Fix protocol downgrade bug in case of fragmented packets

CVE-2014-3511

Reviewed-by: Emilia Käsper <emilia@openssl.org>
Reviewed-by: Bodo Möller <bodo@openssl.org>

diff --git a/ssl/s23_srvr.c b/ssl/s23_srvr.c
index 836dd1f1cf4b9137961436e8b1176343f612a242..a997bb5d60001b66233c6d8d3573007766843e42 100644 (file)
--- a/ssl/s23_srvr.c
+++ b/ssl/s23_srvr.c
@@ -328,23 +328,19 @@ int ssl23_get_client_hello(SSL *s)
                         * Client Hello message, this would be difficult, and we'd have
                         * to read more records to find out.
                         * No known SSL 3.0 client fragments ClientHello like this,
-                        * so we simply assume TLS 1.0 to avoid protocol version downgrade
-                        * attacks. */
+                        * so we simply reject such connections to avoid
+                        * protocol version downgrade attacks. */
                        if (p[3] == 0 && p[4] < 6)
                                {
-#if 0
                                SSLerr(SSL_F_SSL23_GET_CLIENT_HELLO,SSL_R_RECORD_TOO_SMALL);
                                goto err;
-#else
-                               v[1] = TLS1_VERSION_MINOR;
-#endif
                                }
                        /* if major version number > 3 set minor to a value
                         * which will use the highest version 3 we support.
                         * If TLS 2.0 ever appears we will need to revise
                         * this....
                         */
-                       else if (p[9] > SSL3_VERSION_MAJOR)
+                       if (p[9] > SSL3_VERSION_MAJOR)
                                v[1]=0xff;
                        else
                                v[1]=p[10]; /* minor version according to client_version */
@@ -403,14 +399,34 @@ int ssl23_get_client_hello(SSL *s)
                v[0] = p[3]; /* == SSL3_VERSION_MAJOR */
                v[1] = p[4];
 
+               /* An SSLv3/TLSv1 backwards-compatible CLIENT-HELLO in an SSLv2
+                * header is sent directly on the wire, not wrapped as a TLS
+                * record. It's format is:
+                * Byte  Content
+                * 0-1   msg_length
+                * 2     msg_type
+                * 3-4   version
+                * 5-6   cipher_spec_length
+                * 7-8   session_id_length
+                * 9-10  challenge_length
+                * ...   ...
+                */
                n=((p[0]&0x7f)<<8)|p[1];
                if (n > (1024*4))
                        {
                        SSLerr(SSL_F_SSL23_GET_CLIENT_HELLO,SSL_R_RECORD_TOO_LARGE);
                        goto err;
                        }
+               if (n < 9)
+                       {
+                       SSLerr(SSL_F_SSL23_GET_CLIENT_HELLO,SSL_R_RECORD_LENGTH_MISMATCH);
+                       goto err;
+                       }
 
                j=ssl23_read_bytes(s,n+2);
+               /* We previously read 11 bytes, so if j > 0, we must have
+                * j == n+2 == s->packet_length. We have at least 11 valid
+                * packet bytes. */
                if (j <= 0) return(j);
 
                ssl3_finish_mac(s, s->packet+2, s->packet_length-2);