What’s Vulnerable › Vulnerabilities

CVE-2026-41319: MailKit has STARTTLS Response Injection via unflushed stream buffer that enables SASL mechanism downgrade

• Severity: medium (CVSS 5.9 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.2% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2026-04-24 · updated 2026-10-26
• Weakness: CWE-74

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: before 4.16.0 — fixed in 4.16.0

How to fix

Upgrade to a fixed version: jstedfast MailKit 4.16.0. Check the vendor advisory for mitigations.

References

• https://github.com/jstedfast/MailKit/security/advisories/GHSA-9j88-vvj5-vhgr
• https://github.com/jstedfast/MailKit/security/advisories/GHSA-9j88-vvj5-vhgr

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