CVE-2026-2771 Mozilla CVE debrief

Who should care

This vulnerability affects users of Mozilla Firefox, Firefox ESR, and Thunderbird. Specifically, users with versions prior to Firefox 148, Firefox ESR 115.33, Firefox ESR 140.8, Thunderbird 148, and Thunderbird 140.8 are vulnerable. Organizations and individuals using these affected versions should prioritize updating their installations to prevent potential exploitation.

Technical summary

The CVE-2026-2771 vulnerability is caused by undefined behavior in the DOM: Core & HTML component of Mozilla Firefox. This vulnerability has a CVSS score of 9.8, indicating a critical severity level. The CVSS vector is CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H, indicating that the vulnerability can be exploited remotely with low attack complexity and no privileges required. The vulnerability affects multiple Firefox versions, including Firefox, Firefox ESR, and Thunderbird. The CWE associated with this vulnerability is CWE-125, which refers to a 'Buffer Overflow' vulnerability.

Defensive priority

This vulnerability has a high defensive priority due to its critical severity level and potential for remote exploitation. Organizations and individuals using affected Firefox and Thunderbird versions should prioritize updating their installations to prevent potential exploitation.

Recommended defensive actions

• Update Firefox to version 148 or later
• Update Firefox ESR to version 115.33 or later
• Update Firefox ESR to version 140.8 or later
• Update Thunderbird to version 148 or later
• Update Thunderbird to version 140.8 or later

Evidence notes

The CVE-2026-2771 vulnerability was publicly disclosed on February 24, 2026, and has since been modified on June 30, 2026. The vulnerability is considered critical, with a CVSS score of 9.8. Multiple sources, including the CVE record and NVD, provide detailed information about this vulnerability.

Official resources