Re: Format String Attacks

[Rick Perry <perry () NEWS VILL EDU>]

Doug Hughes <Doug.Hughes () eng auburn edu> writes:
> Since I don't recall anybody else posting one, here is a simple, generic,
> setuid wrapper that people could use around, for instance, /usr/bin/eject
> or other setuid programs.
>
> /*
> * This program provided AS IS with no warranty
> * Copyright 2000, doug () eng auburn edu
> * Use freely.
> * The environment from the original program is completely obliviated
> */

I found that completely obliviating the environment prevents one from
logging in using CDE in Solaris 7; actually, if you just truncate all
env strings to 128 bytes it prevents CDE login apparently because
there are some necessary long env strings.  The ones I leave alone
are:

#define MAX_LENGTH 128

/* environment variables which we allow to exceed MAX_LENGTH */

char *longenv[] = {
  "DTAPPSEARCHPATH=",
  "DTDATABASESEARCHPATH=",
  "DTHELPSEARCHPATH=",
  "DTSCREENSAVERLIST=",
  "MANPATH=",
  "NLSPATH=",
  "PATH=",
  "TERMCAP=",
  "XFILESEARCHPATH=",
  "XMICONBMSEARCHPATH=",
  "XMICONSEARCHPATH=",
  NULL
};

I know that NLSPATH is in my list above, but having it set long doesn't seem to
be part of the recent exploits.

> #include <stdio.h>
> #include <stdlib.h>
>
>
> main (int argc, char *argv[]) {
>
>       char *origfile;
>       char *envp[1] = { (char *) NULL };
>
>       if ((origfile = (char *) malloc(strlen(argv[0])+6)) == NULL) {

If this wrapper is setuid, using argv[0] doesn't seem safe.  One could
do something like this: cp /bin/sh sh.orig; ln -s path_to_wrapper sh; ./sh
and get a shell under that uid.

What I do instead is something like: execv( ORIG, argv);
where ORIG is #define'd separately (gcc -DORIG=\"/bin/something.orig\") for each
setuid program being wrapped, i.e. each wrapper has a hard-coded string for
the .orig program to execute, and each .orig has its own wrapper.

>               perror("allocating memory");
>               exit(1);
>       }
>       strcpy(origfile, argv[0]);
>       strcat(origfile, ".orig");

>       execve(origfile, argv, envp);
> }
...


...Rick  perry () ece villanova edu, http://www.ece.villanova.edu/~perry/ [PGP]