Content-Security-Policy enforces the policy and blocks browser actions that violate it. Content-Security-Policy-Report-Only does not block anything, but it records violations so teams can test and tune a policy before enforcement. In practice, report-only is the safer starting point when you want visibility into breakage before you turn on blocking.
What the two directives do differently in Spring Security
Spring Security lets you send either directive as a response header, but they serve different operational purposes. OWASP Cheat Sheet Series is useful here because the practical question is not only what the header means, but how teams stage a policy safely. The enforcement header tells the browser to block disallowed sources, while the report-only header tells the browser to observe and report violations without changing page behaviour.
The difference matters because browser enforcement can break scripts, styles, images, frames, or API calls that your application still relies on. Report-only gives you a safer discovery phase: you can see which page paths, third-party dependencies, or inline behaviours would fail before you force users onto the stricter policy.
Spring’s role is mainly delivery and configuration. The application does not interpret the policy in a special way, it writes the header and relies on the browser to act on it. That means the key decision is not “which one is stronger” in the abstract, but whether you are still learning the application’s real resource dependencies or have already validated that the policy is ready to block.
- Use enforcement when the policy has been tested against real traffic and the remaining violations are understood and acceptable.
- Use report-only when you expect breakage, have third-party assets to map, or need evidence before changing user-facing behaviour.
- Remember that report-only can expose gaps in your intended policy, but it does not protect users by itself.
Why report-only is usually the first safe step
Report-only is the better starting point when a policy is new, especially in applications with legacy templates, inline JavaScript, or multiple frontend build paths. It lets you observe violations without taking the site down for users. That is why security teams commonly use it as a tuning phase before switching to blocking.
It is also the right choice when you are coordinating changes across development, test, and production. A policy that looks fine in a controlled environment can still fail in production because of CDN differences, embedded widgets, analytics tags, or browser quirks. Running in report-only helps you separate policy design errors from deployment drift.
For teams that manage lots of frontend dependencies, the useful evidence is not the existence of a violation itself, but the pattern of what is violating and where. Repeated reports from the same template or route often point to a stable allowlist problem, while one-off reports may indicate a transient integration issue or a user-specific condition that should be verified before broadening the policy.
How to choose the mode during rollout
A sensible rollout rule is to start with report-only, baseline the violations, then move to enforcement once the remaining exceptions are intentional and documented. If you cannot explain a violation clearly, you usually do not yet have enough confidence to block it. If the same violation keeps appearing after a policy change, treat that as a sign that the application still depends on a resource path the policy does not allow.
NHI Mgmt Group’s Ultimate Guide to NHIs is relevant at the implementation level because modern web apps often depend on tokens, API keys, and service-backed integrations behind the scenes. If those components are embedded into frontend workflows or misaligned with your content policy, you can create failures that only appear once enforcement is turned on.
What to verify: Check that your application has no remaining inline execution dependencies that were only “working” because the browser was previously permissive. Verify the report stream is actionable, meaning you can trace each violation to a page, source, and change request.
Practitioner takeaway: Use report-only to prove the policy, not to postpone the decision. The mature state is a policy you can enforce without surprise breakage because the violations have already been explained, triaged, and either removed or explicitly accepted.
Standards & Framework Alignment
This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.
OWASP Agentic AI Top 10 address the attack and risk surface, while CIS Controls v8 and NIST CSF 2.0 set the governance and control requirements practitioners need to meet.
| Framework | Control / Reference | Relevance |
|---|---|---|
| CIS Controls v8 | CIS 16 — Application Software Security | CSP is a browser-facing application security control for preventing script abuse. |
| CIS 8 — Audit Log Management | Report-only depends on collecting and reviewing violation reports to tune policy. | |
| Recommendation — Apply secure coding and testing practices to validate CSP before enforcing it. Collect and review CSP violation reports to identify broken or risky content paths. | ||
| OWASP Agentic AI Top 10 | A1 — Prompt Injection | Browser content controls help reduce injected script pathways that can alter page behaviour. |
| A4 — Tool Misuse | Misconfigured content rules can permit unintended execution or resource use in the browser. | |
| Recommendation — Restrict executable content paths so injected browser-side code cannot run freely. Validate allowed execution and resource sources before moving from observation to blocking. | ||
| NIST CSF 2.0 | PR.DS-2 — Data-in-Transit is Protected | CSP helps constrain what browser-delivered code and resources can be loaded and executed. |
| Recommendation — Harden browser-delivered content paths so only approved sources can influence page execution. | ||
Related resources from NHI Mgmt Group
- What is the difference between Content Security Policy report only mode and enforcement mode?
- What is the difference between Content Security Policy and Subresource Integrity in JavaScript security?
- What is the difference between X-Frame-Options and Content Security Policy frame-ancestors?
- What is the difference between using a nonce and using a hash for inline Content Security Policy in Django?
Deepen Your Knowledge
Reviewed and updated by the NHIMG editorial team on September 17, 2026.
NHI Mgmt Group — the #1 independent authority on Non-Human Identity, IAM, and Agentic AI security. nhimg.org
