Synopsis: OpenSSH CBC cipher mode plain text disclosure vulnerability NetBSD versions: 5.0 Thanks to: Martin Albrecht, Kenny Paterson and Gaven Watson Reported in NetBSD Security Advisory: NetBSD-SA2009-005 Index: crypto/dist/ssh/cipher.c =================================================================== RCS file: /cvsroot/src/crypto/dist/ssh/cipher.c,v retrieving revision 1.21 diff -u -r1.21 cipher.c --- cipher.c 23 Jun 2008 14:51:31 -0000 1.21 +++ cipher.c 29 Jun 2009 22:41:39 -0000 @@ -62,38 +62,39 @@ u_int block_size; u_int key_len; u_int discard_len; + u_int cbc_mode; const EVP_CIPHER *(*evptype)(void); } ciphers[] = { - { "none", SSH_CIPHER_NONE, 8, 0, 0, EVP_enc_null }, - { "des", SSH_CIPHER_DES, 8, 8, 0, EVP_des_cbc }, - { "3des", SSH_CIPHER_3DES, 8, 16, 0, evp_ssh1_3des }, - { "blowfish", SSH_CIPHER_BLOWFISH, 8, 32, 0, evp_ssh1_bf }, - - { "3des-cbc", SSH_CIPHER_SSH2, 8, 24, 0, EVP_des_ede3_cbc }, - { "blowfish-cbc", SSH_CIPHER_SSH2, 8, 16, 0, EVP_bf_cbc }, - { "cast128-cbc", SSH_CIPHER_SSH2, 8, 16, 0, EVP_cast5_cbc }, - { "arcfour", SSH_CIPHER_SSH2, 8, 16, 0, EVP_rc4 }, - { "arcfour128", SSH_CIPHER_SSH2, 8, 16, 1536, EVP_rc4 }, - { "arcfour256", SSH_CIPHER_SSH2, 8, 32, 1536, EVP_rc4 }, - { "aes128-cbc", SSH_CIPHER_SSH2, 16, 16, 0, EVP_aes_128_cbc }, - { "aes192-cbc", SSH_CIPHER_SSH2, 16, 24, 0, EVP_aes_192_cbc }, - { "aes256-cbc", SSH_CIPHER_SSH2, 16, 32, 0, EVP_aes_256_cbc }, + { "none", SSH_CIPHER_NONE, 8, 0, 0, 0, EVP_enc_null }, + { "des", SSH_CIPHER_DES, 8, 8, 0, 1, EVP_des_cbc }, + { "3des", SSH_CIPHER_3DES, 8, 16, 0, 1, evp_ssh1_3des }, + { "blowfish", SSH_CIPHER_BLOWFISH, 8, 32, 0, 0, evp_ssh1_bf }, + + { "3des-cbc", SSH_CIPHER_SSH2, 8, 24, 0, 1, EVP_des_ede3_cbc }, + { "blowfish-cbc", SSH_CIPHER_SSH2, 8, 16, 0, 1, EVP_bf_cbc }, + { "cast128-cbc", SSH_CIPHER_SSH2, 8, 16, 0, 1, EVP_cast5_cbc }, + { "arcfour", SSH_CIPHER_SSH2, 8, 16, 0, 0, EVP_rc4 }, + { "arcfour128", SSH_CIPHER_SSH2, 8, 16, 1536, 0, EVP_rc4 }, + { "arcfour256", SSH_CIPHER_SSH2, 8, 32, 1536, 0, EVP_rc4 }, + { "aes128-cbc", SSH_CIPHER_SSH2, 16, 16, 0, 1, EVP_aes_128_cbc }, + { "aes192-cbc", SSH_CIPHER_SSH2, 16, 24, 0, 1, EVP_aes_192_cbc }, + { "aes256-cbc", SSH_CIPHER_SSH2, 16, 32, 0, 1, EVP_aes_256_cbc }, { "rijndael-cbc@lysator.liu.se", - SSH_CIPHER_SSH2, 16, 32, 0, EVP_aes_256_cbc }, + SSH_CIPHER_SSH2, 16, 32, 0, 1, EVP_aes_256_cbc }, #ifdef AES_CTR_MT - { "aes128-ctr", SSH_CIPHER_SSH2, 16, 16, 0, evp_aes_ctr_mt }, - { "aes192-ctr", SSH_CIPHER_SSH2, 16, 24, 0, evp_aes_ctr_mt }, - { "aes256-ctr", SSH_CIPHER_SSH2, 16, 32, 0, evp_aes_ctr_mt }, + { "aes128-ctr", SSH_CIPHER_SSH2, 16, 16, 0, 0, evp_aes_ctr_mt }, + { "aes192-ctr", SSH_CIPHER_SSH2, 16, 24, 0, 0, evp_aes_ctr_mt }, + { "aes256-ctr", SSH_CIPHER_SSH2, 