The signature verification routine in the Airmail GPG-PGP Plugin, versions 1.0 (9) and earlier, does not verify the status of the signature at all, which allows remote attackers to spoof arbitrary email signatures by crafting a signed email with an invalid signature. Also, it does not verify the validity of the signing key, which allows remote attackers to spoof arbitrary email signatures by crafting a key with a fake user ID (email address) and injecting it into the user’s keyring.
packetstormsecurity.com/files/152703/Johnny-You-Are-Fired.html
seclists.org/fulldisclosure/2019/Apr/38
github.com/Airmail/AirmailPlugIn-Framework/commits/master
github.com/RUB-NDS/Johnny-You-Are-Fired
github.com/RUB-NDS/Johnny-You-Are-Fired/blob/master/paper/johnny-fired.pdf
www.openwall.com/lists/oss-security/2019/04/30/4