[MFSA-2021-55] Security Vulnerabilities fixed in Thunderbird 91.4.1

CVE-2021-4126: OpenPGP signature status doesn't consider additional message content (moderate)
When receiving an OpenPGP/MIME signed email message that contains an
additional outer MIME message layer, for example a message footer added by a
mailing list gateway, Thunderbird only considered the inner signed message for
the signature validity. This gave the false impression that the additional contents
were also covered by the digital signature. Starting with Thunderbird version 91.4.1,
only the signature that belongs to the top level MIME part will be considered for
the displayed status.

CVE-2021-44538: Matrix chat library libolm bundled with Thunderbird vulnerable to a buffer overflow (moderate)
Thunderbird users who use the Matrix chat protocol were vulnerable
to a buffer overflow in libolm, that an attacker may trigger by a crafted
sequence of messages. The overflow content is partially controllable
by the attacker and limited to ASCII spaces and digits.