Date: Wed, 25 Feb 2004 10:54:31 -0800 From: Tim Kientzle <tim@kientzle.com> To: freebsd-security@freebsd.org, das@freebsd.org Subject: Environment Poisoning and login -p Message-ID: <403CEF67.5040004@kientzle.com>
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There's been an ongoing discussion (started by
Colin Percival's recent work on nologin) about
environment-poisoning attacks via "login -p".
I thought I saw a way to address this, but the more I learn,
the uglier this looks. Maybe some of the good folks who read
freebsd-security can puzzle this one out:
Problem: login -p can be used to propagate environment flags
in order to trojan the user shell of the target account. There
are several significant cases:
* Dynamically-linked target shell.
* Target account shell is a shell script.
Both of these are quite common in practice. (However,
I will note that in -CURRENT, both "nologin" and /bin/sh
are statically-linked and thus currently immune to this.)
The particular thread that started me looking at this
concerned "nologin" scripts or programs that attempt
to block access to certain accounts. David Schultz
has demonstrated that login -p can be used to circumvent
a dynamically-linked nologin program, for example. If
your friend's account hasn't been blocked, you may be able
use that to circumvent any blocks placed on your own account.
Possible fix: Ignore "-p" flag if target shell is not
in /etc/shells. In this scenario, a nologin program would
not be listed in /etc/shells, and thus such attacks would
be blocked.
Problem: One common use of nologin scripts is to create
ftp-only accounts. However, ftpd(8) limits ftp access to
accounts with "standard" shells. As a result, ftp-only
accounts must have their shell listed in /etc/shells.
Possible fix: Have login unconditionally discard LD_LIBRARY_PATH
and LD_PRELOAD from the environment, even if "-p" is specified.
I'm unsure that this is sufficient. I'm also unsure whether
this blocks legitimate use of "login -p." I am generally
distrustful of blacklist security approaches; I'd prefer
a whitelist approach that only passed through selected environment
variables.
Possible fix: Eliminate the "-p" option to login.
This would certainly close the hole, but could introduce
complications elsewhere. (I've looked at telnetd, which
uses this flag. However, it appears to use it only to
propagate the TERM flag, which our login program does even
without -p.)
Part of the issue here is an underlying disagreement about
the meaning of "standard shell." Skimming the -CURRENT source
tree reveals several different definitions of "standard shell":
* A group of "equivalent" shells from which
users can choose. (from chpass(1))
* An indicator that the user is allowed ftp
access (from ftpd(8))
* An indicator that su is permitted
* An indicator that mail include files should
be honored (in sendmail; based on a very quick
skimming of the source)
These are not entirely consistent interpretations,
resulting in security problems with "ftp-only"
access, for example.
I've attached a patch to "login" that implements
the two "Possible Fixes" above. I'm not entirely
happy with it, though, for the reasons I've indicated.
Suggestions?
Tim Kientzle
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Index: login.c
===================================================================
RCS file: /home/ncvs/src/usr.bin/login/login.c,v
retrieving revision 1.98
diff -r1.98 login.c
86a87
> static int chshell(const char *);
468c469,472
< * preservation - but preserve TERM in all cases
---
> * preservation or the user has a non-standard shell. In
> * particular, this "non-standard shell" check blocks certain
> * environment-poisoning exploits against nologin scripts.
> * Preserve TERM in all cases.
471c475
< if (!pflag)
---
> if (!pflag || !chshell(shell))
476a481,491
> * The chshell() check above isn't sufficient, though.
> * For example, consider a custom nologin script used
> * to limit accounts to ftp-only access. ftpd(8) requires
> * the user shell to be in /etc/shells, thus crippling
> * the above check. The following provides some modest
> * additional security limits in such cases.
> */
> unsetenv("LD_LIBRARY_PATH");
> unsetenv("LD_PRELOAD");
>
> /*
935a951,968
> }
>
> /*
> * Return TRUE if the shell is a "standard" shell.
> * (That is, one listed in /etc/shells.)
> */
> static int
> chshell(const char *sh)
> {
> int r;
> const char *cp;
>
> r = 0;
> setusershell();
> while ((cp = getusershell()) != NULL && !r)
> r = (strcmp(cp, sh) == 0);
> endusershell();
> return (r);
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