CSP Violations in Enforce Mode

Quick Problem Overview

I noticed it first in our Application Performance Monitoring (APM) tool — a steady stream of Content Security Policy violations coming from a third-party CDN. Not a test environment. Not a staging misconfiguration. Production traffic, real users, and a CSP running in full enforcement mode.

The question that immediately came up was a practical one: will fixing these CSP errors actually improve page performance? It sounds simple, but the answer depends entirely on which mode your CSP is running in — and the implications are different enough that it changes the entire remediation strategy. After digging into the violation types, the enforcement model, and our compliance constraints, I landed on a clear recommendation: don't just patch the header. Migrate to an internal Nexus Sonatype repository and eliminate the CDN dependency entirely.

Here's how I reasoned through it with my team.

What is a Content Security Policy?

A Content Security Policy is an HTTP response header that tells the browser which external origins are trusted to load scripts, styles, images, fonts, and network connections. It's one of the most effective defenses against cross-site scripting (XSS) and data injection attacks because it removes the browser's implicit trust in inline code and third-party origins.

There are two operating modes:

• Report-Only mode (Content-Security-Policy-Report-Only): The policy is evaluated but not enforced. Resources load normally; violations are logged and optionally sent to a reporting endpoint. Fixing violations in this mode improves your security posture but has essentially zero performance impact — the resources were loading anyway.

• Enforce mode (Content-Security-Policy): The policy is enforced. Violations aren't just reported — the browser blocks the resource. This is the mode we were in.

That distinction matters enormously when you're trying to answer a question about performance. In enforce mode, a CSP violation has real costs associated with it since its a blocked request, it isn't a diagnostic signal you can defer later on.

The Violations

The two directives generating violations were script-src-elem and connect-src. Each covers a different category of browser behavior, and each has a different performance footprint when blocked.

Table summary

The DynaTrace Layer Compounds the Problem

On top of the browser-level costs, every policy violation triggers a report notification — usually an asynchronous POST request back to the reporting endpoint. DynaTrace is hooking into these events, processing telemetry, and adding a modest but real runtime overhead to each one of these violations.

Individually, these are small costs. At scale — many concurrent users, each generating multiple violation events per page load — the noise compounds. It shows up in your metrics, inflates your error counts, and makes it harder to distinguish real signals from compliance artifact.

Two Options, One Clear Winner

Once it was clear that enforce-mode CSP violations carry genuine performance and reliability costs, the remediation options came down to two choices.

Option 1: Update the CSP header to whitelist the CDN

This is the fastest path. Add the CDN origin to script-src-elem and connect-src, and the violations stop immediately. But you've just permanently opened your policy to a third-party domains that you don't control. That means:

• In a FISMA and NIST 800-53 compliance environment like VA.gov, justifying a third-party script execution origin to an ATO reviewer is a real conversation — and not a comfortable one
• CDN availability and latency variability are now baked into your application's reliability profile
• You're trusting an external host to serve scripts that execute in your users' browsers
• Any compromise of that CDN becomes your compromise

Why isn't this the ideal solution? Given the attack vector of bad actors compromising supply-chains (like the most recent Shai-Hulud worm) it is increasingly more ideal for engineers to secure code and minimize attack vector surfaces.

Option 2: Migrate to an internal Nexus Sonatype repository

Nexus Sonatype is an artifact repository manager used across government and enterprise environments to host, proxy, and control internal software dependencies. By migrating the offending CDN-hosted scripts and fetch endpoints to an internal Nexus instance:

• All requests become same-origin or trusted-internal-origin — neither script-src-elem nor connect-src need to be relaxed at all
• Third-party CDN availability is no longer in your application's critical path
• You have full control over versioning, dependency pinning, and update cadence
• Your security headers remain restrictive, which is exactly the posture FISMA and NIST require
• DynaTrace noise drops to zero for these violation types and you save some money

This is the holistically correct solution. It fixes the performance problem, eliminates the security trade-off, and improves reliability — all without loosening a single security control. On top of that, your DynaTrace bill might go down as the log ingestion goes down.

The Mental Model Worth Keeping

There's a broader principle here that applies well beyond this specific scenario:

"Patching a security header to accommodate an insecure dependency is managing a symptom, not solving a problem."

When a CDN origin shows up in your CSP violations, the right question isn't "How do I make the browser accept this CDN?" It's "Why does my application depend on an external CDN at all, and is that dependency justified?" In most compliance-heavy environments, the answer to the second question will lead you toward internal artifact management rather than header whitelisting.

CSP violations in enforce mode are expensive — in performance, in reliability, and in the observability noise they generate. But they're also diagnostic. They're telling you something about your dependency graph that you should already know.

Takeaways

If you're working in a government or enterprise context and have been sitting on a pile of CSP violations because "it's just reporting," check your Content-Security-Policy headers. If report-only mode isn't set, your users are already paying the cost.