CVE-2026-41319 json
MailKit has STARTTLS Response Injection via unflushed stream buffer that enables SASL mechanism downgrade
published 6mo ago modified 14d ago also GHSA-9j88-vvj5-vhgr , EUVD-2026-25388
- cvss
- 6.5
- version 3.1, CNA
- cisa kev
- Not listed
- not in CISA's catalog
- epss
- 0.2%
- EPSS 0.2% chance of exploitation in 30 days, percentile 12%
- public exploits
- 0
- none known
Summary
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.
Patch / fix
- nuget MailKit 4.16.0
- jstedfast mailkit 4.16.0
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Check with FencerCISA SSVC
- Exploitation
- Proof of concept is public
- Automatable
- no
- Technical impact
- partial
- Assessed
- 6mo ago
Exploit prediction
0.22% chance of exploitation in the next 30 days, higher than 12% of all scored CVEs (FIRST EPSS, 2026-11-09).
Exploits and checks
- github.com https://github.com/jstedfast/MailKit/security/advisories/GHSA-9j88-vvj5-vhgr referenced PoC, tagged "exploit" by a source, unverified
CVSS scores
| assigned by | version | score | vector |
|---|---|---|---|
| CNA | 3.1 | 6.5 | CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N |
| NVD | 3.1 | 5.9 | CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N |
| GitHub | 3.1 | 6.5 | CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N |
Affected packages
| ecosystem | package | affected | fixed in |
|---|---|---|---|
| nuget | MailKit | <4.16.0 |
4.16.0 |
Affected products
- jstedfast MailKit
Classification
- Weakness
-
- CWE-74 Improper Neutralization of Special Elements in Output Used by a Downstream Component
References
-
github.com
/jstedfast/MailKit/security/advisories/GHSA-9j88-vvj5-vhgr
exploit, web, vendor-advisory
-
nvd.nist.gov
/vuln/detail/CVE-2026-41319
advisory
-
github.com
/jstedfast/MailKit
package
Sources
- CVE List CVE-2026-41319
- GitHub Advisory GHSA-9j88-vvj5-vhgr
- NVD CVE-2026-41319
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