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high advisory

Mozilla Firefox and Thunderbird Canvas2D Improper Boundary Conditions Vulnerability (CVE-2026-4686)

CVE-2026-4686 is a high-severity vulnerability due to incorrect boundary conditions in the Canvas2D component of Mozilla Firefox and Thunderbird, potentially leading to a denial-of-service condition.

CVE-2026-4686 describes a vulnerability affecting Mozilla Firefox and Thunderbird due to improper boundary conditions within the Graphics: Canvas2D component. This flaw can be triggered in Firefox versions prior to 149, Firefox ESR versions less than 115.34 and 140.9, and Thunderbird versions less than 149 and 140.9. An unauthenticated remote attacker could potentially exploit this vulnerability to cause a denial-of-service condition by crafting malicious web content that triggers the improper boundary conditions within the Canvas2D rendering engine. This could lead to resource exhaustion or application crashes, impacting availability for legitimate users. Defenders should prioritize patching affected versions of Firefox and Thunderbird.

Attack Chain

  1. The attacker crafts a malicious HTML page containing JavaScript that leverages the Canvas2D API.
  2. The malicious JavaScript generates specific Canvas2D drawing operations designed to trigger the incorrect boundary condition.
  3. The victim visits the malicious webpage using a vulnerable version of Firefox or Thunderbird.
  4. The browser's Canvas2D rendering engine attempts to process the malformed drawing operations.
  5. The improper boundary conditions lead to excessive resource consumption (e.g., memory allocation or CPU usage).
  6. The affected process becomes unresponsive or crashes due to resource exhaustion.
  7. The user experiences a denial-of-service condition, where the browser or email client becomes unusable.

Impact

Successful exploitation of CVE-2026-4686 can lead to a denial-of-service condition in Firefox and Thunderbird. While the exact number of potential victims is unknown, any user running vulnerable versions of the software is at risk. This can disrupt productivity, especially for users who rely on these applications for web browsing or email communication. The vulnerability can be triggered remotely without requiring user interaction beyond visiting a malicious webpage.

Recommendation

  • Upgrade Mozilla Firefox to version 149 or later to remediate CVE-2026-4686.
  • Upgrade Mozilla Firefox ESR to version 115.34 or 140.9 or later.
  • Upgrade Thunderbird to version 149 or later to remediate CVE-2026-4686.
  • Deploy the Sigma rule "Detect Canvas2D DoS Exploit Attempt" to identify potential exploitation attempts by monitoring JavaScript execution.

Detection coverage 2

Detect Canvas2D DoS Exploit Attempt

high

Detects potential attempts to exploit Canvas2D vulnerabilities leading to denial-of-service by monitoring for excessive or unusual Canvas API calls within a short timeframe.

sigma tactics: denial_of_service techniques: T1499 sources: webserver, linux

Detect Malicious HTML Containing Canvas2D API Calls

medium

Detects potential exploitation attempts of Canvas2D by detecting specific keywords associated with the Canvas2D API calls within HTML content.

sigma tactics: denial_of_service techniques: T1499 sources: webserver, linux

Detection queries are available on the platform. Get full rules →