[MFSA-2021-13] Security Vulnerabilities fixed in Thunderbird 78.9.1

CVE-2021-23991: An attacker may use Thunderbird's OpenPGP key refresh mechanism to poison an existing key (moderate)
If a Thunderbird user has previously imported Alice's OpenPGP key, and Alice has extended the validity period of her key, but Alice's updated key has not yet been imported, an attacker may send an email containing a crafted version of Alice's key with an invalid subkey, Thunderbird might subsequently attempt to use the invalid subkey, and will fail to send encrypted email to Alice.

CVE-2021-23992: A crafted OpenPGP key with an invalid user ID could be used to confuse the user (moderate)
Thunderbird did not check if the user ID associated with an OpenPGP key has a valid self signature. An attacker may create a crafted version of an OpenPGP key, by either replacing the original user ID, or by adding another user ID. If Thunderbird imports and accepts the crafted key, the Thunderbird user may falsely conclude that the false user ID belongs to the correspondent.

CVE-2021-23993: Inability to send encrypted OpenPGP email after importing a crafted OpenPGP key (moderate)
An attacker may perform a DoS attack to prevent a user from sending encrypted email to a correspondent. If an attacker creates a crafted OpenPGP key with a subkey that has an invalid self signature, and the Thunderbird user imports the crafted key, then Thunderbird may try to use the invalid subkey, but the RNP library rejects it from being used, causing encryption to fail.

CVE-2021-29949: Thunderbird might execute an alternative OTR library (low)
When loading the shared library that provides the OTR protocol implementation, Thunderbird will initially attempt to open it using a filename that isn't distributed by Thunderbird. If a computer has already been infected with a malicious library of the alternative filename, and the malicious library has been copied to a directory that is contained in the search path for executable libraries, then Thunderbird will load the incorrect library.