Foiling a Magecart-Style Web Skimmer in a Local Retail Chain

A Case Study in Browser Threat Detection, Digital Forensics, and Client-Side Security

Introduction: The Hidden Cost of Convenience

JavaScript has revolutionized the online shopping experience. From real-time inventory updates to seamless checkout flows, it powers nearly every modern retail website. But that same flexibility comes with an often-overlooked risk: the browser is a battleground, and JavaScript is a frontline vulnerability.

In this article, we take you inside a real-world incident response engagement conducted under a strict non-disclosure agreement where a regional retail chain’s website was silently compromised by a Magecart-style credit card skimmer.

This case didn’t make headlines. The business isn’t a household name. And yet the implications were just as severe: over 1,000 customers unknowingly had their payment data stolen, and the compromise lasted nearly three weeks before it was detected.

What follows is a step-by-step breakdown of how our team at Bylinear identified, investigated, and helped remediate this threat—and what lessons any eCommerce operator can apply to prevent becoming the next victim.

Background: A Growing Retail Brand Goes Digital

The company in question referred to here as “RetailCo” operates over 40 physical stores across the Midwest. In early 2025, RetailCo launched a localized eCommerce initiative, building individual store-level web portals under a shared architecture. The sites were mobile-friendly, integrated with inventory and point-of-sale systems, and critically used a modern JavaScript-heavy frontend to deliver a “frictionless checkout” experience.

This initiative was widely seen as a success internally. Online sales rose 38% within two months. The marketing team was already preparing to roll out targeted promotions and loyalty integrations. Then, without warning, complaints started trickling in.

The First Clue: A Pattern in the Chargebacks

At first, it seemed like a coincidence. A few customers reported suspicious activity on their credit cards shortly after shopping online. RetailCo’s finance team flagged a minor increase in card-not-present chargebacks, but nothing outside seasonal norms.

But within ten days, the fraud team noticed something strange: nearly every affected card had been used on RetailCo’s eCommerce checkout page and almost all of them within a narrow 14-day window. Customers weren’t reporting unauthorized use of their RetailCo accounts, just the payment card numbers.

That raised a red flag.

RetailCo’s head of IT escalated the concern. They brought in our team for a full-scope digital forensics and browser-side security analysis.

Phase One: Incident Containment and Environment Freeze

Upon engagement, our first priority was containment. We advised the client to:

• Restrict write access to production web servers
• Suspend CI/CD pipelines until source and deployment integrity could be verified
• Snapshot server images, including memory dumps for process analysis
• Preserve access logs from web servers, load balancers, CDN endpoints, and WAF appliances

The client’s infrastructure used a mix of cloud-hosted services and traditional hosting. We worked alongside internal developers to capture a forensic image of the frontend and checkout flows, as they appeared to real users.

Nothing seemed wrong until we looked at what the browser was doing.

Discovery: A JavaScript Skimmer Hidden in Plain Sight

The checkout page included over a dozen third-party scripts, ranging from payment gateway integrations to analytics and support chat tools. While the core source code appeared untouched in version control, the production-rendered HTML included a suspicious script reference that wasn’t present in any dev or staging environment:

<script src=”https://cdn-tagmanager-analytics[.]com/lib.js”></script>

This domain looked like a typo of a known analytics vendor, but on inspection, it was newly registered and unrelated. The hosted file was heavily obfuscated, packed with hexadecimal and Base64 encoding, and minimized to hide its purpose.

Using a browser sandbox and behavioral monitoring tools, we decoded the script’s behavior:

• Hooked into checkout form fields, listening for input on card number, CVV, expiration, ZIP, and name
• Cloned the form data and stored it in a local array
• On form submission or shortly afterward, it transmitted the data to a remote exfiltration server
• Transmission was done using navigator.sendBeacon() or XMLHttpRequest, depending on browser support

To the end user, the experience was seamless. The checkout completed normally. But behind the scenes, a shadow transaction occurred quietly, capturing and transmitting sensitive payment data.

