Common Attack Pattern Enumeration and Classification
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This attack utilizes the frequent client-server roundtrips in Ajax conversation to scan a system. While Ajax does not open up new vulnerabilities per se, it does optimize them from an attacker point of view. In many XSS attacks the attacker must get a "hole in one" and successfully exploit the vulnerability on the victim side the first time, once the client is redirected the attacker has many chances to engage in follow on probes, but their is only one first chance. In a widely used web application this is not a major problem because 1 in a 1,000 is good enough in a widely used application.
A common first step for an attacker is to footprint the environment to understand what attacks will work. Since footprinting relies on enumeration, the conversational pattern of rapid, multiple requests and responses that are typical in Ajax applications enable an attacker to look for many vulnerabilities, well known ports, network locations and so on.
Skill or Knowledge Level: Medium
Design: Use browser technologies that do not allow client side scripting.
Design: Utilize strict type, character, and encoding enforcement
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: 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.
Payload delivered through standard communication protocols, such as Ajax application.
Shreeraj Shah, "Ajax footprinting for Web 2.0 applications", http://www.net-security.org/dl/articles/Ajax_fingerprinting.pdf