HTTP Parameter Pollution

A new class of Injection Vulnerability called HTTP Parameter Pollution (HPP) is less known

– Has not received much attention

– First presented by S. di Paola and L. Carettoni at OWASP 2009

• Attack consists of injecting encoded query string delimiters into existing HTTP parameters (e.g. GET/POST/Cookie)

– If application does not sanitize its inputs, HPP can be used to launch client-side or server-side attacks

– Attacker may be able to override existing parameter values, inject a new parameter or exploit variables out of a direct reach

“An attacker overrides or adds HTTP GET/POST parameters by injecting query string delimiters. Via HPP it may be possible to override existing hardcoded HTTP parameters, modify the application behaviors, access and, potentially exploit, uncontrollable variables, and bypass input validation checkpoints and WAF rules”.

HTTP Parameter Handling

•During interaction with web application, client provides parameters via GET/POST/Cookie

– http://www.site.com/login?login=alice

• HTTP allows the same parameter to be provided twice

– E.g., in a form checkbox

http://www.w3schools.com/html/tryit.asp?filename=tryhtml_form_checkbox

• What happens when the same parameter is provided twice?

– http://demo.testfire.net/search.aspx?txtSearch=alice&txtSearch=bob

demo.testfire.net

Yahoo

• An HTTP Parameter Pollution (HPP) attack occurs

– When a malicious parameter P(inj), preceded by an encoded query string delimiter (e.g. %26), is injected into an existing parameter P(host)

• Typical scenario (client-side)

– Web application for election for two candidates

Url : http://host/election.jsp?poll_id=4568

Link1: <a href="vote.jsp?poll_id=4568&candidate=white”> Vote for Mr.White </a>

Link2:<a href="vote.jsp?poll_id=4568&candidate=green”>Vote for Mrs.Green </a>

• The two links are built from the URL

ID = Request.getParameter(“poll_id”)

href_link = “vote.jsp?poll_id=” + ID + ”&candidate=xyz”

• No sanitization

• poll_id is vulnerable and Attacker creates URL:

http://host/election.jsp?poll_id=4568%26candidate%3Dgreen

• The resulting page now contains injected links:

<a href=vote.jsp?poll_id=4568&candidate=green&candidate=white>

Vote for Mr. White </a>

<a href=vote.jsp?poll_id=4568&candidate=green&candidate=green>

Vote for Mrs. Green </a>

• If the developer expects to receive a single value

– Jsp’s Request.getParameter(“candidate”)returns the 1st value

– The parameter precedence is consistent…

• Candidate Mrs. Green is always voted!

Consequence

• Override existing (hardcoded) values

• Inject a new parameter

• Exploit a parameter out of a direct reach

• Client-side (user) or server-side (webapplication) attack

Solutions and Mitigations

• Configuration: If using a Web Application Firewall (WAF), filters should be carefully configured to detect abnormal HTTP requests

• Design: Perform URL encoding

• Implementation: Use strict regular expressions in URL rewriting

• Implementation: Beware of multiple occurrences of a parameter in a Query String

Cheers!!!!!!!!!