CVE-2026-126133

When a CalDAV or ICS calendar was accessed through an HTTP proxy that requested authentication, Thunderbird could answer the proxy with the calendar server's username and password. This affected both setting up a new calendar and syncing existing calendars, where a saved password could be sent without any user interaction. A malicious or compromised proxy could use this to obtain the calendar credentials, even for calendars served over HTTPS. This vulnerability was fixed in Thunderbird 160 and Thunderbird 153.7.

• https://bugzilla.mozilla.org/show_bug.cgi?id=2083485
• https://www.mozilla.org/security/advisories/mfsa2026-128/
• https://www.mozilla.org/security/advisories/mfsa2026-129/