Mozilla Firefox and Thunderbird Improper Boundary Condition Vulnerability (CVE-2026-4699)

CVE-2026-4699 describes a vulnerability in the Layout: Text and Fonts component of Mozilla Firefox and Thunderbird products. This vulnerability stems from incorrect boundary conditions, which can lead to unexpected behavior and potentially a denial-of-service. The vulnerability affects Firefox versions prior to 149, Firefox ESR versions prior to 115.34 and 140.9, and Thunderbird versions prior to 149 and 140.9. This was reported to Mozilla and assigned bug ID 2021863. Successful exploitation could result in instability or termination of the affected application. Defenders should prioritize patching affected Firefox and Thunderbird versions.

Attack Chain

1. An attacker crafts a malicious HTML page or email containing specific text and font formatting.
2. The user opens the crafted HTML page in Firefox or views the malicious email in Thunderbird.
3. The Layout: Text and Fonts component attempts to render the text.
4. Due to the incorrect boundary conditions, the rendering process encounters an unexpected state.
5. The application enters an infinite loop or attempts to access an invalid memory location.
6. This causes the affected process to become unresponsive.
7. The Firefox or Thunderbird application crashes, resulting in a denial-of-service.

Impact

Successful exploitation of CVE-2026-4699 can lead to a denial-of-service condition on the affected system. An attacker could potentially disrupt the user's browsing or email activities. While the vulnerability has a CVSS v3.1 score of 7.5, the impact is limited to application instability. This vulnerability affects users of Firefox (versions < 149, ESR < 115.34, ESR < 140.9) and Thunderbird (versions < 149, < 140.9).

Recommendation

• Upgrade Mozilla Firefox to version 149 or later to patch CVE-2026-4699.
• Upgrade Mozilla Firefox ESR to version 115.3.4 or 140.9 or later to patch CVE-2026-4699.
• Upgrade Mozilla Thunderbird to version 149 or 140.9 or later to patch CVE-2026-4699.