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Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Repair Windows errors before they cause bigger problemsFix Now →Scan for outdated or missing drivers - takes under a minuteDriver Scan →A content security policy (CSP) is a set of rules that a website sends to a browser to control which resources a page can load or execute and how the page may behave. Browsers commonly receive it in the Content-Security-Policy HTTP response header. CSP can reduce the damage caused by injected content, including some cross-site scripting attacks, but it is a layer of defense—not a replacement for secure coding.
What does content security policy mean?
Content Security Policy, usually shortened to CSP, is a browser-enforced security mechanism. A site defines its policy; the browser applies those rules to the page, such as whether it may load scripts, images, or other resources from particular sources.
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CSP is not simply a list of trusted programs. It is a collection of directives that govern specific resource types or behaviors. The W3C Content Security Policy specification describes CSP as a way for developers to control what a page fetches or executes. MDN’s CSP guide explains it as instructions from a website to a browser that restrict what the site’s code may do.
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A policy consists of directives separated by semicolons. A directive names the kind of resource or behavior it governs, and its value specifies the sources or rules that apply. For example:
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Content-Security-Policy: default-src 'self'; img-src 'self' example.comIn this example, default-src 'self' sets a fallback for fetch directives that do not have their own rule. The img-src directive separately allows images from the site’s own origin and the named host. The source expression 'self' refers to the protected site’s own origin.
Common directives include script-src for scripts, img-src for images, and object-src for plugin content. The default-src directive can serve as a fallback for fetch directives, but a policy can also set specific rules for individual resource types.
How does a website deliver its policy?
The usual method is an HTTP response header named Content-Security-Policy. A page can also declare a policy using a <meta http-equiv="Content-Security-Policy"> element, but that method does not support every CSP feature. For a site-wide policy or when full feature support matters, use the response header as the basis for configuration.
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Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →Outbyte Driver Updater FREEFix the driver behind crashes, sound loss and screen glitchesFind Drivers →A browser can apply multiple policies to a protected resource. Adding another policy can only further restrict what that resource is allowed to do; it does not loosen the restrictions already in place.
What is CSP used for?
- Limit resource sources: Directives can restrict where a page loads scripts, images, and other resources from.
- Reduce some injection risks: Script rules can make it harder for injected content to execute, helping limit the impact of cross-site scripting.
- Restrict embedding: The
frame-ancestorsdirective can limit which sites are allowed to embed a page, helping defend against clickjacking. - Upgrade insecure requests: The
upgrade-insecure-requestsdirective can instruct the browser to upgrade insecure resource requests. - Require trusted types: CSP can require trusted types in supported contexts, helping constrain certain ways that potentially unsafe data reaches DOM injection sinks.
What CSP does not do
The W3C specification states, “CSP is not intended as a first line of defense against content injection vulnerabilities.” CSP is best treated as defense in depth: it can reduce the harm an injection causes, but it does not remove the underlying vulnerability or make unsafe input safe.
Sites still need careful input validation, output encoding, and appropriate sanitization. A policy that blocks some unwanted scripts cannot compensate for weaknesses in how an application handles data.
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What do report-only and enforced policies mean?
An enforced CSP blocks actions that violate the policy. A report-only policy, delivered through Content-Security-Policy-Report-Only, lets a site observe policy violations without applying those restrictions. MDN recommends starting in report-only mode, reviewing what would be blocked, and addressing legitimate site behavior before switching to enforcement.
This gradual approach matters because a restrictive policy can also block scripts, styles, or dependencies that a site actually needs. Strict policies commonly use nonce- or hash-based rules for scripts and/or styles, and existing inline code or third-party dependencies may need adjustment. There is no single policy that fits every site.
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When should a site use CSP?
CSP is useful as one part of a broader web security strategy, especially when a site wants to constrain script execution and resource loading or control which pages can embed it. The W3C CSP Level 3 page identifies a Working Draft version dated September 16, 2026, and links to the latest published version. Because CSP syntax, reporting features, and browser behavior can evolve, check current standards and browser support before deploying a policy.
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