Tolerate DTLS alerts with an incorrect version number

In the case of a protocol version alert being sent by a peer the record
version number may not be what we are expecting. In DTLS records with an
unexpected version number are silently discarded. This probably isn't
appropriate for alerts, so we tolerate a mismatch in the minor version
number.

This resolves an issue reported on openssl-users where an OpenSSL server
chose DTLS1.0 but the client was DTLS1.2 only and sent a protocol_version
alert with a 1.2 record number. This was silently ignored by the server.

Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5019)

diff --git a/ssl/d1_pkt.c b/ssl/d1_pkt.c
index f196b8fb2df28b4fab05bc857d7aec0f98b4eb94..685d50a4f10ed9460228f82379d42134b5da618f 100644 (file)
--- a/ssl/d1_pkt.c
+++ b/ssl/d1_pkt.c
@@ -706,8 +706,11 @@ int dtls1_get_record(SSL *s)
 
         n2s(p, rr->length);
 
-        /* Lets check version */
-        if (!s->first_packet) {
+        /*
+         * Lets check the version. We tolerate alerts that don't have the exact
+         * version number (e.g. because of protocol version errors)
+         */
+        if (!s->first_packet && rr->type != SSL3_RT_ALERT) {
             if (version != s->version) {
                 /* unexpected version, silently discard */
                 rr->length = 0;