16, 32, 0, 0, evp_aes_ctr_mt }, #else - { "aes128-ctr", SSH_CIPHER_SSH2, 16, 16, 0, evp_aes_128_ctr }, - { "aes192-ctr", SSH_CIPHER_SSH2, 16, 24, 0, evp_aes_128_ctr }, - { "aes256-ctr", SSH_CIPHER_SSH2, 16, 32, 0, evp_aes_128_ctr }, + { "aes128-ctr", SSH_CIPHER_SSH2, 16, 16, 0, 0, evp_aes_128_ctr }, + { "aes192-ctr", SSH_CIPHER_SSH2, 16, 24, 0, 0, evp_aes_128_ctr }, + { "aes256-ctr", SSH_CIPHER_SSH2, 16, 32, 0, 0, evp_aes_128_ctr }, #endif #ifdef ACSS - { "acss@openssh.org", SSH_CIPHER_SSH2, 16, 5, 0, EVP_acss }, + { "acss@openssh.org", SSH_CIPHER_SSH2, 16, 5, 0, 0, EVP_acss }, #endif - { NULL, SSH_CIPHER_INVALID, 0, 0, 0, NULL } + { NULL, SSH_CIPHER_INVALID, 0, 0, 0, 0, NULL } }; #ifndef ACSS @@ -121,6 +122,12 @@ } u_int +cipher_is_cbc(const Cipher *c) +{ + return (c->cbc_mode); +} + +u_int cipher_mask_ssh1(int client) { u_int mask = 0; Index: crypto/dist/ssh/cipher.h =================================================================== RCS file: /cvsroot/src/crypto/dist/ssh/cipher.h,v retrieving revision 1.2 diff -u -r1.2 cipher.h --- cipher.h 28 Sep 2006 21:22:14 -0000 1.2 +++ cipher.h 29 Jun 2009 22:41:40 -0000 @@ -82,6 +82,7 @@ void cipher_set_key_string(CipherContext *, Cipher *, const char *, int); u_int cipher_blocksize(const Cipher *); u_int cipher_keylen(const Cipher *); +u_int cipher_is_cbc(const Cipher *); u_int cipher_get_number(const Cipher *); void cipher_get_keyiv(CipherContext *, u_char *, u_int); Index: crypto/dist/ssh/packet.c =================================================================== RCS file: /cvsroot/src/crypto/dist/ssh/packet.c,v retrieving revision 1.31 diff -u -r1.31 packet.c --- packet.c 16 Feb 2009 20:53:54 -0000 1.31 +++ packet.c 29 Jun 2009 22:41:41 -0000 @@ -84,6 +84,8 @@ #define DBG(x) #endif +#define PACKET_MAX_SIZE (256 * 1024) + /* * This variable contains the file descriptors used for communicating with * the other side. connection_in is used for reading; connection_out for @@ -160,6 +162,10 @@ /* roundup current message to extra_pad bytes */ static u_char extra_pad = 0; +/* XXX discard incoming data after MAC error */ +static u_int packet_discard = 0; +static Mac *packet_discard_mac = NULL; + struct packet { TAILQ_ENTRY(packet) next; u_char type; @@ -209,6 +215,36 @@ packet_timeout_ms = timeout * count * 1000; } +static void +packet_stop_discard(void) +{ + if (packet_discard_mac) { + char buf[1024]; + + memset(buf, 'a', sizeof(buf)); + while (buffer_len(&incoming_packet) < PACKET_MAX_SIZE) + buffer_append(&incoming_packet, buf, sizeof(buf)); + (void) mac_compute(packet_discard_mac, + p_read.seqnr, + buffer_ptr(&incoming_packet), + PACKET_MAX_SIZE); + } + logit("Finished discarding for %.200s", get_remote_ipaddr()); + cleanup_exit(255); +} + +static