Tracking the Infection Path: From Plugin to Payload

We knew what the skimmer did. The next question was how it got there.

By cross-referencing deployment logs, Git history, and CDN diffs, we traced the injection to a third-party live chat plugin that had been manually updated by a developer three weeks earlier.

Instead of pulling the update from the official vendor CDN, the developer had copied a script from a GitHub mirror. That version, while functional, had been modified by attackers who inserted the rogue <script> tag linking to the skimmer.

This change bypassed the normal CI/CD process and was never code-reviewed. Worse, RetailCo didn’t have a Content Security Policy (CSP) or Subresource Integrity (SRI) in place, which meant the browser blindly executed whatever scripts it was told to load, even from typo-squatted domains.

This allowed the skimmer to run undetected for 18 days.

Browser Behavior Monitoring: What Helped Us Catch It

A turning point in the investigation was the use of browser instrumentation tools that allowed us to monitor how actual browsers executed the site.

By running the site in controlled environments with network traffic sniffing, DevTools logging, and JavaScript behavior analysis, we were able to:

• Capture obfuscated function calls that only executed after user input
• Detect nonstandard domain lookups initiated from the checkout page
• Reconstruct payloads exfiltrated via HTTPS, including decoded Base64 strings with card data formats

We also reviewed recordings from the site’s user experience monitoring platform (RUM). These sessions didn’t show the script source code, but they did confirm the timing and frequency of outbound network calls that should not have existed.

These real-time clues, combined with forensic network log analysis, allowed us to isolate the attack vector quickly.

Forensic Summary

Remediation Actions

Our team worked closely with RetailCo to remove the infection and prevent recurrence. Key steps included:

1. Script Purge and Plugin Audit
   
   • Removed the compromised chat plugin
   • Audited every external script source for authenticity
   • Established allowlists for trusted CDN domains
     
2. Content Security Policy (CSP) Deployment
   
   • Implemented a strict CSP to prevent unauthorized scripts from running
   • Enabled CSP violation reporting via a dedicated endpoint
     
3. Subresource Integrity (SRI) for Scripts
   
   • Added integrity hashes to all third-party scripts
   • Blocked execution of resources with hash mismatches
     
4. JavaScript Monitoring and Logging
   
   • Deployed browser-side anomaly detection tools
   • Integrated real-time JS error and XHR monitoring into security alerts
     
5. Development Policy Reforms
   
   • Prohibited plugin sourcing from non-vendor repositories
   • Enforced code review and approval for frontend changes
   • Integrated automated scanners into the CI/CD pipeline

Lessons Learned: What Every Retailer Should Know

This attack wasn’t the result of sophisticated zero-day exploits. It happened because a common security blind spot, trust in third-party JavaScript, wasn’t addressed. And it’s a mistake any growing business can make.

Here are our key takeaways:

1. Your browser is part of your security perimeter.

Client-side threats are real. If you don’t monitor what runs in your customer’s browser, you’re flying blind.

2. Treat third-party code like untrusted user input.

Every script you include could be a Trojan horse. Review it. Limit its access. Control where it loads from.

3. Use the tools already available to you.

CSP, SRI, browser telemetry, and even DevTools are powerful defenses—when properly configured and monitored.

4. Preventing an attack costs less than responding to one.

The breach cost RetailCo over $150,000 in fraud reimbursements, legal consultations, and emergency response. A CSP policy and script audit could have prevented it entirely.

Final Thoughts: Securing the Modern Checkout Experience

In the age of single-page apps, third-party APIs, and rapid frontend development, attackers are no longer breaking in through your backend. They’re walking in through the front door, right into your customer’s browser.

Magecart-style skimmers aren’t going away. But with the right awareness, monitoring, and development practices, you can make your site a much harder target.

For RetailCo, this was a painful but transformative event. They now maintain one of the most secure client-side environments we’ve seen in their market segment, and they’ve become evangelists for browser security awareness within their region.

Let their story be your reminder: JavaScript is powerful, but it’s not innocent.