Summary

Attack Execution Flow

1. Survey the application for user-controllable inputs:

   Using a browser or an automated tool, an attacker follows all public links and actions on a web site. He records all the links, the forms, the resources accessed and all other potential entry-points for the web application.

   Attack Step Techniques

   ID	Attack Step Technique Description	Environments
   1	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 in the URL.	env-Web
   2	Use a proxy tool to record all links visited during a manual traversal of the web application.	env-Web
   3	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.	env-Web

   Indicators

   ID	type	Indicator Description	Environments
   1	Positive	Inputs are used by the application or the browser (DOM)	env-Web
   2	Inconclusive	Using URL rewriting, parameters may be part of the URL path.	env-Web
   3	Inconclusive	No parameters appear to be used on the current page. Even though none appear, the web application may still use them if they are provided.	env-Web
   4	Negative	Applications that have only static pages or that simply present information without accepting input are unlikely to be susceptible.	env-Web

   Outcomes

   ID	type	Outcome Description
   1	Success	A list of URLs, with their corresponding parameters (POST, GET, COOKIE, etc.) is created by the attacker.
   2	Success	A list of application user interface entry fields is created by the attacker.
   3	Success	A list of resources accessed by the application is created by the attacker.

   Security Controls

   ID	type	Security Control Description
   1	Detective	Monitor velocity of page fetching in web logs. Humans who view a page and select a link from it will click far slower and far less regularly than tools. Tools make requests very quickly and the requests are typically spaced apart regularly (e.g. 0.8 seconds between them).
   2	Detective	Create links on some pages that are visually hidden from web browsers. Using IFRAMES, images, or other HTML techniques, the links can be hidden from web browsing humans, but visible to spiders and programs. A request for the page, then, becomes a good predictor of an automated tool probing the application.
   3	Preventative	Use CAPTCHA to prevent the use of the application by an automated tool.
   4	Preventative	Actively monitor the application and either deny or redirect requests from origins that appear to be automated.

1. 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.

   Attack Step Techniques

   ID	Attack Step Technique Description	Environments
   1	Use a list of XSS probe strings to inject script in parameters of known URLs. If possible, the probe strings contain a unique identifier.	env-Web
   2	Use a proxy tool to record results of manual input of XSS probes in known URLs.	env-Web
   3	Use a list of XSS probe strings to inject script into UI entry fields. If possible, the probe strings contain a unique identifier.	env-Web
   4	Use a list of XSS probe strings to inject script into resources accessed by the application. If possible, the probe strings contain a unique identifier.	env-Web

   Indicators

   ID	type	Indicator Description	Environments
   1	Positive	The output of pages includes some form of a URL parameter. E.g., ?error="<foobar>'(){};=" becomes "<foobar>'(){}=" in the title of the web page.	env-Web
   2	Positive	Input content becomes part of the web page.	env-Web
   3	Inconclusive	Nothing is returned to the web page. It may be a stored XSS. The unique identifier from the probe helps to trace the flow of the possible XSS.	env-Web

   Outcomes

   ID	type	Outcome Description
   1	Success	The attacker's cross-site scripting string is repeated back verbatim at some point in the web site (if not on the same page). Note that sometimes, the payload might be well encoded in the page, but wouldn't be encoded at all in some other section of the same web page (title, script, etc.)
   2	Failure	All HTML-sensitive characters are consistently re-encoded before being sent to the web browser.
   3	Inconclusive	Some sensitive characters are consistently encoded, but others are not.

   Security Controls

   ID	type	Security Control Description
   1	Detective	Monitor input to web servers (not only GET, but all potential inputs), application servers, and other HTTP infrastructure (e.g., load balancers). Alert on standard XSS probes. The majority of attackers use well known strings to check for vulnerabilities. Use the same vulnerability catalogs that hackers use.
   2	Preventative	Apply appropriate input validation to filter all user-controllable input of scripting syntax
   3	Preventative	Appropriately encode all browser output to avoid scripting syntax
   4	Preventative	Actively monitor the application and either deny or redirect requests from origins that appear to be generating XSS probes.

