diff --git a/include/haproxy/h2.h b/include/haproxy/h2.h
index 430774e76..1c76fa4bd 100644
--- a/include/haproxy/h2.h
+++ b/include/haproxy/h2.h
@@ -182,7 +182,8 @@ enum h2_err {
 #define H2_MSGF_RSP_1XX        0x0010    // a 1xx ( != 101) HEADERS frame was received
 #define H2_MSGF_BODYLESS_RSP   0x0020    // response message is known to have no body
                                          // (response to HEAD request or 204/304 response)
-#define H2_MSGF_EXT_CONNECT    0x0040    // Extended CONNECT method from rfc 8441
+#define H2_MSGF_EXT_CONNECT    0x0040    // Extended CONNECT method from rfc 8441 is in use
+#define H2_MSGF_EXT_CONN_OK    0x0080    // Extended CONNECT is permitted
 
 #define H2_MAX_STREAM_ID       ((1U << 31) - 1)
 #define H2_MAX_FRAME_LEN       ((1U << 24) - 1)
diff --git a/src/mux_h2.c b/src/mux_h2.c
index 61e7db54b..4d1bef4b2 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -6285,7 +6285,10 @@ next_frame:
 	msgf = (h2c->dff & H2_F_HEADERS_END_STREAM) ? 0 : H2_MSGF_BODY;
 	msgf |= (*flags & H2_SF_BODY_TUNNEL) ? H2_MSGF_BODY_TUNNEL: 0;
 	/* If an Extended CONNECT has been sent on this stream, set message flag
-	 * to convert 200 response to 101 htx response */
+	 * to convert 200 response to 101 htx response. We only support this if
+	 * the connection supports RFC8441.
+	 */
+	msgf |= (h2c->flags & H2_CF_RCVD_RFC8441) ? H2_MSGF_EXT_CONN_OK : 0;
 	msgf |= (*flags & H2_SF_EXT_CONNECT_SENT) ? H2_MSGF_EXT_CONNECT: 0;
 
 	/* when dealing with trailers, we need to check the content-length */