CVE-2021-23214 (C, published 2022-03-04T16:15Z)
Description: value: When the server is configured to use trust authentication with a clientcert requirement or to use cert authentication, a man-in-the-middle attacker can inject arbitrary SQL queries when a connection is first established, despite the use of SSL certificate verification and encryption.
Severity: MEDIUM (CVSS v3 base score: 8.1)
Weakness: CWE-89 \u2014 Improper Neutralization of Special Elements used in an SQL Command ('SQL Injection')
The product constructs all or part of an SQL command using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the intended SQL command when it is sent to a downstream component. Without sufficient removal or quoting of SQL syntax in user-controllable inputs, the generated SQL query can cause those inputs to be interpreted as SQL instead of ordinary user data.
Repository: https://github.com/postgres/postgres
Files changed:
src/backend/libpq/pqcomm.c
src/backend/postmaster/postmaster.c
src/include/libpq/libpq.h
Commit message: Reject extraneous data after SSL or GSS encryption handshake.

The server collects up to a bufferload of data whenever it reads data
from the client socket.  When SSL or GSS encryption is requested
during startup, any additional data received with the initial
request message 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 be abused to send faked SQL commands to the server,
although that would only work if the server did not demand any
authentication data.  (However, a server relying on SSL certificate
authentication might well not do so.)

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-2021-23214


Vulnerable code:

Fixed code:
/* --------------------------------
 *		pq_buffer_has_data		- is any buffered data available to read?
 *
 * This will *not* attempt to read more data.
 * --------------------------------
 */
bool
pq_buffer_has_data(void)
{
	return (PqRecvPointer < PqRecvLength);
}

		/*
		 * At this point we should have no data already buffered.  If we do,
		 * it was received before we performed the SSL handshake, so it wasn't
		 * encrypted and indeed may have been injected by a man-in-the-middle.
		 * We report this case to the client.
		 */
		if (pq_buffer_has_data())
			ereport(FATAL,
					(errcode(ERRCODE_PROTOCOL_VIOLATION),
					 errmsg("received unencrypted data after SSL request"),
					 errdetail("This could be either a client-software bug or evidence of an attempted man-in-the-middle attack.")));

		/*
		 * At this point we should have no data already buffered.  If we do,
		 * it was received before we performed the GSS handshake, so it wasn't
		 * encrypted and indeed may have been injected by a man-in-the-middle.
		 * We report this case to the client.
		 */
		if (pq_buffer_has_data())
			ereport(FATAL,
					(errcode(ERRCODE_PROTOCOL_VIOLATION),
					 errmsg("received unencrypted data after GSSAPI encryption request"),
					 errdetail("This could be either a client-software bug or evidence of an attempted man-in-the-middle attack.")));

extern bool pq_buffer_has_data(void);