CVE-2026-41319

6.5 MEDIUM

MailKit is a cross-platform mail client library built on top of MimeKit

Published: 2026-04-24 · Last updated: 2026-05-21

Severity and scoring

CVSS: 6.5 MEDIUM
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N
CWE: CWE-74

Affected products

Vendor	Product
jstedfast	mailkit

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.

Source: NVD

References

[NVD]https://nvd.nist.gov/vuln/detail/CVE-2026-41319
[Vendor advisory]https://github.com/jstedfast/MailKit/security/advisories/GHSA-9j88-vvj5-vhgr
[Vendor advisory]https://github.com/jstedfast/MailKit/security/advisories/GHSA-9j88-vvj5-vhgr

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