CVE-2026-41319Public exploitNetwork attack vector

SASL downgrade via unflushed STARTTLS buffers

Published Apr 24, 2026 · Updated Apr 24, 2026

STARTTLS response injection in MailKit before 4.16.0 allows man-in-the-middle attackers to downgrade SASL authentication. During an SMTP, IMAP, or POP3 STARTTLS upgrade, SmtpStream, ImapStream, and Pop3Stream replace the transport with SslStream without clearing buffered plaintext responses, so injected bytes are trusted after TLS begins. Reachability requires StartTls or the default StartTlsWhenAvailable mode and an on-path attacker; successful injection can force weaker authentication or manipulate advertised capabilities.

CVSS severity6.5
Medium
EPSS probability0.22%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

STARTTLS response injection in MailKit before 4.16.0 allows man-in-the-middle attackers to downgrade SASL authentication. During an SMTP, IMAP, or POP3 STARTTLS upgrade, SmtpStream, ImapStream, and Pop3Stream replace the transport with SslStream without clearing buffered plaintext responses, so injected bytes are trusted after TLS begins. Reachability requires StartTls or the default StartTlsWhenAvailable mode and an on-path attacker; successful injection can force weaker authentication or manipulate advertised capabilities.

The record

CVE
CVE-2026-41319
Published
Apr 24, 2026
Updated
Apr 24, 2026
Vendor
Jeffrey Stedfast
Product
MailKit
Classifications
CWE-74, T1557
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 24, 2026CVE publishedPublication date reported by the CVE source.
  2. Apr 24, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 4.16.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Jeffrey Stedfast · MailKitversion=< 4.16.0

Published CVSS scores

6.5GitHub, Inc.CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N
5.9NIST NVDCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-74Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')
T1557Adversary-in-the-Middle

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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