Mutt: failure to check server certificate in SMTP TLS connection

Credit: dave b
Risk: Medium
Local: No
Remote: Yes

CVSS Base Score: 5.8/10
Impact Subscore: 4.9/10
Exploitability Subscore: 8.6/10
Exploit range: Remote
Attack complexity: Medium
Authentication: No required
Confidentiality impact: Partial
Integrity impact: Partial
Availability impact: None

Hi all. It seems that mutt fails to check the validity of a SMTP servers certificate during a TLS connection. In my mutt configuration I have set ssl_starttls = yes set ssl_force_tls = yes However, after performing the steps below I found that mutt did not properly validate the remote servers SMTP tls certificate. This means that an attacker could potentially MITM a mutt user connecting to their SMTP server even when the user has forced a TLS connection. Steps to test this: 1. I set in my hosts file the ip for to be bound to in /etc/hosts 2.Then I changed my set smtp_url = "smtp://MYUSERNAME () smtp gmail com:587/" to be set smtp_url = "smtp://MYUSERNAME () mail lolok com:587/" 3. I opened up mutt and emailed my self. I note that I saw mutt say "connecting to". I feel that this is an issue because mutt _does_ actually perform IMAP server certificate validation (at least it did when I last tested it :P).


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