CVE-2026-41319: Jstedfast Mailkit

Medium severity, CVSS 5.9. EPSS: 0.3% chance of exploitation in the next 30 days.

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.

Affected products

• Jstedfast Mailkit: before 4.16.0 (fixed in 4.16.0)

Published 2026-04-24. Last modified 2026-06-17.