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Pico Server (pServ) Remote Command Injection
From: Claus R. F. Overbeck (bugtraqclausrfoverbeck.de)
Date: Mon May 16 2005 - 09:56:51 CDT
Advisory: Pico Server (pServ) Remote Command Injection
RedTeam found a remote command injection in Pico Server (pServ) which results
in a remote attacker being able to issue arbitrary commands on the server.
Product: Pico Server (pServ)
Affected Version: 3.2(verified), <=3.2 probably too
Immune Version: 3.3
OS affected: all
Security-Risk: very high
Vendor-Status: new version available
Pico Server is a small web server. It is meant to be portable and
* small, portable
* CGI-BIN support
* auto-indexing of directories
* access and error logging (see p-reporter for an analyzer)
* forking or single-connection at choice
Pico Server (pServ) is written in portable C (K&R style so it can compile on
older compilers too) and sports several options that by means of #define
statements can customize the behavior, the performance and the feature set so
to be able to fit better the the requisites.
If pServ is compiled with support for CGI-BIN a remote attacker is able to
execute any program (with pServ permissions) on the server by traversing out
of the cgi-bin directory.
pServ has CGI-BIN support. Only URLs beginning with "cgi-bin" are treated as
To avoid that a user traverses out of the cgi-bin using traditional /../,
pServ parses the requested url. It increases a counter by one if it parses a
/ (new subdir) and decreases the counter if ist parses /../. If the counter
goes below zero the url is rejected as illegal. Unfortunately an attacker can
avoid beeing rejected, just using enough / in the url (without directory
names between them), so he can traverse out of the cgi-bin by adding some
/../ . This lets the attacker execute any program on the server (with pServ
Proof of Concept
The following url downloads a script (or executable) to the server:
This is how the script can be executed afterwards:
The only workaround is to compile pServ without support for cgi-bin.
The Developers have released Version 3.3. This version should fix the
problem. The changes have not been tested by RedTeam, yet.
The security risk is rated very high because a remote attacker can use this
flaw to execute arbitrary code on the server (with the permissions of pServ).
2005-05-02 first attempt to inform developers
2005-05-02 CAN-number assigned
2005-05-04 second attempt to inform developers
2005-05-16 new version released. Advisory published
RedTeam is a penetration testing group working at the Laboratory for
Dependable Distributed Systems at RWTH-Aachen University. You can find more
Information on the RedTeam Project at