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.

Affected: NuGet: MailKit

### Summary

A STARTTLS Response Injection vulnerability in MailKit 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. This is the same vulnerability class as CVE-2021-23993 (Thunderbird), CVE-2021-33515 (Dovecot), and CVE-2011-0411 (Postfix).

### Details

The `Stream` property in `SmtpStream` (line 84-86), `ImapStream`, and `Pop3Stream` is a simple auto-property with no buffer reset:

```csharp public Stream Stream { get; internal set; // ← No buffer reset on set! } ```

During the STARTTLS upgrade in `SmtpClient.cs` (lines 1372-1389):

```csharp // Reads STARTTLS response — "220 Ready" consumed, any extra data stays in buffer response = Stream.SendCommand("STARTTLS\r\n", cancellationToken);

// Swaps to TLS — buffer NOT flushed! var tls = new SslStream(stream, false, ValidateRemoteCertificate); Stream.Stream = tls; SslHandshake(tls, host, cancellationToken);

// Reads EHLO response — processes INJECTED pre-TLS data from buffer first! Ehlo(true, cancellationToken); ```

A MitM appends extra data after the `"220 Ready\r\n"` STARTTLS response. Both arrive in…