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Findings

CVE-2026-41319

MailKit is a cross-platform mail client library built on top of MimeKit.

Published Apr 24, 2026 · updated Jun 17, 2026 · jstedfast mailkit

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 , , and is not flushed when the underlying stream is replaced with during STARTTLS upgrade, causing pre-TLS attacker-injected data to be processed as trusted post-TLS responses. Version 4.16.0 patches the issue.

Code

The project this record applies to. Exploit listings stay in References.

No public source repository is identified for this product.

Timeline

Detection, updates, and resolution for this record.

  1. Apr 24, 2026 · NIST NVD

    Detected

    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 , , and is

  2. Jun 17, 2026 · NIST NVD

    Updated

    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 , , and is

Affected

  • cpe jstedfast/mailkitversion unspecified

Sources

  • CVE.orgVulnerability recordCVE-2026-41319

    Official CVE identifier

    cve.org
  • NIST NVDVulnerability recordCVE-2026-41319

    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 au

    nvd.nist.gov

References