MailKit is a cross-platform mail client library built on top of MimeKit. A STARTTLS Response Injection vulnerability in versions prior to 4.16.0 allows a Man-in-the-Middle attacker to inject arbitrary protocol responses across the plaintext-to-TLS trust boundary, enabling SASL authentication mechanism downgrade (e.g., forcing PLAIN instead of SCRAM-SHA-256). The internal read buffer in `SmtpStream`, `ImapStream`, and `Pop3Stream` is not flushed when the underlying stream is replaced with `SslStream` during STARTTLS upgrade, causing pre-TLS attacker-injected data to be processed as trusted post-TLS responses. Version 4.16.0 patches the issue.
Fix
Patch, vendor advisory and mitigation references on NVD 2
- https://github.com/jstedfast/MailKit/security/advisories/GHSA-9j88-vvj5-vhgr Exploit, Vendor Advisory
- https://github.com/jstedfast/MailKit/security/advisories/GHSA-9j88-vvj5-vhgr Exploit, Vendor Advisory
Every line above is quoted from NVD, CISA, Microsoft, the vendor's own advisory or DISA; nothing is generated.
Change history 1
- First seenCVSS 5.9 · jstedfastNVD modified 2026-10-26 22:10
Affected (CPE criteria) 1
cpe:2.3:a:jstedfast:mailkit:*:*:*:*:*:*:*:*
References 2
- https://github.com/jstedfast/MailKit/security/advisories/GHSA-9j88-vvj5-vhgr Exploit, Vendor Advisory
- https://github.com/jstedfast/MailKit/security/advisories/GHSA-9j88-vvj5-vhgr Exploit, Vendor Advisory