Vulnerability in Jstedfast Mailkit

CVE-2026-41319

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 · last modified .

CVSS v3 metric

CVSS v3 base score 6.5 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N.

EPSS exploit prediction

EPSS: 0.003 (23.9th percentile), scored .

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 24th percentile — 23.9% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS trend (30 days)EPSS over the last 30 days for CVE-2026-41319: held from 0.002 to 0.003.

EPSS over last 30 days · oldest: 0.002 · newest: 0.003 · change: +0.001

Affected products

Weakness classification (CWE)

References

Frequently asked questions

What is CVE-2026-41319?
CVE-2026-41319 is a medium-severity vulnerability in Jstedfast Mailkit, classified under Improper Neutralization of Special Elements in Output Used by a Downstream Component (Injection). CVSS score: 6.5/10. Published 2026-04-24.
How severe is CVE-2026-41319?
Medium severity. CVSS v3 base score is 6.5 out of 10.