Submitted by: Robin Seggelmann <seggelmann@fh-muenster.de>, Michael Tuexen <tuexen@fh-muenster.de>

Reviewed by: steve

Fix for DTLS plaintext recovery attack discovered by Nadhem Alfardan and
Kenny Paterson.

CHANGES

@@ -523,6 +523,20 @@
 
  Changes between 1.0.0e and 1.0.0f [xx XXX xxxx]
 
+  *) Nadhem Alfardan and Kenny Paterson have discovered an extension
+     of the Vaudenay padding oracle attack on CBC mode encryption
+     which enables an efficient plaintext recovery attack against
+     the OpenSSL implementation of DTLS. Their attack exploits timing
+     differences arising during decryption processing. A research
+     paper describing this attack can be found at:
+                  http://www.isg.rhul.ac.uk/~kp/dtls.pdf
+     Thanks go to Nadhem Alfardan and Kenny Paterson of the Information
+     Security Group at Royal Holloway, University of London
+     (www.isg.rhul.ac.uk) for discovering this flaw and to Robin Seggelmann
+     <seggelmann@fh-muenster.de> and Michael Tuexen <tuexen@fh-muenster.de>
+     for preparing the fix. (CVE-2011-4108)
+     [Robin Seggelmann, Michael Tuexen]
+
   *) Clear bytes used for block padding of SSL 3.0 records.
      (CVE-2011-4576)
      [Adam Langley (Google)]

ssl/d1_pkt.c

@@ -383,6 +383,7 @@ dtls1_process_record(SSL *s)
 	SSL3_RECORD *rr;
 	unsigned int mac_size;
 	unsigned char md[EVP_MAX_MD_SIZE];
+	int decryption_failed_or_bad_record_mac = 0;
 
 
 	rr= &(s->s3->rrec);
@@ -417,13 +418,10 @@ dtls1_process_record(SSL *s)
 	enc_err = s->method->ssl3_enc->enc(s,0);
 	if (enc_err <= 0)
 		{
-		/* decryption failed, silently discard message */
-		if (enc_err < 0)
-			{
-			rr->length = 0;
-			s->packet_length = 0;
-			}
-		goto err;
+		/* To minimize information leaked via timing, we will always
+		 * perform all computations before discarding the message.
+		 */
+		decryption_failed_or_bad_record_mac = 1;
 		}
 
 #ifdef TLS_DEBUG
@@ -453,7 +451,7 @@ printf("\n");
 			SSLerr(SSL_F_DTLS1_PROCESS_RECORD,SSL_R_PRE_MAC_LENGTH_TOO_LONG);
 			goto f_err;
 #else
-			goto err;
+			decryption_failed_or_bad_record_mac = 1;
 #endif			
 			}
 		/* check the MAC for rr->input (it's in mac_size bytes at the tail) */
@@ -464,17 +462,25 @@ printf("\n");
 			SSLerr(SSL_F_DTLS1_PROCESS_RECORD,SSL_R_LENGTH_TOO_SHORT);
 			goto f_err;
 #else
-			goto err;
+			decryption_failed_or_bad_record_mac = 1;
 #endif
 			}
 		rr->length-=mac_size;
 		i=s->method->ssl3_enc->mac(s,md,0);
 		if (i < 0 || memcmp(md,&(rr->data[rr->length]),mac_size) != 0)
 			{
-			goto err;
+			decryption_failed_or_bad_record_mac = 1;
 			}
 		}
 
+	if (decryption_failed_or_bad_record_mac)
+		{
+		/* decryption failed, silently discard message */
+		rr->length = 0;
+		s->packet_length = 0;
+		goto err;
+		}
+
 	/* r->length is now just compressed */
 	if (s->expand != NULL)
 		{