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 ee2cd86866da9f71dec0708ecba09d014bc33fb9..93f2e0b81d328da2c720239079e6e2a4e1ae679c 100644 (file)
--- a/src/backend/libpq/pqcomm.c
+++ b/src/backend/libpq/pqcomm.c
@@ -1183,6 +1183,18 @@ 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 5775fc0c0910a8ec091319ba52ff4e6921eab1d9..1e0936e5b48296699331ea168df2396310904e8a 100644 (file)
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -2049,6 +2049,18 @@ retry1:
            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, and a GSS request should only
@@ -2081,6 +2093,18 @@ retry1:
            return STATUS_ERROR;
 #endif
 
+       /*
+        * 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.")));
+
        /*
         * regular startup packet, cancel, etc packet should follow, but not
         * another GSS negotiation request, and an SSL request should only

diff --git a/src/include/libpq/libpq.h b/src/include/libpq/libpq.h
index b1152475ace58c355524044685d135d99070b275..54c5fa7797735c1c7183a50d588c4d0b2ff80989 100644 (file)
--- a/src/include/libpq/libpq.h
+++ b/src/include/libpq/libpq.h
@@ -72,6 +72,7 @@ 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);
 
 /*