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
+         * (https://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:
https://eprint.iacr.org/2003/052/

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

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