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

diff --git a/src/backend/libpq/pqcomm.c b/src/backend/libpq/pqcomm.c
index 6a870b897303eb8247367995901ccf76e4a2335d..16281a1ce09d691d68657d935d54b948c7d6f8d7 100644 (file)
--- a/src/backend/libpq/pqcomm.c
+++ b/src/backend/libpq/pqcomm.c
@@ -1176,6 +1176,18 @@ pq_getstring(StringInfo s)
    }
 }
 
+/* --------------------------------
+ *     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);
+}
+
 
 /* --------------------------------
  *     pq_startmsgread - begin reading a message from the client.

diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 19aa7b765d6413a39f559de3ca3a3c4dc931b17d..7d54e339d0ac8373692e3d08aec3b9fddf35246f 100644 (file)
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -2025,6 +2025,19 @@ retry1:
        if (SSLok == 'S' && secure_open_server(port) == -1)
            return STATUS_ERROR;
 #endif
+
+       /*
+        * 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.")));
+
        /* regular startup packet, cancel, etc packet should follow... */
        /* but not another SSL negotiation request */
        return ProcessStartupPacket(port, true);

diff --git a/src/include/libpq/libpq.h b/src/include/libpq/libpq.h
index 18052cbfef86d51a0d08149d72148e4263ebc921..348292f45815c6e81430af6ca3eccbce8790570c 100644 (file)
--- a/src/include/libpq/libpq.h
+++ b/src/include/libpq/libpq.h
@@ -71,6 +71,7 @@ extern int    pq_getmessage(StringInfo s, int maxlen);
 extern int pq_getbyte(void);
 extern int pq_peekbyte(void);
 extern int pq_getbyte_if_available(unsigned char *c);
+extern bool pq_buffer_has_data(void);
 extern int pq_putbytes(const char *s, size_t len);
 
 /*