CVE Catalog

CVE-2026-41319

MediumCVSS 6.5
Published: Updated: Translated: NVD NIST

Exploitation Probability (EPSS)

Low risk
0.22%

13th percentile - higher than 13% of all known CVEs

Summary

MailKit versions prior to 4.16.0 contain a STARTTLS Response Injection vulnerability. A man-in-the-middle attacker can 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.

Risk Assessment

Organizations using MailKit may be exposed to credential interception or authentication weakening in email communication. A network attacker can exploit this to hijack sessions or steal login credentials.

Recommendation

Update MailKit to version 4.16.0 or later, which fixes this vulnerability. Verify that applications using MailKit are updated accordingly.

Related vulnerabilities

Original NVD description (English source)

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.

Vulnerability data from NVD (NIST) · CISA KEV · EPSS