27th Sep 2002 [SBWID-5713]
COMMAND
Remote system compromise in WASD OpenVMS http server
SYSTEMS AFFECTED
WASD 7.1, 7.2 (up to 7.2.3), 8.0, and possibly earlier versions.
PROBLEM
Thanks to Jean-loup Gailly [http://gailly.net] advisory, this document
is available at [http://jl.gailly.net/security/wasd-vuln-2002-09.txt] :
WASD VMS Hypertext Services is a popular http server for OpenVMS
released under the GNU GPL. See http://wasd.vsm.com.au/WASD/ The
default installation of the WASD server allows:
- universal directory traversal
- instant access to the entire web server tree
- trivial bypassing of access control rules
- getting the location of the document root
- read access to the whole web server configuration
- read access to all web server logs
- disclosure of directories supposed to be hidden
- getting the list of all cgi scripts
- getting the sources of all cgi scripts
- read access to OpenVMS system files
- user home directories might be readable
- one very serious flaw in a cgi script enabled by default
- some problems with other cgi-scripts enabled by default
--snipp--
1.1 The builtin "tree" script
By default the entire directory tree of the web server can be
seen with http://webserver/tree/
Documentation: http://wasd.vsm.com.au/ht_root/doc/env/env_0400.html#43
Example: http://wasd.vsm.com.au/tree/
.1.2 "/*.*" directory traversal
Universal directory traversal is builtin with
http://webserver/dirname/*.* even if there is a page /dirname/index.html
Documentation: http://wasd.vsm.com.au/ht_root/doc/env/env_0400.html
.1.3 The builtin "upd" script
A nice graphical interface for directory traversal is builtin with
http://webserver/upd/dirname/ even if there is a page /dirname/index.html
Documentation: http://wasd.vsm.com.au/httpd/-/updhelp.html
.1.4 The builtin search
All documents can be searched by default.
Documentation: http://wasd.vsm.com.au/ht_root/doc/env/env_0700.html#97
.1.5 Most of the server root is accessible by default
The document root is the main server root, which includes everything
(configuration files, scripts, executables, etc...). Directory traversal
is enabled on most of the directories and access to the other
directories is possible anyway (see later).
The WASD documentation states that the liberal option was on purpose:
http://wasd.vsm.com.au/ht_root/doc/htd/htd_0600.html#98
"The default configuration is fairly liberal, providing information
of use in a technical environment, but that may be superfluous or
less-than-desirable in other, possibly commercial environments."
OpenVMS has a reputation of being a very secure operating system. This may
explain why VMS system administrators may have a false sense of security
when installing the WASD web server and leave the default configuration
almost untouched.
The default configuration in WASD 8.1 will be much more restrictive.
2 The access control rules can be trivially bypassed
All the liberal features described above can be used to bypass
the few restrictions set by the web server.
.2.1 Bypassing access restrictions set by the web server
The configuration file httpd$map.conf contains rules such as:
pass /ht_root/wwwroot*
fail /ht_root/*
fail /-/*
In this example, the server root is ht_root and the document root is
wwwroot. The "fail" rules attempt to restrict access to the server
root but can be trivially bypassed with:
http://theserver/ht_root/wwwroot/-/*.*
(note: "-" is the VMS equivalent of ".."). So the only real protection
comes from the directory protections and ACLs (access control lists)
imposed by the operating system. (See section "Solutions" below.)
.2.2 The location of the document root can easily be obtained
The location of the document root can easily be obtained with the "where"
builtin script.
Documentation: http://wasd.vsm.com.au/ht_root/doc/htd/htd_2100.html#364
Even if the "where" script is not enabled, the "404 not found" message
gives away the document root. On a server where the document root was
(correctly) not the entire web server root (logical name HT_ROOT), the
real document root could still be obtained with
http://theserver/notfound which gives "Document not found ...
/ht_root/wwwroot/notfound" This allows getting the entire web server
root with http://theserver/ht_root/wwwroot/-/*.* as described above.
The full physical path may also be given hidden as a comment in the html
returned in the "404 not found" message:
<!-- sts: %X00018292 "$1$DUA2:[HT_ROOT.][WWWROOT]NOTFOUND" -->
.2.3 The full web server configuration can easily be obtained
The web server configuration file in ht_root/local/httpd$map.conf
is generally supposed to be protected by a fail rule:
fail /ht_root/local/*
However this can often be trivially bypassed as shown above:
http://theserver/ht_root/wwwroot/-/local/httpd$map.conf
On one of the machines tested, there was even no "fail" rule so the
configuration could be obtained directly as
http://theserver/local/httpd$map.conf
The configuration file httpd$map.conf gives a lot of information to
intruders, in particular all the access control rules, all the script
directories, all the virtual domains handled, all the accessible
directories not under ht_root, etc...
