Crosssite

Crosssite is a term used in web security to describe phenomena that occur when data, behavior, or requests cross from one site or domain to another. It is used to describe issues arising from interactions across sites, including how browsers enforce security boundaries like the same-origin policy and how attackers can exploit trust relationships between sites. The term is most often encountered in discussions of vulnerabilities such as cross-site scripting and cross-site request forgery.

Cross-site scripting (XSS) is a class of vulnerabilities that allow an attacker to inject and execute malicious scripts in the context of a trusted website. The exploit can run in the victim’s browser and access data such as cookies or local storage. XSS is commonly categorized as stored, reflected, or DOM-based, depending on where the payload resides and how it is triggered.

Cross-site request forgery (CSRF) is a different class, where an attacker entices a user to perform an action on a web application in which they are authenticated. The site treats the request as legitimate because the user’s browser presents valid credentials. Defenses include anti-CSRF tokens, SameSite cookies, and requiring explicit user actions for state-changing operations.

Mitigation of crosssite risks involves defense in depth: input validation and output encoding, the use of a content security policy (CSP) to restrict script execution, proper handling of authentication, session management, and secure cookie flags. Web frameworks often provide built-in protections and developers should follow security best practices.

See also: cross-site scripting, content security policy, same-origin policy, cross-site request forgery.