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libpq: reject extraneous data after SSL or GSS encryption handshake.
libpq collects up to a bufferload of data whenever it reads data from the socket. When SSL or GSS encryption is requested during startup, any additional data received with the server's yes-or-no reply remained in the buffer, and would be treated as already-decrypted data once the encryption handshake completed. Thus, a man-in-the-middle with the ability to inject data into the TCP connection could stuff some cleartext data into the start of a supposedly encryption-protected database session. This could probably be abused to inject faked responses to the client's first few queries, although other details of libpq's behavior make that harder than it sounds. A different line of attack is to exfiltrate the client's password, or other sensitive data that might be sent early in the session. That has been shown to be possible with a server vulnerable to CVE-2021-23214. To fix, throw a protocol-violation error if the internal buffer is not empty after the encryption handshake. Our thanks to Jacob Champion for reporting this problem. Security: CVE-2021-23222
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