https://arstechnica.com/information-technology/2018/06/decades-old-pgp-bug-allowed-hackers-to-spoof-just-about-anyones-signature/

For their entire existence, some of the world's most widely used email encryption tools have been vulnerable to hacks that allowed attackers to spoof the digital signature of just about any person with a public key, a researcher said Wednesday. GnuPG, Enigmail, GPGTools, and python-gnupg have all been updated to patch the critical vulnerability. Enigmail and the Simple Password Store have also received patches for two related spoofing bugs.

Digital signatures are used to prove the source of an encrypted message, data backup, or software update. Typically, the source must use a private encryption key to cause an application to show that a message or file is signed. But a series of vulnerabilities dubbed SigSpoof makes it possible in certain cases for attackers to fake signatures with nothing more than someone’s public key or key ID, both of which are often published online. The spoofed email shown at the top of this post can't be detected as malicious without doing forensic analysis that's beyond the ability of many users.