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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.

PUBLISHED Reserved 2026-04-20 | Published 2026-04-24 | Updated 2026-04-25 | Assigner GitHub_M




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

Problem types

CWE-74: Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')

Product status

< 4.16.0
affected

References

github.com/...ailKit/security/advisories/GHSA-9j88-vvj5-vhgr exploit

github.com/...ailKit/security/advisories/GHSA-9j88-vvj5-vhgr

cve.org (CVE-2026-41319)

nvd.nist.gov (CVE-2026-41319)

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