# CVE-2026-41319

> Vulnerability · severity: **MEDIUM** (CVSS 6.5).

## 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.

## Key facts

- **CVE ID:** CVE-2026-41319
- **Published:** 2026-04-24
- **CVSS severity:** MEDIUM
- **CVSS base score:** 6.5
- **CWE codes:** CWE-74

## Primary sources

- NIST NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-41319

## Citation

> AI Analytics. CVE-2026-41319. Retrieved 2026-07-15 from https://api.ai-analytics.org/cve/CVE-2026-41319. Derived from NIST NVD. Licensed CC0.

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