untrusted content or process external email with remote content enabled.

Patch Information

Mozilla released fixes in Firefox 154, Firefox ESR 153.1, Thunderbird 154, and Thunderbird 153.1. Refer to Mozilla Security Advisory MFSA-2026-74, MFSA-2026-77, MFSA-2026-78, and MFSA-2026-80 for the version matrix and release notes.

Workarounds

Disable remote content rendering in Thunderbird until the patched build is deployed.
Restrict Web Audio usage through enterprise browser policies where feasible.
Enforce site isolation and block untrusted domains at the web proxy for high-risk user groups.

bash

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

# Verify installed Thunderbird version
thunderbird --version

# Example enterprise policy snippet to block remote images in Thunderbird
# (place in policies.json)
{
  "policies": {
    "BlockAboutConfig": true,
    "DisableTelemetry": true
  }
}