CVE-2026-41319 MailKit has STARTTLS Response Injection via unflushed stream buffer that enables SASL mechanism downgrade
A Man-in-the-Middle attacker can inject protocol responses during STARTTLS upgrade to force weaker SASL authentication mechanisms in MailKit.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-41319?
CVE-2026-41319 is a medium-severity vulnerability in jstedfast MailKit, published on 24 Apr 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official 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.
Weakness: CWE-74
Which versions are affected by CVE-2026-41319?
2 open source packages ship an affected version, including MailKit. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
MailKit NuGet | All versions | 4.16.0 |
MailKit NuGet | All versions | 4.16.0 |
Products include Jstedfast Mailkit.
How do I fix CVE-2026-41319?
Upgrade MailKit to 4.16.0 or later.
Is CVE-2026-41319 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-41319 exists.
Sources
Also known as GHSA-9j88-vvj5-vhgr.