Subject: security/9528: xterm vulnerable to root exploit
To: None <>
From: Bill Squier <>
List: netbsd-bugs
Date: 03/03/2000 18:03:36
>Number:         9528
>Category:       security
>Synopsis:       xterm vulnerable to root exploit
>Confidential:   yes
>Severity:       critical
>Priority:       high
>Responsible:    security-officer (NetBSD Security Officer)
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Mar  3 18:03:00 2000
>Originator:     Bill Squier
Bill Squier (
>Release:        <NetBSD-current source date>
System: NetBSD nyarlathotep 1.4 NetBSD 1.4 (NYARLATHOTEP) #10: Wed Dec 15 20:43:55 EST 1999 groo@nyarlathotep:/sysn/arch/i386/compile/NYARLATHOTEP i386


Forwarded message, from BUGTRAQ:

---------- Forwarded message ----------
Date: Tue, 29 Feb 2000 17:39:25 +0100
From: Morten Welinder <terra@DIKU.DK>
Subject: xterm log file vulnerability

It used to be Well Known that xterm's way of opening a log file
was insecure.  Well, that was 5+ years ago so I decided to take
a look at the current state of affairs.  Things have changed,
but mostly to "different" rather than "better".

Problem: when log files are enabled, they are created in the
following way (checking in XFree86 3.3.6 source; matches Solaris
binaries) and are subject to race conditions:

1. File is checked for existance using access.
2. If file does not exist, it is created in a subprocess using user's
   real uid/gid.  [ok]
3. File is checked for existance using access.
4. File is checked for write permission using access.
5. File is opened O_WRONLY | O_APPEND.  [plonk]

A little symlink magic between 4 and 5 and you have write access to
any file if your xterm is setuid/setgid.

General attack idea:

ls -lL `which xterm`
# If not setuid/setgid, you are safe
touch dummy
symlink-flipflop link dummy /.rhosts
xterm -l -lf link -e echo + +

Moral: access() is totally useless for security purposes.  Use it
only as a means of providing better error messages (as it might not
be easy to get an error message out from a subprocess).


----- End forwarded message -----