Home > CAPEC List > CAPEC-18: Embedding Scripts in Non-Script Elements (Version 2.6)  

CAPEC-18: Embedding Scripts in Non-Script Elements

 
Embedding Scripts in Non-Script Elements
Definition in a New Window Definition in a New Window
Attack Pattern ID: 18
Abstraction: Standard
Status: Draft
Completeness: Complete
Presentation Filter:
+ Summary

This attack is a form of Cross-Site Scripting (XSS) where malicious scripts are embedded in elements that are not expected to host scripts such as image tags (<img>), comments in XML documents (< !-CDATA->), etc. These tags may not be subject to the same input validation, output validation, and other content filtering and checking routines, so this can create an opportunity for an attacker to tunnel through the application's elements and launch a XSS attack through other elements.

As with all remote attacks, it is important to differentiate the ability to launch an attack (such as probing an internal network for unpatched servers) and the ability of the remote attacker to collect and interpret the output of said attack.

+ Attack Execution Flow
Explore
  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 non-script element. These non-script elements can be located in the HTML content (head, body, comments), in an HTML tag, XML, CSS, etc.

    Attack Step Techniques

    IDAttack Step Technique DescriptionEnvironments
    1

    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.

    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

    IDTypeIndicator DescriptionEnvironments
    1Positive

    At least one input is reflected in a non-script element.

    env-Web
    2Inconclusive

    Using URL rewriting, parameters may be part of the URL path and still used in a non-script element.

    env-Web
    3Inconclusive

    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
    4Negative

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

    env-Web

    Outcomes

    IDTypeOutcome Description
    1Success
    A list of URLs, with their corresponding parameters (POST, GET, COOKIE, etc.) is created by the attacker. These parameters are all used in, possibly, client-side non-scripts elements.
    2Success
    A list of application user interface entry fields is created by the attacker.
    3Success
    A list of resources accessed by the application is created by the attacker.

    Security Controls

    IDTypeSecurity Control Description
    1Detective
    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).
    2Detective
    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.
    3Preventative
    Use CAPTCHA to prevent the use of the application by an automated tool.
    4Preventative
    Actively monitor the application and either deny or redirect requests from origins that appear to be automated.
Experiment
  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

    IDAttack Step Technique DescriptionEnvironments
    1

    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 non-script elements context and observe system behavior to determine if script was executed. Since these probes may have to be injected in many different types of non-script elements, they should cover a variety of possible contexts (CSS, HTML tag, XML, etc.).

    env-Web
    2

    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 non-script elements context and observe system behavior to determine if script was executed. Since these probes may have to be injected in many different types of non-script elements, they should cover a variety of possible contexts (CSS, HTML tag, XML, etc.).

    env-Web
    3

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

    env-Web

    Indicators

    IDTypeIndicator DescriptionEnvironments
    1Positive

    User-controllable input is output back to the browser

    env-Web
    2Positive

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

    env-Web

    Outcomes

    IDTypeOutcome Description
    1Success
    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.)
    2Failure
    All context-sensitive characters are consistently re-encoded before being sent to the web browser. For example, in a HTML tag element, the payload may not be able to evade the quotes in order to inject another attribute.
    3Inconclusive
    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.

    Security Controls

    IDTypeSecurity Control Description
    1Detective
    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.
    2Preventative
    Apply appropriate input validation to filter all user-controllable input of scripting syntax
    3Preventative
    Appropriately encode all browser output to avoid scripting syntax
    4Preventative
    Actively monitor the application and either deny or redirect requests from origins that appear to be generating XSS probes.
Exploit
  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

    IDAttack Step Technique DescriptionEnvironments
    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

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

    Security Controls

    IDTypeSecurity Control Description
    1Detective
    Monitor server logs for scripting parameters.
    2Detective
    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.
    3Preventative
    Apply appropriate input validation to filter all user-controllable input of scripting syntax
    4Preventative
    Appropriately encode all browser output to avoid scripting syntax
    5Preventative
    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

    IDAttack Step Technique DescriptionEnvironments
    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

    IDTypeOutcome Description
    1Success
    The attacker indirectly controls the user's browser and makes it performing actions exploiting CSRF.
    2Success
    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

    IDTypeSecurity Control Description
    1Detective
    Monitor server logs for scripting parameters.
    2Detective
    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.
    3Preventative
    Apply appropriate input validation to filter all user-controllable input of scripting syntax
    4Preventative
    Appropriately encode all browser output to avoid scripting syntax
    5Preventative
    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

    IDAttack Step Technique DescriptionEnvironments
    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

    IDTypeOutcome Description
    1Success
    The user sees a page containing wrong information

    Security Controls

    IDTypeSecurity Control Description
    1Detective
    Monitor server logs for scripting parameters.
    2Detective
    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.
    3Preventative
    Apply appropriate input validation to filter all user-controllable input of scripting syntax
    4Preventative
    Appropriately encode all browser output to avoid scripting syntax
    5Preventative
    Actively monitor the application and either deny or redirect requests from origins that appear to be generating XSS probes.
+ Attack Prerequisites
  • Target client software must be a client that allows script execution based on scripts generated by remote hosts.

+ Typical Severity

Very High

+ Typical Likelihood of Exploit

Likelihood: High

+ Methods of Attack
  • Injection
  • API Abuse
+ Examples-Instances

Description

In this example, the attacker adds script to HTML tags other than <script> tags, when the victim's standard content is appended with a malicious script. For example a link to http://myfavoritewebsite/getMyHomePage/content?maliciousscript.js

The victim clicks on the link, which directs them to their home page (so that the victim does not notice anything is amiss) and simultaneously executes a script on their machine.

+ 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 content such as XML payload or HTML files 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.

+ Resources Required

Ability to include malicious script in document, e.g. HTML file, or XML document. Ability to deploy a custom hostile service for access by targeted clients. Ability to communicate synchronously or asynchronously with client machine

+ Solutions and Mitigations

Design: Use browser technologies that do not allow client side scripting.

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: Service provider should not use the XMLHttpRequest method to create a local proxy for content from other sites, because the client will not be able to discern what content comes from which host.

+ Attack Motivation-Consequences
ScopeTechnical ImpactNote
Confidentiality
Integrity
Availability
Execute unauthorized code or commands
Run Arbitrary Code
Confidentiality
Read application data
+ Injection Vector

Malicious input delivered through standard document formats, e.g. XML document or HTML file to the client.

+ Payload

Varies with instantiation of attack pattern. In the case of HTML files they may not be visible to the end user via a browser.

+ Activation Zone

Client software and its component libraries

+ Payload Activation Impact

Enables attacker to execute scripts to launch attacks on remote client machine and environment

+ Purposes
  • Penetration
+ CIA Impact
Confidentiality Impact: MediumIntegrity Impact: MediumAvailability Impact: Low
+ Technical Context
Architectural Paradigms
All
Frameworks
All
Platforms
All
Languages
All
+ References
[R.18.1] [REF-2] G. Hoglund and G. McGraw. "Exploiting Software: How to Break Code". Addison-Wesley. February 2004.
+ Content History
Submissions
SubmitterOrganizationDateSource
CAPEC Content TeamThe MITRE Corporation2014-06-23Internal_CAPEC_Team

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Page Last Updated: December 04, 2014