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.
Affected products
jstedfast: mailkit
References2
- github.com/jstedfast/MailKit/security/advisories/GHSA-9j88-vvj5… ExploitVendor Advisory
- github.com/jstedfast/MailKit/security/advisories/GHSA-9j88-vvj5… ExploitVendor Advisory
Metrics
- CVSS 3.1
-
5.9
MEDIUM
Score source: NVD
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N - EPSS — exploit probability
- 0% percentile: 13 · as of 13 November 2026
- CWE
- CWE-74
Could this be exploited in your environment?
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