Don't fail on an out-of-order CCS in DTLS

Fixes #4929

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6170)

1 files changed, 14 insertions, 0 deletions

diff --git a/ssl/statem/statem_clnt.c b/ssl/statem/statem_clnt.c
index 91b986fc89..60e987adb1 100644
--- a/ssl/statem/statem_clnt.c
+++ b/ssl/statem/statem_clnt.c
@@ -375,6 +375,20 @@ int ossl_statem_client_read_transition(SSL *s, int mt)
 
  err:
     /* No valid transition found */
+    if (SSL_IS_DTLS(s) && mt == SSL3_MT_CHANGE_CIPHER_SPEC) {
+        BIO *rbio;
+
+        /*
+         * CCS messages don't have a message sequence number so this is probably
+         * because of an out-of-order CCS. We'll just drop it.
+         */
+        s->init_num = 0;
+        s->rwstate = SSL_READING;
+        rbio = SSL_get_rbio(s);
+        BIO_clear_retry_flags(rbio);
+        BIO_set_retry_read(rbio);
+        return 0;
+    }
     SSLfatal(s, SSL3_AD_UNEXPECTED_MESSAGE,
              SSL_F_OSSL_STATEM_CLIENT_READ_TRANSITION,
              SSL_R_UNEXPECTED_MESSAGE);