1 files changed, 24 insertions, 2 deletions

diff --git a/deattack.c b/deattack.c
index 8b55d6686..d174abc76 100644
--- a/deattack.c
+++ b/deattack.c
@@ -27,6 +27,24 @@ RCSID("$OpenBSD: deattack.c,v 1.19 2003/09/18 08:49:45 markus Exp $");
 #include "xmalloc.h"
 #include "deattack.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)
@@ -87,7 +105,7 @@ detect_attack(u_char *buf, u_int32_t len, u_char *IV)
         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;
 
@@ -133,17 +151,21 @@ detect_attack(u_char *buf, u_int32_t len, u_char *IV)
         if (IV)
                 h[HASH(IV) & (n - 1)] = HASH_IV;
 
-        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 (h[i] == HASH_IV) {
                                 if (!CMP(c, IV)) {
+                                        if (++same > MAX_IDENTICAL)
+                                                return (DEATTACK_DOS_DETECTED);
                                         if (check_crc(c, buf, len, IV))
                                                 return (DEATTACK_DETECTED);
                                         else
                                                 break;
                                 }
                         } else if (!CMP(c, buf + h[i] * SSH_BLOCKSIZE)) {
+                                if (++same > MAX_IDENTICAL)
+                                        return (DEATTACK_DOS_DETECTED);
                                 if (check_crc(c, buf, len, IV))
                                         return (DEATTACK_DETECTED);
                                 else