1. 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.

   Attack Step Techniques

   ID	Attack Step Technique Description	Environments
   1	Develop malicious JavaScript that is injected through vectors identified during the Experiment Phase and loaded by the victim's browser and sends document information to the attacker.	env-Web
   2	Develop malicious JavaScript that injected through vectors identified during the Experiment Phase and takes commands from an attacker's server and then causes the browser to execute appropriately.	env-Web

   Outcomes

   ID	type	Outcome Description
   1	Success	The attacker gets the user's cookies or other session identifiers.
   2	Success	The attacker gets the content of the page the user is viewing.
   3	Success	The attacker causes the user's browser to visit a page with malicious content.

   Security Controls

   ID	type	Security Control Description
   1	Detective	Monitor server logs for scripting parameters.
   2	Detective	Monitor server logs for referrers. If users are being tricked into clicking XSS links through forums or other web postings, their web browsers will be providing Referrer headers most of the time. These can help indicate that the actual request is illegitimate.
   3	Preventative	Apply appropriate input validation to filter all user-controllable input of scripting syntax
   4	Preventative	Appropriately encode all browser output to avoid scripting syntax
   5	Preventative	Actively monitor the application and either deny or redirect requests from origins that appear to be generating XSS probes.

2. 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).

   Attack Step Techniques

   ID	Attack Step Technique Description	Environments
   1	Develop malicious JavaScript that is injected through vectors identified during the Experiment Phase and loaded by the victim's browser and performs actions on the same web site	env-Web
   2	Develop malicious JavaScript that injected through vectors identified during the Experiment Phase and takes commands from an attacker's server and then causes the browser to execute request to other web sites (especially the web applications that have CSRF vulnerabilities).	env-Web

   Outcomes

   ID	type	Outcome Description
   1	Success	The attacker indirectly controls the user's browser and makes it performing actions exploiting CSRF.
   2	Success	The attacker manipulates the browser through the steps that he designed in his attack. The user, identified on a website, is now performing actions he is not aware of.

   Security Controls

   ID	type	Security Control Description
   1	Detective	Monitor server logs for scripting parameters.
   2	Detective	Monitor server logs for referrers. If users are being tricked into clicking XSS links through forums or other web postings, their web browsers will be providing Referrer headers most of the time. These can help indicate that the actual request is illegitimate.
   3	Preventative	Apply appropriate input validation to filter all user-controllable input of scripting syntax
   4	Preventative	Appropriately encode all browser output to avoid scripting syntax
   5	Preventative	Actively monitor the application and either deny or redirect requests from origins that appear to be generating XSS probes.

3. Content spoofing:

   By manipulating the content, the attacker targets the information that the user would like to get from the website.

   Attack Step Techniques

   ID	Attack Step Technique Description	Environments
   1	Develop malicious JavaScript that is injected through vectors identified during the Experiment Phase and loaded by the victim's browser and exposes attacker-modified invalid information to the user on the current web page.	env-Web

   Outcomes

   ID	type	Outcome Description
   1	Success	The user sees a page containing wrong information

   Security Controls

   ID	type	Security Control Description
   1	Detective	Monitor server logs for scripting parameters.
   2	Detective	Monitor server logs for referrers. If users are being tricked into clicking XSS links through forums or other web postings, their web browsers will be providing Referrer headers most of the time. These can help indicate that the actual request is illegitimate.
   3	Preventative	Apply appropriate input validation to filter all user-controllable input of scripting syntax
   4	Preventative	Appropriately encode all browser output to avoid scripting syntax
   5	Preventative	Actively monitor the application and either deny or redirect requests from origins that appear to be generating XSS probes.

Likelihood: High

Description

Skill or Knowledge Level: Low

To achieve a redirection and use of less trusted source, an attacker can simply place a script in bulletin board, blog, wiki, or other user-generated content site that are echoed back to other client machines.

Skill or Knowledge Level: High

Exploiting a client side vulnerability to inject malicious scripts into the browser's executable process.

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.