Vulnerability in Mozilla Thunderbird

[ Description ]Official description

When logging in to a Matrix account, Thunderbird used the homeserver address provided in the server's login response without validating it. A malicious or compromised homeserver could provide a file: URL, causing Thunderbird to send all further Matrix requests of that session to local files or, on Windows, to an attacker-controlled network share, which could leak the user's NTLM credentials. The NTLM credential leak only affects Thunderbird on Windows. Other operating systems are unaffected. This vulnerability was fixed in Thunderbird 159 and Thunderbird 153.6.

[ Fixes ]Fixes and patches

No fix or interim patch is linked from our sources yet. Check the vendor references below and limit exposure of affected systems until a fix is available.

[ Advisories ]Government CERT advisories

• CERT-FR28 Oct 2026CERTFR-2026-AVI-1406

  Multiple vulnerabilities in Mozilla products Translated from French

  Multiple vulnerabilities have been discovered in Mozilla products. Some of them allow an attacker to cause remote arbitrary code execution, privilege escalation and remote denial of service.

  CVE-2026-117907CVE-2026-117889CVE-2026-117988+177 more CVEs
• BSI CERT-Bund27 Oct 2026WID-SEC-2026-4216

  Mozilla Firefox, Firefox ESR, and Thunderbird: Multiple vulnerabilities Translated from German

  Risk highCVE-2026-117930CVE-2026-117957CVE-2026-117890+177 more CVEs

[ References ]References

• bugzilla.mozilla.org/show_bug.cgi
• www.mozilla.org/security/advisories/mfsa2026-121
• www.mozilla.org/security/advisories/mfsa2026-122