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Re: security hole in mailx
From: casper () HOLLAND SUN COM (Casper Dik)
Date: Thu, 25 Jun 1998 12:05:58 +0200


I've discovered a rather serious security hole in mailx, the good
old Berkeley mail program. It's somehow present at least in the
last versions I've checked (mailx-8.1.1 in Linux, mailx 5.0 in
Solaris). The bug is an exploitable buffer overflow (using the
HOME environment variable) that allows any local user to acquire
the privileges under which the program runs, usually group "mail"
(on many of the modern versions I've seen, mailx is setgid
"mail"). So the bug allows you to get "mail" group privileges,
and this means that at least you can play with other people's
mail. Although this wouldn't be very nice from you, this
shouldn't be a big problem for the integrity of the system. But
there could be a much more serious implication: being able to
write in /var/spool/mail is usually an open door to the root
account, for example by using races and such in mail delivery
programs.


Let me start of saying that I've filed a bug in the Solaris bug database,
it's #4152234.  (I say this mainly to prevent duplicate filings)


About the bug: it is in "fio.c", in the "xname" variable of the
"expand" function:

 char xname[PATHSIZE];
 [...]
 sprintf(xname, "%s%s", homedir, name + 1);


It should be noted that homedir itself, at least on Solaris,,
is a char homedir[PATHSIZE] which is copied from the environment.

(This never stops to amaze me; why *copy* the result from getenv()?)

You'd want to fix the overflow of homedir too; looks like there
are a few other overflows as well.

Two attachments are included in this message:

-A patch against mailx-8.1.1 that solves the problem. There
are a lot of buffer overflows in the sources of mailx,
although only the one I mention seems to be exploitable. The
patch is dirty and simple: replace sprintf and strcpy by
snprintf and strncpy almost everywhere. I haven't tested it
a lot, use it at your own risk.


I don't particularly care for arguments as "seem exploitable".

The homedir data segment buffer overflow may well be exploitable;
in the Solaris sources, there is at least one other buffer overflow
on the stack.


BTW, the person who tested the bug under Solaris (I don't have
direct access to any Solaris machine) told me that he had a hard
time:

tcsh$ setenv HOME `perl -e 'print "A"x10000'`
connection lost

!!! Seems like tcsh doesn't like huge homes like this. Second try:


Interesting.

This should work for all shells:


        env HOME=`perl -e 'print "x" x 10000'` ...


Tcsh calls abort().  Weird.

Casper



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