An issue was discovered in Enigmail before 1.9.9. Regular expressions are exploitable for Denial of Service, because of attempts to match arbitrarily long strings, aka TBE-01-003.
enigmail.net/download/other/Enigmail%20Pentest%20Report%20by%20Cure53%20-%20Excerpt.pdf
lists.debian.org/debian-lts-announce/2017/12/msg00021.html
lists.debian.org/debian-security-announce/2017/msg00333.html
www.debian.org/security/2017/dsa-4070
www.mail-archive.com/enigmail-users%40enigmail.net/msg04280.html