diff options
| -rw-r--r-- | usr.bin/ssh/deattack.c | 26 | ||||
| -rw-r--r-- | usr.bin/ssh/deattack.h | 3 | ||||
| -rw-r--r-- | usr.bin/ssh/packet.c | 15 |
3 files changed, 36 insertions, 8 deletions
diff --git a/usr.bin/ssh/deattack.c b/usr.bin/ssh/deattack.c index f28f33d7415..591c9559fa9 100644 --- a/usr.bin/ssh/deattack.c +++ b/usr.bin/ssh/deattack.c @@ -1,4 +1,4 @@ -/* $OpenBSD: deattack.c,v 1.29 2006/08/03 03:34:42 deraadt Exp $ */ +/* $OpenBSD: deattack.c,v 1.30 2006/09/16 19:53:37 djm Exp $ */ /* * Cryptographic attack detector for ssh - source code * @@ -30,6 +30,24 @@ #include "crc32.h" #include "misc.h" +/* + * CRC attack detection has a worst-case behaviour that is O(N^3) over + * the number of identical blocks in a packet. This behaviour can be + * exploited to create a limited denial of service attack. + * + * However, because we are dealing with encrypted data, identical + * blocks should only occur every 2^35 maximally-sized packets or so. + * Consequently, we can detect this DoS by looking for identical blocks + * in a packet. + * + * The parameter below determines how many identical blocks we will + * accept in a single packet, trading off between attack detection and + * likelihood of terminating a legitimate connection. A value of 32 + * corresponds to an average of 2^40 messages before an attack is + * misdetected + */ +#define MAX_IDENTICAL 32 + /* SSH Constants */ #define SSH_MAXBLOCKS (32 * 1024) #define SSH_BLOCKSIZE (8) @@ -85,7 +103,7 @@ detect_attack(u_char *buf, u_int32_t len) static u_int16_t *h = (u_int16_t *) NULL; static u_int32_t n = HASH_MINSIZE / HASH_ENTRYSIZE; u_int32_t i, j; - u_int32_t l; + u_int32_t l, same; u_char *c; u_char *d; @@ -122,10 +140,12 @@ detect_attack(u_char *buf, u_int32_t len) } memset(h, HASH_UNUSEDCHAR, n * HASH_ENTRYSIZE); - for (c = buf, j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++) { + for (c = buf, same = j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++) { for (i = HASH(c) & (n - 1); h[i] != HASH_UNUSED; i = (i + 1) & (n - 1)) { if (!CMP(c, buf + h[i] * SSH_BLOCKSIZE)) { + if (++same > MAX_IDENTICAL) + return (DEATTACK_DOS_DETECTED); if (check_crc(c, buf, len)) return (DEATTACK_DETECTED); else diff --git a/usr.bin/ssh/deattack.h b/usr.bin/ssh/deattack.h index 6275981040c..0316fb28543 100644 --- a/usr.bin/ssh/deattack.h +++ b/usr.bin/ssh/deattack.h @@ -1,4 +1,4 @@ -/* $OpenBSD: deattack.h,v 1.9 2006/03/25 22:22:43 djm Exp $ */ +/* $OpenBSD: deattack.h,v 1.10 2006/09/16 19:53:37 djm Exp $ */ /* * Cryptographic attack detector for ssh - Header file @@ -25,6 +25,7 @@ /* Return codes */ #define DEATTACK_OK 0 #define DEATTACK_DETECTED 1 +#define DEATTACK_DOS_DETECTED 2 int detect_attack(u_char *, u_int32_t); #endif diff --git a/usr.bin/ssh/packet.c b/usr.bin/ssh/packet.c index eacbd89642f..278948c8351 100644 --- a/usr.bin/ssh/packet.c +++ b/usr.bin/ssh/packet.c @@ -1,4 +1,4 @@ -/* $OpenBSD: packet.c,v 1.143 2006/08/05 08:34:04 dtucker Exp $ */ +/* $OpenBSD: packet.c,v 1.144 2006/09/16 19:53:37 djm Exp $ */ /* * Author: Tatu Ylonen <ylo@cs.hut.fi> * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland @@ -991,9 +991,16 @@ packet_read_poll1(void) * (C)1998 CORE-SDI, Buenos Aires Argentina * Ariel Futoransky(futo@core-sdi.com) */ - if (!receive_context.plaintext && - detect_attack(buffer_ptr(&input), padded_len) == DEATTACK_DETECTED) - packet_disconnect("crc32 compensation attack: network attack detected"); + if (!receive_context.plaintext) { + switch (detect_attack(buffer_ptr(&input), padded_len)) { + case DEATTACK_DETECTED: + packet_disconnect("crc32 compensation attack: " + "network attack detected"); + case DEATTACK_DOS_DETECTED: + packet_disconnect("deattack denial of " + "service detected"); + } + } /* Decrypt data to incoming_packet. */ buffer_clear(&incoming_packet); |