CAPEC-19 - Embedding Scripts within Scripts

An attack of this type exploits a programs' vulnerabilities that are brought on by allowing remote hosts to execute scripts. The attacker leverages this capability to execute scripts to execute his/her own script by embedding it within other scripts that the target software is likely to execute. The attacker must have the ability to inject script into script that is likely to be executed. If this is done, then the attacker can potentially launch a variety of probes and attacks against the web server's local environment, in many cases the so-called DMZ, back end resources the web server can communicate with, and other hosts.

With the proliferation of intermediaries, such as Web App Firewalls, network devices, and even printers having JVMs and Web servers, there are many locales where an attacker can inject malicious scripts. Since this attack pattern defines scripts within scripts, there are likely privileges to execute said attack on the host.

Of course, these attacks are not solely limited to the server side, client side scripts like Ajax and client side JavaScript can contain malicious scripts as well. In general all that is required is for there to be sufficient privileges to execute a script, but not protected against writing.

Severity

Likelihood

Confidentiality

Integrity

Availability

  • Attack Methods 2
  • Injection
  • API Abuse
  • Purposes 1
  • Penetration
  • Scopes 2
  • Execute unauthorized code or commands
  • Availability
  • Integrity
  • Confidentiality
  • Gain privileges / assume identity
  • Authorization
  • Access_Control
  • Confidentiality

Low level: To load malicious script into open, e.g. world writable directory

Medium level: Executing remote scripts on host and collecting output

Target software must be able to execute scripts, and also allow attacker to write/upload script

Ability to deploy a custom script on host

Step 1 - Spider

Using a browser or an automated tool, an attacker records all entry points for inputs that happen to be reflected in a client-side script element. These script elements can be located in the HTML content (head, body, comments), in an HTML tag, XML, CSS, etc..

Tecnique ID: 1 - Environment(s) env-Web

Use a spidering tool to follow and record all non-static links that are likely to have input parameters (through forms, URL, fragments, etc.) actively used by the Web application.

Tecnique ID: 2 - Environment(s) env-Web

Use a proxy tool to record all links visited during a manual traversal of the web application.

Tecnique ID: 3 - Environment(s) env-Web

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.

Indicator ID: 1 - Environment(s) env-Web

Type: Positive

Inputs are used in a script element (script tag, DOM, etc.) and not in another type of element.

Indicator ID: 2 - Environment(s) env-Web

Type: Inconclusive

Using URL rewriting, parameters may be part of the URL path or the URL fragment.

Indicator ID: 3 - Environment(s) env-Web

Type: 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.

Indicator ID: 4 - Environment(s) env-Web

Type: Negative

Applications that have only static pages or that simply present information without accepting input are unlikely to be susceptible.


Security Control ID: 1

Type: 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).

Security Control ID: 2

Type: 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.

Security Control ID: 3

Type: Preventative

Use CAPTCHA to prevent the use of the application by an automated tool.

Security Control ID: 4

Type: Preventative

Actively monitor the application and either deny or redirect requests from origins that appear to be automated.


Outcome ID: 1

Type: Success

A list of URLs, with their corresponding parameters (POST, GET, COOKIE, etc.) is created by the attacker. These parameters are possibly used in client-side scripts elements.

Outcome ID: 2

Type: Success

A list of application user interface entry fields is created by the attacker.

Outcome ID: 3

Type: Success

A list of resources accessed by the application is created by the attacker.



Step 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..

Tecnique ID: 1 - Environment(s) env-Web

Manually inject various script payloads into each identified entry point using a list of common script injection probes that typically work in a client-side script elements context and observe system behavior to determine if script was executed.

Tecnique ID: 2 - Environment(s) env-Web

Manually inject various script payloads into each identified entry point using a list of common script injection probes that typically work in a server-side script elements context and observe system behavior to determine if script was executed.

Tecnique ID: 3 - Environment(s) env-Web

Use an automated injection attack tool to inject various script payloads into each identified entry point using a list of common script injection probes that typically work in a client-side script elements context and observe system behavior to determine if script was executed.

