An issue was discovered in Enigmail before 1.9.9. In a variant of CVE-2017-17847, signature spoofing is possible for multipart/related messages because a signed message part can be referenced with a cid: URI but not actually displayed. In other words, the entire containing message appears to be signed, but the recipient does not see any of the signed text.
packetstormsecurity.com/files/152703/Johnny-You-Are-Fired.html
seclists.org/fulldisclosure/2019/Apr/38
www.openwall.com/lists/oss-security/2019/04/30/4
github.com/RUB-NDS/Johnny-You-Are-Fired
github.com/RUB-NDS/Johnny-You-Are-Fired/blob/master/paper/johnny-fired.pdf
lists.debian.org/debian-lts-announce/2017/12/msg00021.html
lists.debian.org/debian-security-announce/2017/msg00333.html
sourceforge.net/p/enigmail/bugs/709/
www.debian.org/security/2017/dsa-4070