COMMON ATTACK PATTERN ENUMERATION AND CLASSIFICATIONLive MITRE record
CAPEC-86
XSS Through HTTP Headers
An adversary exploits web applications that generate web content, such as links in a HTML page, based on unvalidated or improperly validated data submitted by other actors. XSS in HTTP Headers attacks target the HTTP headers which are hidden from most users and may not be validated by web applications.
Abstraction
Detailed
Typical severity
Very High
Likelihood of attack
High
Understand the attack
What is this attack pattern?
An adversary exploits web applications that generate web content, such as links in a HTML page, based on unvalidated or improperly validated data submitted by other actors. XSS in HTTP Headers attacks target the HTTP headers which are hidden from most users and may not be validated by web applications.
Execution flow
How does the attack proceed, step by step?
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 entry points (input) that becomes part of generated HTTP header (not only GET/POST/COOKIE, but also Content-Type, etc.)
Techniques used
- 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 used in the HTTP headers.
- Look for HTML meta tags that could be injectable
- Use a proxy tool to record all links visited during a manual traversal of the web application.
- 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.
Experiment
Probe identified potential entry points for XSS vulnerability
Techniques used
- Manually inject various script payloads into each identified entry point using a list of common script injection probes and observe system behavior to determine if script was executed.
- Use an automated injection attack tool to inject various script payloads into each identified entry point using a list of common script injection probes and observe system behavior to determine if script was executed.
- 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 which is used in an HTTP header to include a malicious script tag. Because it is in the header it may bypass validation.
- Send information gathered from the malicious script to a remote endpoint.
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 software must be a client that allows scripting communication from remote hosts.
Skills required
- Low: To achieve a redirection and use of less trusted source, an adversary can simply edit HTTP Headers that are sent to client machine.
- High: Exploiting a client side vulnerability to inject malicious scripts into the browser's executable process.
Resources required
- The adversary must have the ability to deploy a custom hostile service for access by targeted clients and the abbility to communicate synchronously or asynchronously with client machine. The adversary must also control a remote site of some sort to redirect client and data to.
Impact
What does a successful attack lead to?
Execute Unauthorized Commands
Affected scopes: Confidentiality, Integrity, Availability
Read Data
Affected scopes: Confidentiality
Gain Privileges
Affected scopes: Confidentiality, Access Control, Authorization
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.
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.
Real world
MITRE example instances
Example instance 1Detail
Utilize a remote style sheet set in the HTTP header for XSS attack. When the adversary is able to point to a remote stylesheet, any of the variables set in that stylesheet are controllable on the client side by the remote adversary. Like most XSS attacks, results vary depending on browser that is used [ REF-97 ].
<META HTTP-EQUIV="Link" Content="<http://ha.ckers.org/xss.css>; REL=stylesheet">
Example instance 2Detail
Google's 404 redirection script was found vulnerable to this attack vector.
Google's 404 file not found page read
* Response headers: "Content-Type: text/html; charset=[encoding]".
* Response body: <META http-equiv="Content-Type" (...) charset=[encoding]/>
If the response sends an unexpected encoding type such as UTF-7, then no enforcement is done on the payload and arbitrary XSS code will be transported along with the standard HTTP response. [ REF-476 ]
Example instance 3Detail
XSS can be used in variety of ways, because it is scripted and executes in a distributed, asynchronous fashion it can create its own vector and openings. For example, the adversary can use XSS to mount a DDoS attack by having series of different computers unknowingly executing requests against a single host.