Software Engineering Institute

ibrow NewsDesk is a Perl CGI script that is designed to create and display news articles on a web site. The code for NewsDesk is implemented in the file newsdesk.cgi. This code accepts input from a URL and uses this data in an argument to the Perl open() command. Since NewsDesk does not adequately filter URL input, it is possible to construct a URL containing shell meta-characters such as "../" and "|" (pipe), and these characters will be processed with the privileges of the web server.

By submitting a specially crafted URL, an attacker can execute operating system commands and traverse the file system and read files with the permission of the web server.

The CERT/CC is currently unaware of a practical solution to this problem.

Modify NewsDesk

• Modify newsdesk.cgi to only allow acceptable characters in URL input that is passed to open(). This type of input validation is referred to as "positive" or "whitelist" filtering. See Laundering and Detecting Tainted Data in perlsec for more information.
• Enabling Perl "taint" checking will prevent execution of commands using the pipe character ("|"). See perlsec and the Secure Programming for Linux and Unix HOWTO for more information.
• Rewrite the vulnerable open() calls in newsdesk.cgi to be safer, possibly by using sysopen(). See perlopentut for more information.