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