.2.4 All the web server logs can easily be obtained
This is just a variation on the above. There is generally a rule
fail /ht_root/log/*
but all the logs can generally be obtained with
http://theserver/upd/ht_root/src/-/log/
unless they are protected with adequate ACLs.
If the logs are protected by ACLs but you have a user account on
the machine hosting the web server, then some of the logs may still
be obtained because the last request may be available in the
logical name (environment variable) HTTPD80$REQUEST :
$ show log HTTPD80$REQUEST
"HTTPD80$REQUEST" = "08 11:56:42.200.430.5287.0.9000.http://theserver:
80.ip.address.of.caller.GET /filename"
The logical name is dynamically updated at each request. However this
was observed on only one system with an old version of WASD so the
problem may be fixed in 8.0.
.2.5 The "tree" builtin script shows directories supposed to be hidden
The server logs are actually in a subdirectory /ht_root/log/server/
Since there is generally a fail rule for /ht_root/log/*
the subdirectory server is supposed to be hidden, but the "tree"
builtin script happily shows it:
http://theserver/tree/ht_root/
.2.6 Directory protection can be bypassed anyway
Some directories are better protected and are not even visible with
the "tree" builtin scripts. On one tested site, the directory
/ht_root/script_local/ exists but cannot be seen with
/ht_root/script_local/*.* or with /tree/. This site has probably used
the following configuration described in
http://wasd.vsm.com.au/ht_root/doc/htd/htd_0600.html#99
[DirAccess] - Make "disabled" to completely remove the ability
to generate directory listings under any circumstances.
However the search feature comes to rescue, and the directory
(together will all its scripts, see next section) can still be seen
with http://theserver/.../*.com?search=$
If even the standard search feature has been disabled, one can also
try the default glist script, which is supposed to give only a list
of images but also gives all subdirectories:
http://theserver/cgi-bin/glist/ht_root/?list=now
In any case, on this particular server the directory script_local
is given in /robots.txt so it is not difficult to see that it exists.
.2.7 The list of cgi scripts can easily be obtained
It is not possible to see the list of cgi scripts with the url
http://theserver/cgi-bin/*.* however the scripts are generally in
ht_root/script/ or ht_root/script_local. If a configuration rule is
supposed to block access to these directories (either because the
document root is correctly separated from the server root, or because
there is an explicit rule blocking script/*), it is generally possible
to get the list of scripts and their sources with an url such as:
http://theserver/ht_root/wwwroot/-/script_local/*.*
For cgi scripts of users, which can be run with url
http://theserver/~username/cgi-bin/scriptname
it is a bit less trivial to obtain the list of scripts because
the obvious attempts fail:
http://theserver/~username/cgi-bin/*.*
but the following generally works:
http://theserver/~username/xxx/-/cgi-bin/*.*
where xxx doesn't have to be an existing directory.
If it doesn't work, then the search feature can generally be used:
http://theserver/~username/.../*.com?search=$
(note: on OpenVMS "..." indicates directory recursion). This searches
all the *.com scripts for the character $ which always starts a command
on OpenVMS, and so gives as a result the full list of all .com scripts
under all subdirectories of the home directory. It is even possible to
search *.* instead of *.com.
.2.8 The sources of cgi scripts can easily be obtained
After the list of scripts has been obtained as above, it is trivial
to get the sources of the scripts by clicking on a link. For general
server scripts the link can be of the form:
http://theserver/ht_root/wwwroot/-/script_local/scriptname
For user scripts obtained by the search feature, the link is:
http://theserver/extract/~username/cgi-bin/scriptname.com?highlight=$
Here "extract" is a script provided by default by WASD. It allows
getting the source of a script instead of executing it, even if the
directory containing the script is named cgi-bin.
.2.9 OpenVMS system files can generally be read
On several tested sites, a configuration rule is supposed to
give access to only a selected portion of system files:
pass /sys$common/syslib/* /sys$common/syslib/*
However other system files can easily be read, for example
http://theserver/sys$common/syslib/-/sysmgr/systartup_vms.com
.2.10 User home directories might be readable
It is common practice to map url http://theserver/~username/ to
a subdirectory /user_disk/username/web/ of the home directory
/user_disk/username/. But if the OpenVMS protections and ACLs on
the home directory are not set correctly, it is possible to traverse
it with:
http://theserver/~username/-/*.*
Actually this often returns an error because of a strange mapping rule:
pass /*/-/* /ht_root/runtime/*/*
but it is easy to work around this rule with:
http://theserver/~username/x/--/*.*
where x doesn't have to be the name of an existing directory,
and "--" represents for VMS the equivalent of "../.."
