MailKit versions prior to 4.16.0 contain a STARTTLS Response Injection flaw in the stream handling code. An attacker who can perform a man‑in‑the‑middle (MITM) on the email session can inject arbitrary protocol responses before the TLS upgrade. Those injected responses are processed after the stream is wrapped in SSL, allowing the attacker to force the client to authenticate with weaker SASL mechanisms such as PLAIN instead of stronger SCRAM series. This can lead to credential theft or unauthorized access if weak authentication is used.

The affected product is jstedfast MailKit. All releases before 4.16.0 are vulnerable; the issue is fixed in MailKit 4.16.0 and later. Any application that references an older MailKit library is at risk, regardless of the underlying mail protocol (SMTP, IMAP, POP3).

The CVSS score of 6.5 indicates a moderate severity in the context of authentication downgrade. The EPSS score of less than 1% shows that current exploitation activity appears very low. MailKit is not listed in the CISA KEV catalog. The likely attack vector requires a MITM that can intercept the plaintext phase of the connection; the attacker does not need any special privileges on the client machine. Although exploitation probability is low, the impact of successfully downgrading to a weak SASL mechanism is significant for systems relying on strong authentication.