Summary

Attack Execution Flow

1. Spider:

   Using a browser or an automated tool, an attacker follows all public links on a web site. He records all the links he finds.

   Attack Step Techniques

   ID	Attack Step Technique Description	Environments
   1	Use a spidering tool to follow and record all links. 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. Make special note of any links that include parameters in the URL. Manual traversal of this type is frequently necessary to identify forms that are GET method forms rather than POST forms.	env-Web
   3	Use a browser to manually explore the website and analyze how it is constructed. Many browser's plugins are available to facilitate the analysis or automate the URL discovery.	env-Web

   Indicators

   ID	type	Indicator Description	Environments
   1	Positive	URL parameters 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 on the URL. Even though none appear, the web application may still use them if they are provided.	env-Web
   4	Inconclusive	Application could use POST variable as GET inside the application. Therefore, looking for POST parameters and adding them to the query string.	env-Web
   5	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 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. Attempt variations on input parameters:

   Possibly using an automated tool, an attacker requests variations on the URLs he spidered before. He sends parameters that include variations of payloads. He records all the responses from the server that include unmodified versions of his script.

   Attack Step Techniques

   ID	Attack Step Technique Description	Environments
   1	Use a list of XSS probe strings to inject 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

   Indicators

   ID	type	Indicator Description	Environments
   1	Positive	The output of pages includes some form of a URL parameter. E.g., ?error="File not Found" becomes "File not Found" in the title of the web page.	env-Web
   2	Positive	Input parameters become part of JavaScript, VBScript, or other script in a 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, 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	Do not embed user-controllable input generated HTTP headers
   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.
   1	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.
   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.

Likelihood: High

Description

Description

Skill or Knowledge Level: Low

To place malicious payload on server via HTTP

Skill or Knowledge Level: High

Exploiting any information gathered by HTTP Query on script host

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, including remote and user-generated 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.

Implementation: Privileges are constrained, if a script is loaded, ensure system runs in chroot jail or other limited authority mode