void +packet_start_discard(Enc *enc, Mac *mac, u_int packet_length, u_int discard) +{ + if (!cipher_is_cbc(enc->cipher)) + packet_disconnect("Packet corrupt"); + if (packet_length != PACKET_MAX_SIZE && mac && mac->enabled) + packet_discard_mac = mac; + if (buffer_len(&input) >= discard) + packet_stop_discard(); + packet_discard = discard - buffer_len(&input); +} + /* Returns 1 if remote host is connected via socket, 0 if not. */ int @@ -1126,6 +1162,9 @@ Mac *mac = NULL; Comp *comp = NULL; + if (packet_discard) + return SSH_MSG_NONE; + if (newkeys[MODE_IN] != NULL) { enc = &newkeys[MODE_IN]->enc; mac = &newkeys[MODE_IN]->mac; @@ -1147,11 +1186,15 @@ block_size); cp = buffer_ptr(&incoming_packet); packet_length = get_u32(cp); - if (packet_length < 1 + 4 || packet_length > 256 * 1024) { + if (packet_length < 1 + 4 || packet_length > PACKET_MAX_SIZE) { #ifdef PACKET_DEBUG buffer_dump(&incoming_packet); #endif - packet_disconnect("Bad packet length %u.", packet_length); + logit("Bad packet length %-10u.", + packet_length); + packet_start_discard(enc, mac, packet_length, + PACKET_MAX_SIZE); + return SSH_MSG_NONE; } DBG(debug("input: packet len %u", packet_length+4)); buffer_consume(&input, block_size); @@ -1160,9 +1203,13 @@ need = 4 + packet_length - block_size; DBG(debug("partial packet %d, need %d, maclen %d", block_size, need, maclen)); - if (need % block_size != 0) - fatal("padding error: need %d block %d mod %d", + if (need % block_size != 0) { + logit("padding error: need %d block %d mod %d", need, block_size, need % block_size); + packet_start_discard(enc, mac, packet_length, + PACKET_MAX_SIZE - block_size); + return SSH_MSG_NONE; + } /* * check if the entire packet has been received and * decrypt into incoming_packet @@ -1184,11 +1231,19 @@ macbuf = mac_compute(mac, p_read.seqnr, buffer_ptr(&incoming_packet), buffer_len(&incoming_packet)); - if (memcmp(macbuf, buffer_ptr(&input), mac->mac_len) != 0) - packet_disconnect("Corrupted MAC on input."); + if (memcmp(macbuf, buffer_ptr(&input), mac->mac_len) != 0) { + logit("Corrupted MAC on input."); + if (need > PACKET_MAX_SIZE) + fatal("internal error need %d", need); + packet_start_discard(enc, mac, packet_length, + PACKET_MAX_SIZE - need); + return SSH_MSG_NONE; + } + DBG(debug("MAC #%d ok", p_read.seqnr)); buffer_consume(&input, mac->mac_len); } + /* XXX now it's safe to use fatal/packet_disconnect */ if (seqnr_p != NULL) *seqnr_p = p_read.seqnr; if (++p_read.seqnr == 0) @@ -1321,6 +1376,13 @@ void packet_process_incoming(const char *buf, u_int len) { + if (packet_discard) { + keep_alive_timeouts = 0; /* ?? */ + if (len >= packet_discard) + packet_stop_discard(); + packet_discard -= len; + return; + } buffer_append(&input, buf, len); }