No. A Content Security Policy is a second layer that limits what injected code can do. It is not a substitute for cleaning untrusted input before it reaches the page. MDN states that setting a CSP is not an alternative to sanitizing input.
Myth: "We have a CSP, so reviews and search boxes are safe"
A review field, a search box, a coupon form and a profile name all take text from visitors and put it back on a page. If that text is not handled safely, it can become code. A strict policy may stop the injected script from running, but the underlying flaw is still there.
The MDN guide on CSP says the policy works by controlling which resources load and which scripts run: it can block untrusted hosts, disable inline script tags unless they carry the correct nonce or hash, disable inline event handlers, disable javascript: URLs and disable dangerous APIs such as eval(). It then adds the note that a CSP is not an alternative to sanitizing input.
Fact: the two layers do different jobs
| Layer | What it does | What it cannot do |
|---|---|---|
| Output encoding and sanitizing | Stops untrusted text from becoming markup or script | Cannot stop a compromised vendor script |
| CSP | Limits which scripts run and where data can go | Cannot fix a missing check on input |
| Trusted Types | Forces data through a policy before dangerous browser calls | Does not itself supply a policy |
MDN's Trusted Types page explains that the API does not supply a policy or transformation functions. The developer defines the policy, often using a sanitizer library. A CSP directive can then require it.
Common gaps on store sites
- Product reviews and questions. Rendered back to other shoppers.
- Search results pages. The query is echoed in the page.
- Order notes and gift messages. Shown in admin tools and emails.
- Rich text in product descriptions. Edited by staff and sometimes pasted from other sources.
- Return and redirect parameters. Used to send people back after login.
Steps for both layers
- Encode output by default in your templates, and sanitize rich text with a maintained sanitizer.
- Add a CSP that avoids unsafe-inline. MDN says developers should avoid it because inline JavaScript is one of the most common XSS vectors.
- Roll the policy out in report-only mode and fix what it reports.
- Use dynamic and static testing to look for places where input reaches a page without handling. Our DAST demo shows a scan on a web app, and a code scan can find the unsafe calls in source.
- Retest after each release that touches templates.
What the policy is good at
Used well, a CSP shrinks the blast radius. If an injected script cannot load from an unknown host and cannot post data to an unexpected address, many skimming and data theft attempts fail. That is valuable on a payment page, where a policy is one of several controls alongside script inventory and change detection.
Try it yourself
Click through it at your own pace, or open it full screen. Look at how a finding points to the source line where untrusted input is handled.
Where Safeguard fits
Safeguard scans dependencies, and its policies and gates can stop a pull request that breaks your rules. Pair that with the CSP and output encoding described above. A free scan is at gold.safeguard.sh/scan.
Quick check
Can we skip sanitizing if our CSP is strict?
No. The CSP limits damage from injected code but does not remove the flaw, and some attacks do not rely on scripts at all.
Which should we do first?
Encode and sanitize output first. Add the policy as a second layer in report-only mode.
Sources
Last reviewed: 19 October 2026.
