TL;DR: first-affected-session skimmer detection
- A Content Security Policy is a doorman, not a bodyguard. It stops new scripts loading but waves through an approved tag that a vendor pushed a skimmer into overnight.
- cside establishes a behavioral baseline for every payment-page script and alerts when a permitted script starts touching form fields it never touched before, in the same session it changes, VikingCloud-validated for PCI DSS 4.0.1 Req 6.4.3 and 11.6.1.
- If mandatory PCI DSS 4.0.1 script integrity from 2025-03-31 is still an unresolved audit finding, close it here. If your only checkout script is Stripe's iframe, a well-tuned CSP might be enough.
The best Magecart protection software watches every script on your payment page in real time and raises an alert the instant any script, first-party or third-party, reads a payment field or ships data to an endpoint it should not. A Content Security Policy stops unknown scripts from loading, but it cannot catch a behavior change inside a script you already approved, which is why PCI DSS 4.0.1 now requires runtime script integrity monitoring on payment pages.
Magecart attacks compromise the JavaScript supply chain on payment pages. An attacker injects skimming code into a third-party script your site already loads and trusts (an analytics tag, a chat widget, a marketing pixel), and that script starts harvesting payment card data from checkout form fields with no visible change to the page. The average checkout loads dozens of third-party scripts, and each one is a potential injection point.
The PCI Security Standards Council made this risk concrete in PCI DSS 4.0.1. Requirement 6.4.3 mandates that all scripts on payment pages be inventoried and authorized. Requirement 11.6.1 mandates tamper detection, meaning automated alerts when any payment page script changes behavior. Both requirements became mandatory on 2025-03-31.
| Tool | Real-time script monitoring | PCI DSS 4.0.1 validated | Detects behavior changes in approved scripts | CSP management | Free tier |
|---|---|---|---|---|---|
| cside | Yes | Yes (VikingCloud, Req 6.4.3 + 11.6.1) | Yes | Yes | Yes (1,000 API calls/mo) |
| Reflectiz | Yes | Yes | Yes | No | No |
| Content Security Policy alone | No (prevents new scripts, not behavior changes) | No | No | Yes | Free (browser native) |
Why a Content Security Policy is necessary but not sufficient
A Content Security Policy (CSP) is a browser-enforced allowlist that blocks scripts from loading unless their source is explicitly permitted. For Magecart, a CSP prevents an attacker from injecting a new foreign script onto a payment page.
It does not prevent an attacker from compromising a script that is already on the allowlist. If an attacker gains access to the CDN or code repository of a third-party vendor your CSP already permits, they can modify that vendor's script to include skimming code. The CSP sees the script loading from the same permitted source and allows it, so the skimmer runs inside an approved script.
Requirement 11.6.1 exists because the PCI SSC recognized this gap. It asks you to detect when an approved script changes what it does, which requires runtime behavior monitoring. A CSP cannot provide that.
cside: real-time Magecart detection
cside Magecart protection monitors every script loaded on the payment page in real time. It establishes a behavioral baseline for each approved script (which DOM elements it accesses, which network destinations it calls, what data it reads) and alerts as soon as any script deviates from that baseline.
When a compromised third-party script begins accessing payment form fields it has not previously touched, cside detects the change in the same session in which it occurs. The alert identifies the specific script, the specific behavioral change, and the timestamp, so you have evidence you can use in a PCI assessment.
cside also maintains the script inventory required by Requirement 6.4.3: a complete, continuously updated list of every script present on the payment page, with authorization status for each. Because inventory and tamper detection ship in a single validated deployment, one tool satisfies both requirements. cside is validated by VikingCloud for PCI DSS 4.0.1 Requirements 6.4.3 and 11.6.1.
For context, IBM put the global average cost of a data breach at $4.88 million in 2024, and payment page skimming remains a primary driver of web-based card data compromise for merchants. Detecting the Magecart injection in the first affected session is what keeps a single compromised vendor from becoming a mass card-data incident.
Reflectiz
Reflectiz is a website security platform that monitors third-party scripts for behavioral changes and risk. It provides real-time detection of unauthorized script behavior and supports PCI DSS 4.0.1 compliance evidence.
Reflectiz is a credible option for organizations that need dedicated script monitoring. The main differences when comparing it against cside are that Reflectiz does not include a free tier for evaluation, it does not combine script monitoring with device fingerprinting and fraud detection in a single integration, and its pricing is enterprise-oriented without self-serve options.
For teams that need only Magecart protection, Reflectiz is functional. For teams that also want AI agent detection, device fingerprinting, and chargeback evidence from a single script tag, cside covers those use cases in one deployment.
Content Security Policy alone
A CSP is a free, browser-native control and should be deployed on every payment page as a baseline. It is not a replacement for runtime monitoring software.
A well-configured CSP with nonces or hashes on inline scripts prevents most opportunistic Magecart injection. It does not monitor the runtime behavior of approved scripts, it does not alert when an approved script changes what it does, and it does not produce the audit trail required by PCI DSS 4.0.1 Requirement 11.6.1. Treat a CSP as the starting point, not the whole solution.
