CVE-2026-118174

When a message containing an OpenPGP key attachment was displayed, Thunderbird automatically loaded the attachment's content to offer importing the key. A sender could mark the attachment as an external attachment pointing to an arbitrary URL, which Thunderbird then loaded without any user interaction, bypassing remote content blocking. On Windows, a URL pointing to an attacker-controlled network share could leak the user's NTLM credentials. The NTLM credential leak only affects Thunderbird on Windows. On other operating systems, the issue allowed a remote request that bypassed remote content blocking. This vulnerability was fixed in Thunderbird 159 and Thunderbird 153.6.

• https://bugzilla.mozilla.org/show_bug.cgi?id=2080910
• https://www.mozilla.org/security/advisories/mfsa2026-121/
• https://www.mozilla.org/security/advisories/mfsa2026-122/