An issue was discovered in Enigmail before 1.9.9. Signature spoofing is possible because the UI does not properly distinguish between an attachment signature, and a signature that applies to the entire containing message, aka TBE-01-021. This is demonstrated by an e-mail message with an attachment that is a signed e-mail message in message/rfc822 format.
https://www.mail-archive.com/enigmail-users%40enigmail.net/msg04280.html
https://www.debian.org/security/2017/dsa-4070
https://sourceforge.net/p/enigmail/bugs/709/
https://lists.debian.org/debian-security-announce/2017/msg00333.html
https://lists.debian.org/debian-lts-announce/2017/12/msg00021.html
https://enigmail.net/download/other/Enigmail%20Pentest%20Report%20by%20Cure53%20-%20Excerpt.pdf