EUVD-2026-25388CVSS VectorNVD
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N
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2DescriptionNVD
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.
AnalysisAI
STARTTLS response injection in MailKit prior to version 4.16.0 allows man-in-the-middle attackers to downgrade SASL authentication mechanisms by injecting malicious protocol responses before TLS negotiation completes. The vulnerability stems from failure to flush the internal read buffer when upgrading from plaintext to encrypted connections, enabling attackers to force weaker authentication (e.g., PLAIN instead of SCRAM-SHA-256) on affected SMTP, IMAP, and POP3 connections. …
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