diff --git a/doc/src/sgml/protocol.sgml b/doc/src/sgml/protocol.sgml
index e26619e1b5..b692648fca 100644
--- a/doc/src/sgml/protocol.sgml
+++ b/doc/src/sgml/protocol.sgml
@@ -1471,6 +1471,20 @@ SELCT 1/0;<!-- this typo is intentional -->
     and proceed without requesting <acronym>SSL</acronym>.
    </para>
 
+   <para>
+    When <acronym>SSL</acronym> encryption can be performed, the server
+    is expected to send only the single <literal>S</literal> byte and then
+    wait for the frontend to initiate an <acronym>SSL</acronym> handshake.
+    If additional bytes are available to read at this point, it likely
+    means that a man-in-the-middle is attempting to perform a
+    buffer-stuffing attack
+    (<ulink url="https://www.postgresql.org/support/security/CVE-2021-23222/">CVE-2021-23222</ulink>).
+    Frontends should be coded either to read exactly one byte from the
+    socket before turning the socket over to their SSL library, or to
+    treat it as a protocol violation if they find they have read additional
+    bytes.
+   </para>
+
    <para>
     An initial SSLRequest can also be used in a connection that is being
     opened to send a CancelRequest message.
@@ -1532,6 +1546,20 @@ SELCT 1/0;<!-- this typo is intentional -->
     encryption.
    </para>
 
+   <para>
+    When <acronym>GSSAPI</acronym> encryption can be performed, the server
+    is expected to send only the single <literal>G</literal> byte and then
+    wait for the frontend to initiate a <acronym>GSSAPI</acronym> handshake.
+    If additional bytes are available to read at this point, it likely
+    means that a man-in-the-middle is attempting to perform a
+    buffer-stuffing attack
+    (<ulink url="https://www.postgresql.org/support/security/CVE-2021-23222/">CVE-2021-23222</ulink>).
+    Frontends should be coded either to read exactly one byte from the
+    socket before turning the socket over to their GSSAPI library, or to
+    treat it as a protocol violation if they find they have read additional
+    bytes.
+   </para>
+
    <para>
     An initial GSSENCRequest can also be used in a connection that is being
     opened to send a CancelRequest message.
diff --git a/src/interfaces/libpq/fe-connect.c b/src/interfaces/libpq/fe-connect.c
index f80f4e98d8..57aee95183 100644
--- a/src/interfaces/libpq/fe-connect.c
+++ b/src/interfaces/libpq/fe-connect.c
@@ -3076,6 +3076,19 @@ keep_going:						/* We will come back to here until there is
 				pollres = pqsecure_open_client(conn);
 				if (pollres == PGRES_POLLING_OK)
 				{
+					/*
+					 * 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.
+					 */
+					if (conn->inCursor != conn->inEnd)
+					{
+						appendPQExpBufferStr(&conn->errorMessage,
+											 libpq_gettext("received unencrypted data after SSL response\n"));
+						goto error_return;
+					}
+
 					/* SSL handshake done, ready to send startup packet */
 					conn->status = CONNECTION_MADE;
 					return PGRES_POLLING_WRITING;
@@ -3175,6 +3188,19 @@ keep_going:						/* We will come back to here until there is
 				pollres = pqsecure_open_gss(conn);
 				if (pollres == PGRES_POLLING_OK)
 				{
+					/*
+					 * 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.
+					 */
+					if (conn->inCursor != conn->inEnd)
+					{
+						appendPQExpBufferStr(&conn->errorMessage,
+											 libpq_gettext("received unencrypted data after GSSAPI encryption response\n"));
+						goto error_return;
+					}
+
 					/* All set for startup packet */
 					conn->status = CONNECTION_MADE;
 					return PGRES_POLLING_WRITING;