CVE-2026-41319
MailKit has STARTTLS Response Injection via unflushed stream buffer that enables SASL mechanism downgrade
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.
INFO
Published Date :
April 24, 2026, 4:16 a.m.
Last Modified :
April 25, 2026, 3:16 a.m.
Remotely Exploit :
Yes !
Source :
[email protected]
Affected Products
The following products are affected by CVE-2026-41319
vulnerability.
Even if cvefeed.io is aware of the exact versions of the
products
that
are
affected, the information is not represented in the table below.
No affected product recoded yet
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS 3.1 | MEDIUM | [email protected] |
Solution
- Update MailKit to version 4.16.0 or later.
- Ensure SmtpStream, ImapStream, and Pop3Stream buffers are flushed.
- Validate STARTTLS upgrade process.
Public PoC/Exploit Available at Github
CVE-2026-41319 has a 1 public
PoC/Exploit available at Github.
Go to the Public Exploits tab to see the list.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-41319.
| URL | Resource |
|---|---|
| https://github.com/jstedfast/MailKit/security/advisories/GHSA-9j88-vvj5-vhgr | |
| https://github.com/jstedfast/MailKit/security/advisories/GHSA-9j88-vvj5-vhgr |
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-41319 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-41319
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Community reconstruction of the legacy JSON NVD Data Feeds. This project uses and redistributes data from the NVD API but is neither endorsed nor certified by the NVD.
Shell
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-41319 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-41319 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0
Apr. 25, 2026
Action Type Old Value New Value Added Reference https://github.com/jstedfast/MailKit/security/advisories/GHSA-9j88-vvj5-vhgr -
New CVE Received by [email protected]
Apr. 24, 2026
Action Type Old Value New Value Added 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. Added CVSS V3.1 AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N Added CWE CWE-74 Added Reference https://github.com/jstedfast/MailKit/security/advisories/GHSA-9j88-vvj5-vhgr