Security and the Web CascadeLinks Regional Web Summit Namaste Retreat & Conference Center Wilsonville, Oregon October 19th, 2000 Security and the Web The web was once considered very safe, essentially a high-level read-only medium The web is now extraordinarily complex and an extreme security risk What changed?
CGI/dynamically generated web pages have low-level access to the host operating system, databases, etc Web browsers with client-side scripting capabilities and tight integration with the operating system What's the danger? Design of the web introduces new types of attacks which are not well understood Tight integration between web browser/server and the host operating system opens a near infinite number of issues Loose integration between the web browser
and the web server, using the stateless HTTP protocol, reduces the degree of trust which can be achieved Underlying theme You must understand the different levels of abstraction Every layer has unique weaknesses and issues Security is only as strong as the weakest link: all weaknesses must be addressed in order to be "secure" Layers of abstraction HTTP protocol (stateless API between
client & server) Server: retrieves files off the hard drive, executes a program using the CGI API, or interprets a script using an internal module (e.g. PHP, ASP) file access limited to the web root, possibly restricted by .htaccess, etc - these restrictions are enforced by the web server! CGI or internal scripts are not limited to the web root, auth restrictions, etc. CGI/scripts have access to the OS, execute programs, access databases, etc.
Weaknesses of HTTP HTTP is a stateless, clear-text protocol Statelessness makes it much easier for man-inthe middle type attacks Clear-text means that it is trivial for a man-inthe-middle to analyze the interaction between the client and server in real-time. Can capture cookies used for authentication, login names, passwords and other information passed in forms It's not easy to properly escape HTML tags in text that is intended to be displayed "as-is" Web server weaknesses
Directory recursion, breaking out of the web root http://buggyserver.com/../../../../../../etc/password http://okserver.com/bad.cgi?file=../../../etc/password Trusting user-supplied input Never, never, never trust user-supplied input! Filter all input for html injection, SQL commands
Other issues Keep on top of vendor patches How do you upload files to the server? Clear-text protocols such as FTP can expose your username/password If you allow ftp and http to access the same directories, make sure ftp uploads are not permitted! Make sure your filesystem/database permissions are sane Change all default passwords
CERT http://www.cert.org http://www.cert.org/advisories/CA-2000-02.html Cracked site mirrors (this could be you)
http://www.securityfocus.com http://www.attrition.org/mirror/attrition/ Reflections on Trusting Trust http://www.acm.org/classics/sep95 Examples Real-life example of cross-site scripting:
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