Tecnique ID: 4 - Environment(s) env-Web

Use an automated injection attack tool to inject various script payloads into each identified entry point using a list of common script injection probes that typically work in a server-side script elements context and observe system behavior to determine if script was executed.

Tecnique ID: 5 - Environment(s) env-Web

Use a proxy tool to record results of the created requests.

Indicator ID: 1 - Environment(s) env-Web

Type: Positive

User-controllable input is output back to the browser

Indicator ID: 2 - Environment(s) env-Web

Type: Positive

User-controllable input is embedded as part of script elements

Indicator ID: 3 - Environment(s) env-Web

Type: Positive

Output to the browser is not encoded to remove executable scripting syntax

Indicator ID: 4 - Environment(s) env-Web

Type: Positive

Server-side components execute script elements containing user-controllable input


Security Control ID: 1

Type: Detective

Monitor input to web servers (not only GET, but all potential inputs like COOKIES, POST, HEADER), 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 adversaries use.

Security Control ID: 2

Type: Preventative

Apply appropriate input validation to filter all user-controllable input of scripting syntax

Security Control ID: 3

Type: Preventative

Do not embed user-controllable input in script elements.

Security Control ID: 4

Type: Preventative

Actively monitor the application and either deny or redirect requests from origins that appear to be generating XSS probes.


Outcome ID: 1

Type: Success

The attacker's script string is being reflected 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, etc.)

Outcome ID: 2

Type: Success

The attacker's script string is executed by the server-side component.

Outcome ID: 3

Type: Failure

All context-sensitive characters are consistently re-encoded before being sent to the web browser.

Outcome ID: 4

Type: Inconclusive

Some sensitive characters are consistently encoded, but others are not. Depending on which type of non-script element the payload is injected in, it may be possible to evade the encodings.



Step 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..

Tecnique ID: 1 - Environment(s) env-Web

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.

Tecnique ID: 2 - Environment(s) env-Web

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.

Security Control ID: 1

Type: Detective

Monitor server logs for scripting parameters.

Security Control ID: 2

Type: 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.

Security Control ID: 3

Type: Preventative

Apply appropriate input validation to filter all user-controllable input of scripting syntax

Security Control ID: 4

Type: Preventative

Appropriately encode all browser output to avoid scripting syntax

Security Control ID: 5

Type: Preventative

Actively monitor the application and either deny or redirect requests from origins that appear to be generating XSS probes.


Outcome ID: 1

Type: Success

The attacker gets the user's cookies or other session identifiers.

Outcome ID: 2

Type: Success

The attacker gets the content of the page the user is viewing.

Outcome ID: 3

Type: Success

The attacker causes the user's browser to visit a page with malicious content.


Step 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)..

Tecnique ID: 1 - Environment(s) env-Web

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

Tecnique ID: 2 - Environment(s) env-Web

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).

Security Control ID: 1

Type: Detective

Monitor server logs for scripting parameters.

Security Control ID: 2

Type: 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.

Security Control ID: 3

Type: Preventative

Apply appropriate input validation to filter all user-controllable input of scripting syntax

Security Control ID: 4

Type: Preventative

Appropriately encode all browser output to avoid scripting syntax

Security Control ID: 5

Type: Preventative

Actively monitor the application and either deny or redirect requests from origins that appear to be generating XSS probes.


Outcome ID: 1

Type: Success

The attacker indirectly controls the user's browser and makes it performing actions exploiting CSRF.

Outcome ID: 2

Type: 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.


Step 3 - Content spoofing

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

Tecnique ID: 1 - Environment(s) env-Web

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.

Security Control ID: 1

Type: Detective

Monitor server logs for scripting parameters.

Security Control ID: 2

Type: 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.

Security Control ID: 3

Type: Preventative

Apply appropriate input validation to filter all user-controllable input of scripting syntax

Security Control ID: 4

Type: Preventative

Appropriately encode all browser output to avoid scripting syntax

Security Control ID: 5

Type: Preventative

Actively monitor the application and either deny or redirect requests from origins that appear to be generating XSS probes.


Outcome ID: 1

Type: Success

The user sees a page containing wrong information



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.

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