MailKit has STARTTLS Response Injection via unflushed stream buffer that enables SASL mechanism downgrade

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

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

EPSS

• CVE-2026-41319: 0.22% estimated exploitation activity in the next 30 days; percentile 12.96 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary A STARTTLS Response Injection vulnerability in MailKit 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. This is the same vulnerability class as CVE-2021-23993 (Thunderbird), CVE-2021-33515 (Dovecot), and CVE-2011-0411 (Postfix). Details The Stream property in SmtpStream (line 84-86), ImapStream, and Pop3Stream is a simple auto-property with no buffer reset: During the STARTTLS upgrade in SmtpClient.cs (lines 1372-1389): A MitM appends extra data after the "220 Ready\r\n" STARTTLS response. Both arrive in one TCP read into SmtpStream's 4096-byte internal buffer.…

Published by the source

18 Apr 2026, 01:13 UTC

Last modified by the source

24 Apr 2026, 21:02 UTC

Source last checked

26 Oct 2026, 17:04 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-9j88-vvj5-vhgr

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# Patch Next advisory brief: CVE-2026-41319

Advisory: MailKit has STARTTLS Response Injection via unflushed stream buffer that enables SASL mechanism downgrade
IDs: CVE-2026-41319, GHSA-9j88-vvj5-vhgr
Record: https://patchnext.com/advisories/cve-2026-41319 (revision 1, source checked 26 Oct 2026, 17:04 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 6.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: mailkit 4.16.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- NuGet mailkit: affected < 4.16.0; fixed in 4.16.0

## Suggested inspection (read-only)
- NuGet mailkit: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on mailkit.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-9j88-vvj5-vhgr (GitHub-reviewed advisory GHSA-9j88-vvj5-vhgr lists affected ranges: NuGet mailkit >=0 <4.16.0)

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