CAPEC-86 XSS Through HTTP Headers

An adversary exploits web applications that generate web content, such as links in a HTML page, based on unvalidated or improperly validated data submitted by other actors. XSS in HTTP Headers attacks target the HTTP headers which are hidden from most users and may not be validated by web applications.

1. [Explore] [Survey the application for public links] Using a browser or an automated tool, an adversary follows all public links on a web site. They record all the entry points (input) that becomes part of generated HTTP header (not only GET/POST/COOKIE, but also Content-Type, etc.) 2. [Experiment] [Probe identified potential entry points for XSS vulnerability] The adversary uses the entry points gathered in the "Explore" phase as a target list and injects various common script payloads to determine if an entry point actually represents a vulnerability and to characterize the extent to which the vulnerability can be exploited. They record all the responses from the server that include unmodified versions of their script. The adversary tries also to inject extra-parameter to the HTTP request to see if they are reflected back in the web page or in the HTTP response. 3. [Experiment] [Craft malicious XSS URL] Once the adversary has determined which parameters are vulnerable to XSS, they will craft a malicious URL containing the XSS exploit. The adversary can have many goals, from stealing session IDs, cookies, credentials, and page content from the victim. 4. [Exploit] [Get victim to click URL] In order for the attack to be successful, the victim needs to access the malicious URL.

• Target software must be a client that allows scripting communication from remote hosts.

• [Low] To achieve a redirection and use of less trusted source, an adversary can simply edit HTTP Headers that are sent to client machine. • [High] Exploiting a client side vulnerability to inject malicious scripts into the browser's executable process.

• Confidentiality, Integrity, Availability: Execute Unauthorized Commands • Confidentiality: Read Data • Confidentiality, Access Control, Authorization: Gain Privileges

• Design: Use browser technologies that do not allow client side scripting. • Design: Utilize strict type, character, and encoding enforcement • Design: Server side developers should not proxy content via XHR or other means, if a http proxy for remote content is setup on the server side, the client's browser has no way of discerning where the data is originating from. • Implementation: Ensure all content that is delivered to client is sanitized against an acceptable content specification. • Implementation: Perform input validation for all remote content. • Implementation: Perform output validation for all remote content. • Implementation: Disable scripting languages such as JavaScript in browser • Implementation: Session tokens for specific host • Implementation: Patching software. There are many attack vectors for XSS on the client side and the server side. Many vulnerabilities are fixed in service packs for browser, web servers, and plug in technologies, staying current on patch release that deal with XSS countermeasures mitigates this.