Neutralizing Cross-Site Scripting (XSS) with Strict Content Security Policy (CSP)
Why output encoding alone is insufficient and how to construct nonce-based Content Security Policies that block arbitrary JavaScript execution.
The Ongoing Persistence of XSS
Cross-Site Scripting allows attackers to inject malicious JavaScript into victim browsers, stealing session tokens, capturing keystrokes, and executing unauthorized actions on behalf of the user.
The Failure of Legacy CSP Directives
Old-school Content Security Policies that whitelist domain names (like `script-src https:`) are trivially bypassed through JSONP endpoints and open redirectors hosted on whitelisted CDNs.
Implementing Nonce-Based Strict CSP
A modern strict CSP generates a cryptographically random cryptographic nonce per HTTP response (`script-src 'nonce-{RANDOM}' 'strict-dynamic'`). Any script tag without the matching server nonce is immediately blocked by the browser.
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