OpenSSL Security Advisory [19 March 2003]

Klima-Pokorny-Rosa attack on RSA in SSL/TLS
===========================================

Czech cryptologists Vlastimil Klima, Ondrej Pokorny, and Tomas Rosa
have come up with an extension of the "Bleichenbacher attack" on RSA
with PKCS #1 v1.5 padding as used in SSL 3.0 and TLS 1.0.  Their
attack requires the attacker to open millions of SSL/TLS connections
to the server under attack; the server's behaviour when faced with
specially made-up RSA ciphertexts can reveal information that in
effect allows the attacker to perform a single RSA private key
operation on a ciphertext of its choice using the server's RSA key.
Note that the server's RSA key is not compromised in this attack.

This problem affects all applications using the OpenSSL SSL/TLS library.
OpenSSL releases up to 0.9.6i and 0.9.7a are vulnerable. The enclosed
patch modifies SSL/TLS server behaviour to avoid the vulnerability.


Security Patch
--------------

The following patch can be applied to OpenSSL releases 0.9.6b up to 0.9.6i,
0.9.7, and 0.9.7a.

--- s3_srvr.c	29 Nov 2002 11:31:51 -0000	1.85.2.14
+++ s3_srvr.c	19 Mar 2003 18:00:00 -0000
@@ -1447,7 +1447,7 @@
 		if (i != SSL_MAX_MASTER_KEY_LENGTH)
 			{
 			al=SSL_AD_DECODE_ERROR;
-			SSLerr(SSL_F_SSL3_GET_CLIENT_KEY_EXCHANGE,SSL_R_BAD_RSA_DECRYPT);
+			/* SSLerr(SSL_F_SSL3_GET_CLIENT_KEY_EXCHANGE,SSL_R_BAD_RSA_DECRYPT); */
 			}
 
 		if ((al == -1) && !((p[0] == (s->client_version>>8)) && (p[1] == (s->client_version & 0xff))))
@@ -1463,30 +1463,29 @@
 				(p[0] == (s->version>>8)) && (p[1] == (s->version & 0xff))))
 				{
 				al=SSL_AD_DECODE_ERROR;
-				SSLerr(SSL_F_SSL3_GET_CLIENT_KEY_EXCHANGE,SSL_R_BAD_PROTOCOL_VERSION_NUMBER);
-				goto f_err;
+				/* SSLerr(SSL_F_SSL3_GET_CLIENT_KEY_EXCHANGE,SSL_R_BAD_PROTOCOL_VERSION_NUMBER); */
+
+				/* The Klima-Pokorny-Rosa extension of Bleichenbacher's attack
+				 * (http://eprint.iacr.org/2003/052/) exploits the version
+				 * number check as a "bad version oracle" -- an alert would
+				 * reveal that the plaintext corresponding to some ciphertext
+				 * made up by the adversary is properly formatted except
+				 * that the version number is wrong.  To avoid such attacks,
+				 * we should treat this just like any other decryption error. */
+				p[0] = (char)(int) "CAN-2003-0131 patch 2003-03-19";
 				}
 			}
 
 		if (al != -1)
 			{
-#if 0
-			goto f_err;
-#else
 			/* Some decryption failure -- use random value instead as countermeasure
 			 * against Bleichenbacher's attack on PKCS #1 v1.5 RSA padding
-			 * (see RFC 2246, section 7.4.7.1).
-			 * But note that due to length and protocol version checking, the
-			 * attack is impractical anyway (see section 5 in D. Bleichenbacher:
-			 * "Chosen Ciphertext Attacks Against Protocols Based on the RSA
-			 * Encryption Standard PKCS #1", CRYPTO '98, LNCS 1462, pp. 1-12).
-			 */
+			 * (see RFC 2246, section 7.4.7.1). */
 			ERR_clear_error();
 			i = SSL_MAX_MASTER_KEY_LENGTH;
 			p[0] = s->client_version >> 8;
 			p[1] = s->client_version & 0xff;
 			RAND_pseudo_bytes(p+2, i-2); /* should be RAND_bytes, but we cannot work around a failure */
-#endif
 			}
 	
 		s->session->master_key_length=


References
----------

Report "Attacking RSA-based Sessions in SSL/TLS" by V. Klima, O. Pokorny,
and T. Rosa:
http://eprint.iacr.org/2003/052/

The Common Vulnerabilities and Exposures project (cve.mitre.org) has
assigned the name CAN-2003-0131 to this issue.
http://cve.mitre.org/cgi-bin/cvename.cgi?name=CAN-2003-0131

URL for this Security Advisory:
http://www.openssl.org/news/secadv_20030319.txt
