PT-2026-34845 · Jstedfast · Emailkit

Published 2026-04-24 · Updated 2026-04-24 · CVE-2026-41319

NVD

CVSS v3.1

6.5

Medium

AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N

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.

Fix

Special Elements Injection

Weakness Enumeration

CWE-74

Related Identifiers

CVE-2026-41319

Affected Products

Emailkit

References · 2

• https://nvd.nist.gov/vuln/detail/CVE-2026-41319 · Security Note
• https://github.com/jstedfast/MailKit/security/advisories/GHSA-9j88-vvj5-vhgr · Note