What is Content Security Policy? — UK Software Development Guide

4 min readJune 2025ClickMasters Technical TeamReviewed by James Whitmore, CTO
what is content security policy

Direct Answer

Content Security Policy (CSP) is a browser security mechanism delivered via HTTP response headers that helps prevent Cross-Site Scripting (XSS) and data injection attacks. CSP allows a web server to specify which sources of content (scripts, styles, images, fonts) are permitted to be loaded by the browser, blocking any content from unauthorised sources.

Csp in the UK

CSP is an important UK Cyber Essentials and OWASP alignment control. OWASP Top 10 includes XSS (Cross-Site Scripting) as a critical web vulnerability — CSP is the primary defence. Cyber Essentials Secure Configuration: CSP headers are a secure configuration control for web applications. UK GDPR: XSS attacks can exfiltrate personal data from the browser — CSP reduces this risk. ClickMasters configures CSP headers for all web applications as standard, starting with a strict policy and relaxing only where genuinely required. Common challenge: strict CSP often breaks third-party analytics and marketing scripts — the solution is nonces or hashes rather than blanket unsafe-inline permissions.

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