In one of the sites tested, there was even no mapping to a subdirectory
so the whole user home directory was available with
http://theserver/~username/*.*
3 CGI scripts are run by default under the identity of the main server
By default the server runs the image httpd.exe (or httpd_ssl.exe)
under the identity of user http$server. CGI scripts are also run by
default under the same identity. Thus a flaw in one CGI script can affect
the entire server.
On Unix, the main server is typically run as root but CGI scripts are
typically run under user "nobody". Thus a bad script cannot affect the
entire server.
The WASD server allows running CGI scripts as a user other than
http$server. This will be the default in WASD 8.1.
4 Problems with some cgi scripts enabled by default
I have not studied all scripts provided in a default installation
of WASD. But at least one is very dangerous and there are some bugs
in others.
.4.1 Write to an arbitrary file on the web server
One script enabled by default allows writing contents of the attacker's
choice to an arbitrary file on the server, as long as the VMS ACLs
allows it. This flaw can be exploited to get a remote SYSTEM (root)
compromise.
Given the severity of this flaw, no details are given here. See the
"Solutions" section below for temporary workarounds.
.4.2 Severe leakage of information in cgi_process.com
Running http://theserver/cgi-bin/cgi_process gives a *lot* of useful
information for an intruder. In particular it gives all the privileges
owned by the script while running. (For Unix users: a script run with
privileges is somewhat equivalent to a setuid or setgid program, but
with much finer control on the actions allowed for the program.)
On one system, the script could be run with the SETPRV privilege, which
is equivalent to setuid root on Unix. This tells an intruder that
efforts should be concentrated on this particularly vulnerable server.
.4.3 Format string bug in PerlRTE_example1.pl
See http://wasd.vsm.com.au/ht_root/src/perl/readmore.html
for a description of this script and its sources. It contains
in particular:
printf ("$name=\"$ENV{$name}\"\n");
The variable $name comes from the user and is not filtered, so a
classic format string attack is possible. For example:
http://wasd.vsm.com.au/plrte/PerlRTE_example1/%25x%25x%25x
(where %25 is the hex encoding of the character '%') gives:
PATH_INFO="/000"
PATH_TRANSLATED="HT_ROOT:[000000]000"
REQUEST_URI="/plrte/PerlRTE_example1/ 0 0 0"
where the number of zeroes is the number of %x format indicators.
This bug is probably not exploitable, but this should be checked.
This script also gives away a lot of potentially useful information
for an intruder (all logical names).
.4.4 Potential denial of service in print.com
The cgi script print.com allows printing a file on the server
from a remote location. This script is enabled by default. The source
is available at http://wasd.vsm.com.au/script/print.com
This script attempts to restrict the IP addresses allowed to print:
$ HPRINTS_ALLOWED = "131.185.250.*"
Anyone in this IP range can force the printer to run out of paper.
Anyway, it should be fairly easy to spoof the source IP address.
--snipp--
SOLUTION
Patch
=====
WASD 8.1, and the update versions 8.0.1 and 7.2.4, will fix the known
vulnerabilities.
Workaround
==========
Use a separate document root, not the whole web server root
Put the document root in /ht_root/wwwroot, not /ht_root, with mapping
rules such as:
pass /* /ht_root/wwwroot/*
fail /ht_root/*
You can add a rule such as
fail /-/*
but unfortunately it will not be very effective because it can be
trivially bypassed with all versions of the WASD server up to 8.0.
Use a subdirectory for a user document root, not the home directory
Use rules such as:
user /~*/* /user_disk/*/web/*
redirect /~* /~*/
Set file protections and ACLs correctly
Since the WASD access restrictions can be bypassed, the only effective
protection is that provided by the system itself, OpenVMS. The web server
runs by default as user http$server, so make sure that directories
supposed to be protected are not readable by this user. Take at least
the following minimum precautions:
. Make sure that all directories under ht_root are owned by user
SYSTEM, not by user HTTP$SERVER. Add specific ACLs for directories
and files which must be readable or writable by HTTP$SERVER, but only
these:
$ set file /owner=system /prot=(s:rwed,o:rwed,g,w) -
ht_root:[000000]local.dir, ht_root:[local...]*.*;* , -
ht_root:[000000]http$server.dir, ht_root:[http$server...]*.*;*
$ set security /acl=((ident=http$server,access=e) /delete=all -
ht_root:[000000]local.dir
$ set security /acl=((ident=http$server,access=r+e) /delete=all -
ht_root:[local]*.com;*
$ set security /acl=((ident=http$server,access=r+e) /delete=all -
ht_root:[http$server]*.com;*
Be particularly careful about ACLs on files in directory [local].
