COMMON ATTACK PATTERN ENUMERATION AND CLASSIFICATIONLive MITRE record

CAPEC-32

XSS Through HTTP Query Strings

An adversary embeds malicious script code in the parameters of an HTTP query string and convinces a victim to submit the HTTP request that contains the query string to a vulnerable web application. The web application then procedes to use the values parameters without properly validation them first and generates the HTML code that will be executed by the victim's browser.

Abstraction

Detailed

Typical severity

High

Likelihood of attack

High

Understand the attack

What is this attack pattern?

An adversary embeds malicious script code in the parameters of an HTTP query string and convinces a victim to submit the HTTP request that contains the query string to a vulnerable web application. The web application then procedes to use the values parameters without properly validation them first and generates the HTML code that will be executed by the victim's browser.

Execution flow

How does the attack proceed, step by step?

1

Explore

Survey the application for public links

Using a browser or an automated tool, an adversary follows all public links on a web site. They record all the links they find.

Techniques used

  • Use a spidering tool to follow and record all links. Make special note of any links that include parameters in the URL.
  • 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.
  • 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.
2

Experiment

Probe public links for XSS vulnerability

The adversary uses the public links gathered in the "Explore" phase as a target list and requests variations on the URLs they spidered before. They send parameters that include variations of payloads. They record all the responses from the server that include unmodified versions of their script.

Techniques used

  • Use a list of XSS probe strings to inject in parameters of known URLs. If possible, the probe strings contain a unique identifier.
  • Use a proxy tool to record results of manual input of XSS probes in known URLs.

Craft malicious XSS URL

Once the adversary has determined which parameters are vulnerable to XSS, they will craft a malicious URL containing the XSS exploit. The adversary can have many goals, from stealing session IDs, cookies, credentials, and page content from the victim.

Techniques used

  • Change a URL parameter to include a malicious script tag.
  • Send information gathered from the malicious script to a remote endpoint.
3

Exploit

Get victim to click URL

In order for the attack to be successful, the victim needs to access the malicious URL.

Techniques used

  • Send a phishing email to the victim containing the malicious URL. This can be hidden in a hyperlink as to not show the full URL, which might draw suspicion.
  • Put the malicious URL on a public forum, where many victims might accidentally click the link.

Requirements

What does the attack require?

Prerequisites

  • Target client software must allow scripting such as JavaScript. Server software must allow display of remote generated HTML without sufficient input or output validation.

Skills required

  • Low: To place malicious payload on server via HTTP
  • High: Exploiting any information gathered by HTTP Query on script host

Resources required

  • Ability to send HTTP post to scripting host and collect output

Impact

What does a successful attack lead to?

Read Data

Affected scopes: Confidentiality

Execute Unauthorized Commands

Affected scopes: Confidentiality, Integrity, Availability

Defence

How is it prevented and mitigated?

1
Design: Use browser technologies that do not allow client side scripting.
2
Design: Utilize strict type, character, and encoding enforcement
3
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.
4
Implementation: Ensure all content that is delivered to client is sanitized against an acceptable content specification.
5
Implementation: Perform input validation for all remote content, including remote and user-generated content
6
Implementation: Perform output validation for all remote content.
7
Implementation: Disable scripting languages such as JavaScript in browser
8
Implementation: Session tokens for specific host
9
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.
10
Implementation: Privileges are constrained, if a script is loaded, ensure system runs in chroot jail or other limited authority mode

Real world

MITRE example instances

Example instance 1Detail
http://user:host@example.com:8080/oradb<script>alert('Hi')</script>
Example instance 2Detail
Web applications that accept name value pairs in a HTTP Query string are inherently at risk to any value (or name for that matter) that an adversary would like to enter in the query string. This can be done manually via web browser or trivially scripted to post the query string to multiple sites. In the latter case, in the instance of many sites using similar infrastructure with predictable http queries being accepted and operated on (such as blogging software, Google applications, and so on), a single malicious payload can be scripted to target a wide variety of sites. Web 2.0 type sites like Technorati and del.icio.us rely on user generated content like tags to build http links that are displayed to other users. del.icio.us allows users to identify sites, tag them with metadata and provide URL, descriptions and more data. This data is then echoed back to any other web browser that is interested in the link. If the data is not validated by the del.icio.us site properly then an arbitrary code can be added into the standard http string sent to del.icio.us by the adversary, for example formatted as normal content with a URL and description and tagged as Java, and available to be clicked on (and executed by) any user browsing for Java content that clicks on this trojaned content.