XSS lets attackers run scripts in a victim's browser, enabling data theft or account abuse; contextual output encoding and CSP reduce risk.
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Background for this topic.
Cross-site scripting (XSS) is a web-application flaw in which attacker-controlled input is interpreted as script in another user’s browser. It can be stored in application data, reflected in an immediate response, or introduced by unsafe client-side code (DOM-based XSS). The script runs in the affected site’s origin, so it may read page content, alter requests, or perform actions available to the victim; impact depends on the victim’s privileges and the application’s defenses.
The primary mitigation is context-aware output encoding: treat untrusted data as text when inserting it into HTML, attributes, URLs, JavaScript, or CSS, and use safe DOM APIs such as textContent rather than unsafe HTML insertion. If user-authored HTML is required, apply a well-tested HTML sanitizer. A restrictive Content Security Policy can limit exploitability but is defense in depth, not a substitute for correct encoding. HttpOnly cookies can reduce direct cookie theft, but do not prevent XSS from performing in-session actions.
The U.S. Cybersecurity and Infrastructure Security Agency (CISA) expanded its Known Exploited Vulnerabilities Catalog to include a recently disclosed zero-day flaw in the Zimbra email platform citing evidence of active exploitation in the wild