CVE-2026-74934: Firefox CanvasWebGL Information Disclosure

CVE-2026-74934 Overview

CVE-2026-74934 is a site isolation flaw in the Graphics: CanvasWebGL component of Mozilla Firefox and Thunderbird. The weakness maps to [CWE-200] Information Exposure and allows a remote attacker to break site isolation guarantees enforced by the browser's rendering pipeline. Exploitation requires the victim to interact with attacker-controlled content, and successful attacks can compromise data confidentiality, integrity, and availability across origins. Mozilla addressed the issue in Firefox 154, Firefox ESR 115.39, Firefox ESR 140.14, Firefox ESR 153.1, Thunderbird 154, Thunderbird 140.14, and Thunderbird 153.1.

Critical Impact
Attackers can bypass same-origin protections in WebGL rendering to leak or manipulate data from other origins loaded in the browser.

Affected Products

Mozilla Firefox prior to 154
Mozilla Firefox ESR prior to 115.39, 140.14, and 153.1
Mozilla Thunderbird prior to 154, 140.14, and 153.1

Discovery Timeline

2026-08-18 - CVE-2026-74934 published to NVD
2026-08-21 - Last updated in NVD database

Technical Details for CVE-2026-74934

Vulnerability Analysis

The vulnerability resides in the Graphics: CanvasWebGL component, which handles hardware-accelerated 2D and 3D rendering exposed to web content through the WebGL API. Site isolation is the browser mechanism that keeps documents from different origins in separate processes or rendering contexts. When this boundary is not consistently enforced within the WebGL pipeline, content from one origin can observe or influence rendering data belonging to another origin.

The CWE-200 classification indicates that the primary consequence is information exposure. An attacker who lures a user to a malicious page can craft WebGL operations that reveal cross-origin pixel data, texture contents, or other rendering artifacts that should remain isolated. The advisory records impacts to integrity and availability as well, suggesting the flaw is not limited to passive read primitives.

Root Cause

The root cause is an incomplete or incorrect enforcement of site isolation constraints inside the CanvasWebGL rendering path. Mozilla does not disclose the specific code path in the public advisory. The high attack complexity indicates that exploitation depends on specific rendering states or timing conditions that are not trivially reproducible.

Attack Vector

Exploitation is network-based and requires user interaction. A victim must load attacker-controlled content, typically a web page or an HTML email rendered by Thunderbird, that issues crafted WebGL calls. No authentication or elevated privileges are required on the target system. Refer to the Mozilla Security Advisory MFSA-2026-74 and Mozilla Bug Report #2050584 for vendor context.

Detection Methods for CVE-2026-74934

Indicators of Compromise

Firefox or Thunderbird processes running versions earlier than the fixed builds while loading untrusted WebGL content.
Browser telemetry showing repeated WebGL context creation and readback operations from newly registered or low-reputation domains.
HTML email messages in Thunderbird that reference remote scripts invoking WebGL APIs.

Detection Strategies

Inventory installed Firefox and Thunderbird versions across the fleet and flag hosts below the patched releases listed in the Mozilla advisories.
Monitor endpoint process telemetry for firefox.exe, firefox, and thunderbird binaries loading child GPU or content processes tied to unusual outbound connections.
Correlate web proxy logs with browser version data to identify vulnerable clients visiting sites that serve WebGL-heavy content from mixed origins.

Monitoring Recommendations

Enable browser update reporting through enterprise management tools and alert on hosts that fail to reach fixed versions within the patch window.
Ingest DNS and HTTP telemetry into a centralized data lake to identify cross-origin resource loads targeting vulnerable browsers.
Review Thunderbird configurations that permit remote content in messages, since HTML email is a viable delivery channel.

How to Mitigate CVE-2026-74934

Immediate Actions Required

Upgrade Firefox to version 154 or the appropriate ESR release: 115.39, 140.14, or 153.1.
Upgrade Thunderbird to version 154, 140.14, or 153.1.
Restart affected applications after patching to ensure the vulnerable rendering processes are replaced.

Patch Information

Mozilla shipped fixes across the advisories MFSA-2026-74 through MFSA-2026-80. Enterprise administrators should deploy the updates through existing software distribution channels and verify version compliance across managed endpoints.

Workarounds

Disable WebGL in Firefox by setting webgl.disabled to true in about:config where updates cannot be applied immediately.
Block remote content in Thunderbird messages to prevent WebGL invocation from HTML email.
Restrict access to untrusted sites through web filtering while patch deployment is in progress.

bash

# Configuration example: disable WebGL via Firefox enterprise policy (policies.json)
{
  "policies": {
    "Preferences": {
      "webgl.disabled": {
        "Value": true,
        "Status": "locked"
      }
    }
  }
}