Review Webserver Metafiles for Information Leakage (OTG-INFO-003)

OWASP Testing Guide v3 Table of Contents

This article is part of the OWASP Testing Guide v3. The entire OWASP Testing Guide v3 can be downloaded here.

OWASP at the moment is working at the OWASP Testing Guide v4: you can browse the Guide here

This is a draft of a section of the new Testing Guide v3

Brief Summary

This section describes how to test the robots.txt file.

Description of the Issue

Web spiders/robots/crawlers retrieve a web page and then recursively traverse hyperlinks to retrieve further web content. Their accepted behavior is specified by the Robots Exclusion Protocol of the robots.txt file in the web root directory [1].

Within the robots.txt file, the User-Agent directive refers to the specific web spider/robot/crawler, e.g., User-Agent: Googlebot refers to the GoogleBot crawler. User-Agent: * applies to all web spiders/robots/crawlers [2].

The Disallow directive specifies which resources should *not* be retrieved by spiders/robots/crawlers. For example, Disallow: /cgi-bin/ indicates that the /cgi-bin directory and its sub-directories should not be crawled.

Web spiders/robots/crawlers can intentionally ignore the Disallow directives specified in a robots.txt file [3]. Hence, robots.txt should not be considered as a mechanism to enforce restrictions on how web content is accessed, stored, or republished by third parties.

Black Box testing and example

The robots.txt file is retrieved from the web root directory of the web server. For example, the URL "http://www.google.com/robots.txt" is the robots.txt file of www.google.com

To retrieve the robots.txt from www.google.com using wget:

$ wget http://www.google.com/robots.txt
--23:59:24-- http://www.google.com/robots.txt
           => 'robots.txt'
Resolving www.google.com... 74.125.19.103, 74.125.19.104, 74.125.19.147, ...
Connecting to www.google.com|74.125.19.103|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: unspecified [text/plain]

    [ <=>                                 ] 3,425        --.--K/s

23:59:26 (13.67MB/s) - 'robots.txt' saved [3425]

Google provides an "Analyze robots.txt" function as part of its "Google Webmaster Tools", which can assist with testing [4].

Gray Box testing and example

The process is the same as Black Box testing above.

References

Whitepapers

• [1] "The Web Robots Pages" - http://www.robotstxt.org/
• [2] "How do I block or allow Googlebot?" - http://www.google.com/support/webmasters/bin/answer.py?answer=40364&query=googlebot&topic=&type=
• [3] "(ISC)2 Blog: The Attack of the Spiders from the Clouds" - http://blog.isc2.org/isc2_blog/2008/07/the-attack-of-t.html
• [4] "How do I check that my robots.txt file is working as expected?" - http://www.google.com/support/webmasters/bin/answer.py?answer=35237