Security Vulnerabilities fixed in Thunderbird 91.1 — Mozilla

When delegating navigations to the operating system, Thunderbird would accept the mk scheme which might allow attackers to launch pages and execute scripts in Internet Explorer in unprivileged mode. This bug only affects Thunderbird for Windows. Other operating systems are unaffected.
Mozilla developers Tyson Smith, Christian Holler, and Gabriele Svelto reported memory safety bugs present in Thunderbird 78.13.0. Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code.