From 86f8fb0e344d62454f8daf3e15236b2b59210756 Mon Sep 17 00:00:00 2001 From: Emilia Kasper Date: Wed, 4 Mar 2015 09:05:02 -0800 Subject: [PATCH] Fix reachable assert in SSLv2 servers. MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit This assert is reachable for servers that support SSLv2 and export ciphers. Therefore, such servers can be DoSed by sending a specially crafted SSLv2 CLIENT-MASTER-KEY. Also fix s2_srvr.c to error out early if the key lengths are malformed. These lengths are sent unencrypted, so this does not introduce an oracle. CVE-2015-0293 This issue was discovered by Sean Burford (Google) and Emilia Käsper of the OpenSSL development team. Reviewed-by: Richard Levitte Reviewed-by: Tim Hudson --- ssl/s2_lib.c | 2 +- ssl/s2_srvr.c | 57 ++++++++++++++++++++++++++++++++++++++++----------- 2 files changed, 46 insertions(+), 13 deletions(-) diff --git a/ssl/s2_lib.c b/ssl/s2_lib.c index f8a9439303..d55b93f76b 100644 --- a/ssl/s2_lib.c +++ b/ssl/s2_lib.c @@ -493,7 +493,7 @@ int ssl2_generate_key_material(SSL *s) OPENSSL_assert(s->session->master_key_length >= 0 && s->session->master_key_length - < (int)sizeof(s->session->master_key)); + <= (int)sizeof(s->session->master_key)); EVP_DigestUpdate(&ctx, s->session->master_key, s->session->master_key_length); EVP_DigestUpdate(&ctx, &c, 1); diff --git a/ssl/s2_srvr.c b/ssl/s2_srvr.c index daba6dd7ad..4046697b26 100644 --- a/ssl/s2_srvr.c +++ b/ssl/s2_srvr.c @@ -454,11 +454,6 @@ static int get_client_master_key(SSL *s) SSLerr(SSL_F_GET_CLIENT_MASTER_KEY, SSL_R_NO_PRIVATEKEY); return (-1); } - i = ssl_rsa_private_decrypt(s->cert, s->s2->tmp.enc, - &(p[s->s2->tmp.clear]), - &(p[s->s2->tmp.clear]), - (s->s2->ssl2_rollback) ? RSA_SSLV23_PADDING : - RSA_PKCS1_PADDING); is_export = SSL_C_IS_EXPORT(s->session->cipher); @@ -475,23 +470,61 @@ static int get_client_master_key(SSL *s) } else ek = 5; + /* + * The format of the CLIENT-MASTER-KEY message is + * 1 byte message type + * 3 bytes cipher + * 2-byte clear key length (stored in s->s2->tmp.clear) + * 2-byte encrypted key length (stored in s->s2->tmp.enc) + * 2-byte key args length (IV etc) + * clear key + * encrypted key + * key args + * + * If the cipher is an export cipher, then the encrypted key bytes + * are a fixed portion of the total key (5 or 8 bytes). The size of + * this portion is in |ek|. If the cipher is not an export cipher, + * then the entire key material is encrypted (i.e., clear key length + * must be zero). + */ + if ((!is_export && s->s2->tmp.clear != 0) || + (is_export && s->s2->tmp.clear + ek != EVP_CIPHER_key_length(c))) { + ssl2_return_error(s, SSL2_PE_UNDEFINED_ERROR); + SSLerr(SSL_F_GET_CLIENT_MASTER_KEY,SSL_R_BAD_LENGTH); + return -1; + } + /* + * The encrypted blob must decrypt to the encrypted portion of the key. + * Decryption can't be expanding, so if we don't have enough encrypted + * bytes to fit the key in the buffer, stop now. + */ + if ((is_export && s->s2->tmp.enc < ek) || + (!is_export && s->s2->tmp.enc < EVP_CIPHER_key_length(c))) { + ssl2_return_error(s,SSL2_PE_UNDEFINED_ERROR); + SSLerr(SSL_F_GET_CLIENT_MASTER_KEY,SSL_R_LENGTH_TOO_SHORT); + return -1; + } + + i = ssl_rsa_private_decrypt(s->cert, s->s2->tmp.enc, + &(p[s->s2->tmp.clear]), + &(p[s->s2->tmp.clear]), + (s->s2->ssl2_rollback) ? RSA_SSLV23_PADDING : + RSA_PKCS1_PADDING); + /* bad decrypt */ # if 1 /* * If a bad decrypt, continue with protocol but with a random master * secret (Bleichenbacher attack) */ - if ((i < 0) || ((!is_export && (i != EVP_CIPHER_key_length(c))) - || (is_export && ((i != ek) - || (s->s2->tmp.clear + - (unsigned int)i != (unsigned int) - EVP_CIPHER_key_length(c)))))) { + if ((i < 0) || ((!is_export && i != EVP_CIPHER_key_length(c)) + || (is_export && i != ek))) { ERR_clear_error(); if (is_export) i = ek; else i = EVP_CIPHER_key_length(c); - if (RAND_pseudo_bytes(p, i) <= 0) + if (RAND_pseudo_bytes(&p[s->s2->tmp.clear], i) <= 0) return 0; } # else @@ -513,7 +546,7 @@ static int get_client_master_key(SSL *s) # endif if (is_export) - i += s->s2->tmp.clear; + i = EVP_CIPHER_key_length(c); if (i > SSL_MAX_MASTER_KEY_LENGTH) { ssl2_return_error(s, SSL2_PE_UNDEFINED_ERROR); -- 2.34.1