Only the .com files there should be readable by http$server; the
rest contains very sensitive information.
. To prevent all the different ways of reading scripts, set protection
Execute only instead of Read+Execute on all essential scripts
(and delete all other scripts as described below). Check at least
the directories /script, /script_local, /vax and /axp, plus any other
directories mentioned in the logical name CGI-BIN.
$ set file /owner=system /prot=(s:rwed,o:rwed,g,w) -
ht_root:[000000]script*.dir, ht_root:[script*...]*.*;* , -
ht_root:[000000]axp.dir, ht_root:[axp...]*.*;* , -
ht_root:[000000]vax.dir, ht_root:[vax...]*.*;*
$ set security /acl=((ident=http$server,access=e) /delete=all -
ht_root:[000000]script*.dir
$ set security /acl=((ident=http$server,access=e) /delete=all -
ht_root:[script*]*.*;*
$ set security /acl=((ident=http$server,access=e) /delete=all -
ht_root:[000000]axp.dir ! or vax.dir depending on architecture
$ set security /acl=((ident=http$server,access=e) /delete=all -
ht_root:[axp]*.*;* ! or [vax]*.*;* depending on architecture
. To prevent reading the web server logs, set ACLs to allow write-only
access for user http$server on directory [log] and its subdirectories.
. To prevent all the forms of directory traversal, set protection
Execute only instead of Read+Execute for directories which must
be protected from traversal. You can also add rules in httpd$map.conf
fail /tree/
fail /tree/*
fail /upd/*
fail /where/*
fail /query/*
fail /extract/*
but do not rely only on these rules, set ACLs correctly first.
Remove all unused cgi programs
This is a basic principle, but unfortunately it was not followed in
any of the tested sites: remove *all* unused cgi scripts and executables.
Some of the programs provided by default with the WASD server are
very dangerous. So check at least the directories script, script_local,
axp and vax (all under ht_root) and move anything which is unused
somewhere outside ht_root, with very strict ACLs.
Let me insist again: at least one of the programs provided by default
with the WASD server is extremely dangerous. I will not name it to avoid
helping too much the script kiddies, so in doubt remove everything
unless you know that you absolutely need it.
If the machine hosting the web server has some untrusted users with
local accounts on the machine, then forbid execution of arbitrary cgi
scripts in ~untrusted/cgi-bin with a rule such as:
fail /~untrusted/cgi-bin/*
put _before_ the rule
exec /~*/cgi-bin/* /user_disk/*/cgi-bin/*
or better, block cgi access to all local users.
Never run cgi scripts under the account of a privileged user
If you really need privileges in your cgi script, then force the
https protocol and force user authentication. Read the WASD documents
http://wasd.vsm.com.au/ht_root/doc/htd/htd_1200.html
Never do user authentication without SSL (the https protocol).
Do not run cgi scripts as user http$server. Strictly speaking, this
account is not privileged, but since it is also used to run the main
server, faulty cgi scripts can cause more harm than totally
unprivileged users.
If the machine hosting the web servers has untrusted local
users with a ~username/cgi-bin directory, then it is preferable to
either remove the ability to run scripts there, or at least run the
scripts as the user. Running those scripts as user http$server
lets the untrusted users do many things to the web server, such
as reading the server logs if ACLs are not set correctly, or killing
the main server process; it would be the equivalent of giving
the additional right [http$server] to the untrusted users.
Acknowledgments
===============
Thanks to Mark Daniel, author of the WASD server, for making it free
software, for quickly answering to my initial report, and for working
insane hours to fix all the problems I reported.
Thanks to Beave and Doc Cypher for providing free accounts on their
respective OpenVMS servers, for testing the proposed solutions and
suggesting improvements.
Thanks to Jeremy Begg for hosting the WASD demonstration server.
TUCoPS is optimized to look best in Firefox® on a widescreen monitor (1440x900 or better).
Site design & layout copyright © 1986-2024 AOH
