--- openssh-4.3p2/deattack.c.deattack-dos	2003-09-22 13:04:23.000000000 +0200
+++ openssh-4.3p2/deattack.c	2006-09-27 13:31:08.000000000 +0200
@@ -27,6 +27,24 @@
 #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 @@
 	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;
 
@@ -133,7 +151,7 @@
 	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) {
@@ -144,6 +162,8 @@
 						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
--- openssh-4.3p2/deattack.h.deattack-dos	2001-07-04 06:46:57.000000000 +0200
+++ openssh-4.3p2/deattack.h	2006-09-27 13:31:08.000000000 +0200
@@ -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, u_char[8]);
 #endif
--- openssh-4.3p2/packet.c.deattack-dos	2005-11-05 05:15:00.000000000 +0100
+++ openssh-4.3p2/packet.c	2006-09-27 13:32:48.000000000 +0200
@@ -669,6 +669,9 @@
 	 */
 	after_authentication = 1;
 	for (mode = 0; mode < MODE_MAX; mode++) {
+		/* protocol error: USERAUTH_SUCCESS received before NEWKEYS */
+		if (newkeys[mode] == NULL)
+			continue;
 		comp = &newkeys[mode]->comp;
 		if (comp && !comp->enabled && comp->type == COMP_DELAYED) {
 			packet_init_compression();
@@ -978,9 +981,16 @@
 	 * (C)1998 CORE-SDI, Buenos Aires Argentina
 	 * Ariel Futoransky(futo@core-sdi.com)
 	 */
-	if (!receive_context.plaintext &&
-	    detect_attack(buffer_ptr(&input), padded_len, NULL) == DEATTACK_DETECTED)
-		packet_disconnect("crc32 compensation attack: network attack detected");
+	if (!receive_context.plaintext) {
+		switch (detect_attack(buffer_ptr(&input), padded_len, NULL)) {
+		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);