Risk Scores
CVSS v3.1
6.5
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N
Affected Products
| Vendor | Product | Versions |
|---|---|---|
| jstedfast | MailKit | < 4.16.0 |
Timeline
- Apr 18, 2026 CVE Published
- Apr 24, 2026 Security Advisory
- Apr 25, 2026 CVE 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 `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.
| Vendor | Product | Versions |
|---|---|---|
| jstedfast | MailKit | < 4.16.0 |