Vulnerability Description
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.
CVSS Score
MEDIUM
Affected Products
| Vendor | Product | Versions |
|---|---|---|
| Jstedfast | Mailkit | < 4.16.0 |
Related Weaknesses (CWE)
References
- https://github.com/jstedfast/MailKit/security/advisories/GHSA-9j88-vvj5-vhgrExploitVendor Advisory
- https://github.com/jstedfast/MailKit/security/advisories/GHSA-9j88-vvj5-vhgrExploitVendor Advisory
FAQ
What is CVE-2026-41319?
CVE-2026-41319 is a vulnerability with a CVSS score of 6.5 (MEDIUM). 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...
How severe is CVE-2026-41319?
CVE-2026-41319 has been rated MEDIUM with a CVSS base score of 6.5/10. Review the CVSS metrics above for detailed severity breakdown.
Is there a patch for CVE-2026-41319?
Check the references section above for vendor advisories and patch information. Affected products include: Jstedfast Mailkit.