1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
|
/* $OpenBSD: deattack.c,v 1.32 2015/01/20 23:14:00 deraadt Exp $ */
/*
* Cryptographic attack detector for ssh - source code
*
* Copyright (c) 1998 CORE SDI S.A., Buenos Aires, Argentina.
*
* All rights reserved. Redistribution and use in source and binary
* forms, with or without modification, are permitted provided that
* this copyright notice is retained.
*
* THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED
* WARRANTIES ARE DISCLAIMED. IN NO EVENT SHALL CORE SDI S.A. BE
* LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY OR
* CONSEQUENTIAL DAMAGES RESULTING FROM THE USE OR MISUSE OF THIS
* SOFTWARE.
*
* Ariel Futoransky <futo@core-sdi.com>
* <http://www.core-sdi.com>
*/
#include "includes.h"
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include "deattack.h"
#include "crc32.h"
#include "sshbuf.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)
/* Hashing constants */
#define HASH_MINSIZE (8 * 1024)
#define HASH_ENTRYSIZE (2)
#define HASH_FACTOR(x) ((x)*3/2)
#define HASH_UNUSEDCHAR (0xff)
#define HASH_UNUSED (0xffff)
#define HASH_IV (0xfffe)
#define HASH_MINBLOCKS (7*SSH_BLOCKSIZE)
/* Hash function (Input keys are cipher results) */
#define HASH(x) PEEK_U32(x)
#define CMP(a, b) (memcmp(a, b, SSH_BLOCKSIZE))
static void
crc_update(u_int32_t *a, u_int32_t b)
{
b ^= *a;
*a = ssh_crc32((u_char *)&b, sizeof(b));
}
/* detect if a block is used in a particular pattern */
static int
check_crc(const u_char *S, const u_char *buf, u_int32_t len)
{
u_int32_t crc;
const u_char *c;
crc = 0;
for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
if (!CMP(S, c)) {
crc_update(&crc, 1);
crc_update(&crc, 0);
} else {
crc_update(&crc, 0);
crc_update(&crc, 0);
}
}
return crc == 0;
}
void
deattack_init(struct deattack_ctx *dctx)
{
bzero(dctx, sizeof(*dctx));
dctx->n = HASH_MINSIZE / HASH_ENTRYSIZE;
}
/* Detect a crc32 compensation attack on a packet */
int
detect_attack(struct deattack_ctx *dctx, const u_char *buf, u_int32_t len)
{
u_int32_t i, j, l, same;
u_int16_t *tmp;
const u_char *c, *d;
if (len > (SSH_MAXBLOCKS * SSH_BLOCKSIZE) ||
len % SSH_BLOCKSIZE != 0)
return DEATTACK_ERROR;
for (l = dctx->n; l < HASH_FACTOR(len / SSH_BLOCKSIZE); l = l << 2)
;
if (dctx->h == NULL) {
if ((dctx->h = calloc(l, HASH_ENTRYSIZE)) == NULL)
return DEATTACK_ERROR;
dctx->n = l;
} else {
if (l > dctx->n) {
if ((tmp = reallocarray(dctx->h, l, HASH_ENTRYSIZE))
== NULL) {
free(dctx->h);
dctx->h = NULL;
return DEATTACK_ERROR;
}
dctx->h = tmp;
dctx->n = l;
}
}
if (len <= HASH_MINBLOCKS) {
for (c = buf; c < buf + len; c += SSH_BLOCKSIZE) {
for (d = buf; d < c; d += SSH_BLOCKSIZE) {
if (!CMP(c, d)) {
if ((check_crc(c, buf, len)))
return DEATTACK_DETECTED;
else
break;
}
}
}
return DEATTACK_OK;
}
memset(dctx->h, HASH_UNUSEDCHAR, dctx->n * HASH_ENTRYSIZE);
for (c = buf, same = j = 0; c < (buf + len); c += SSH_BLOCKSIZE, j++) {
for (i = HASH(c) & (dctx->n - 1); dctx->h[i] != HASH_UNUSED;
i = (i + 1) & (dctx->n - 1)) {
if (!CMP(c, buf + dctx->h[i] * SSH_BLOCKSIZE)) {
if (++same > MAX_IDENTICAL)
return DEATTACK_DOS_DETECTED;
if (check_crc(c, buf, len))
return DEATTACK_DETECTED;
else
break;
}
}
dctx->h[i] = j;
}
return DEATTACK_OK;
}
|
