ssl: Detect unencrypted heartbeat messages

Records are always unencrypted before the ChangeCipherSpec message. This
patch assumes that conversations without a SSL decoder (i.e. before
calling ssl_change_cipher) are unencrypted. If it turns out that the
contents were encrypted anyway, then there is about 0.8% probability
that the heartbeat message gets recognized wrong
(dissect_ssl3_heartbeat checks if the first byte equals 1 (request) or 2
(response)). Not a big deal, and the advantage that the heartbleed
expert information is triggered overweights that possible mistake.

(Note that ssl_set_master_secret as called by external code will also
invoke ssl_change_cipher, but that should be caught by the if
condition in the line before this hunk.)

An example capture is available at:
https://bugs.wireshark.org/bugzilla/show_bug.cgi?id=9988

Change-Id: I5b14d9e7e8e0c1cd358f3b05c9b84fca1daf1d69
Reviewed-on: https://code.wireshark.org/review/1102
Reviewed-by: Evan Huus <eapache@gmail.com>

1 files changed, 10 insertions, 1 deletions

diff --git a/epan/dissectors/packet-ssl.c b/epan/dissectors/packet-ssl.c
index 4a8ba03933..483bd4af1c 100644
--- a/epan/dissectors/packet-ssl.c
+++ b/epan/dissectors/packet-ssl.c
@@ -1768,7 +1768,16 @@ dissect_ssl3_record(tvbuff_t *tvb, packet_info *pinfo,
             add_new_data_source(pinfo, decrypted, "Decrypted SSL record");
             dissect_ssl3_heartbeat(decrypted, pinfo, ssl_record_tree, 0, conv_version, tvb_length (decrypted), TRUE);
         } else {
-            dissect_ssl3_heartbeat(tvb, pinfo, ssl_record_tree, offset, conv_version, record_length, FALSE);
+            gboolean plaintext = TRUE;
+            /* heartbeats before ChangeCipherSpec are unencrypted */
+            if (ssl) {
+                if (ssl_packet_from_server(ssl, ssl_associations, pinfo)) {
+                    plaintext = ssl->server == NULL;
+                } else {
+                    plaintext = ssl->client == NULL;
+                }
+            }
+            dissect_ssl3_heartbeat(tvb, pinfo, ssl_record_tree, offset, conv_version, record_length, plaintext);
         }
         break;
     }