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Fix reachable assert in SSLv2 servers.

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 <levitte@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>
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1 parent c0334c2 commit 86f8fb0e344d62454f8daf3e15236b2b59210756 @ekasper ekasper committed with mattcaswell Mar 4, 2015
Showing with 46 additions and 13 deletions.
  1. +1 −1 ssl/s2_lib.c
  2. +45 −12 ssl/s2_srvr.c
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@@ -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);
View
@@ -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);

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