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.
Spider: Using a browser or an automated tool, an attacker follows all public links on a web site. He records all the entry points (input) that becomes part of generated HTTP header (not only GET/POST/COOKIE, but also Content-Type, etc.)
Use a spidering tool to follow and record all links and analyze the web pages to find entry points. Make special note of any links that include parameters used in the HTTP headers.
Look for HTML meta tags that could be injectable
Use a proxy tool to record all links visited during a manual traversal of the web application.
Use a browser to manually explore the website and analyze how it is constructed. Many browsers' plugins are available to facilitate the analysis or automate the discovery.
Probe identified potential entry points for XSS vulnerability: The attacker 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. He records all the responses from the server that include unmodified versions of his script.
Manually inject various script payloads into each identified entry point using a list of common script injection probes and observe system behavior to determine if script was executed.
Use an automated injection attack tool to inject various script payloads into each identified entry point using a list of common script injection probes and observe system behavior to determine if script was executed.
Use a proxy tool to record results of manual input of XSS probes in known URLs.
Steal session IDs, credentials, page content, etc.: As the attacker succeeds in exploiting the vulnerability, he can choose to steal user's credentials in order to reuse or to analyze them later on.
Forceful browsing: When the attacker targets the current application or another one (through CSRF vulnerabilities), the user will then be the one who perform the attacks without being aware of it. These attacks are mostly targeting application logic flaws, but it can also be used to create a widespread attack against a particular website on the user's current network (Internet or not).
Content spoofing: By manipulating the content, the attacker targets the information that the user would like to get from the website.
Target software must be a client that allows scripting communication from remote hosts.
Typical Likelihood of Exploit
Methods of Attack
Modification of Resources
Utilize a remote style sheet set in the HTTP header for XSS attack. When the attacker is able to point to a remote stylesheet, any of the variables set in that stylesheet are controllable on the client side by the remote attacker. Like most XSS attacks, results vary depending on browser that is used.
If the response sends an unexpected encoding type such as UTF-7, then no enforcement is done on the payload and arbitrary XSS code will be transported along with the standard HTTP response. [R.86.3]
XSS can be used in variety of ways, because it is scripted and executes in a distributed, asynchronous fashion it can create its own vector and openings. For example, the attacker can use XSS to mount a DDoS attack by having series of different computers unknowingly executing requests against a single host.
Attacker Skills or Knowledge Required
Skill or Knowledge Level: Low
To achieve a redirection and use of less trusted source, an attacker can simply edit HTTP Headers that are sent to client machine.
Skill or Knowledge Level: High
Exploiting a client side vulnerability to inject malicious scripts into the browser's executable process.
The adversary must have the ability to deploy a custom hostile service for access by targeted clients and the abbility to communicate synchronously or asynchronously with client machine. The adversary must also control a remote site of some sort to redirect client and data to.
Solutions and Mitigations
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: 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.
Execute unauthorized code or commands
Run Arbitrary Code
Read application data
Gain privileges / assume identity
Malicious input delivered through HTTP Headers.
Varies with instantiation of attack pattern. In the case of HTTP headers they may not be visible to the end user via a browser
Header processing on the server or Client browser
Payload Activation Impact
Enables attacker to execute scripts to launch attacks on server as well as remote client machine and environment
http_protocol.c in (1) IBM HTTP Server 6.0 before 22.214.171.124 and 6.1 before 126.96.36.199, and (2) Apache HTTP Server 1.3 before 1.3.35, 2.0 before 2.0.58, and 2.2 before 2.2.2, does not sanitize the Expect header from an HTTP request when it is reflected back in an error message, which might allow cross-site scripting (XSS) style attacks using web client components that can send arbitrary headers in requests, as demonstrated using a Flash SWF file.
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Page Last Updated or Reviewed:
July 31, 2017
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