Client-side security means protecting everything your user’s browser loads—from scripts to stylesheets to pixels. If it runs in the browser, it can be abused. That includes JavaScript fetched in real time from third-party sources. Attackers are shifting client-side, and most companies aren’t ready.
Why secure the client-side?
A lot of focus is placed on securing the supply chain. Developers scan NPM packages. Infrastructure is locked down. Firewalls and WAFs are treated as baseline. And cloud security has become an entire category. But one of the biggest and fastest-growing attack surfaces is still being overlooked: the client side.
Client-side security covers everything that runs inside your user’s browser. That includes third-party JavaScript, pixels, iFrames, forms, SDKs, and any code that gets fetched and executed after the page loads. This part of the stack is almost always ignored during audits, yet it’s responsible for some of the most damaging attacks.
Third-party scripts are loaded directly into the browser and executed with full access to the DOM. These scripts can scan forms, log user behavior, modify page content, and exfiltrate data without ever touching your server. This is where client side protection matters most. Not just monitoring where the scripts come from, but what they actually do.
And it goes wrong. Often. The Polyfill supply chain attack targeted over 490,000 wegsites by injecting malicious code into a previously trusted open-source library. Before that, Kaiser Permanente leaked the data of 13.4 million patients due to embedded client-side scripts sending data to unauthorized destinations.
If you’re not doing active client side scanning, you’re not protecting your users. Because when something does go wrong, the responsibility falls on you, not necessarily on the vendor who served the script.
Client side security is no longer optional. If your site processes payments, collects personal information, or requires users to log in, you’re a target. And the browser is where those attacks happen.
Client-Side Security Threats and Types of Attack
1. Broken Client-Side Access Control
Access control is often thought of as a server-side concern. But client-side access control failures are real, and they’re often overlooked. JavaScript in the browser can be used to manipulate the DOM and access data or functions that were never meant to be exposed. If a script is not properly isolated (or if tokens are left in memory, for example) it becomes easy for attackers to get access without triggering backend checks.
This is one of the more silent types of client side vulnerabilities. And since it happens in the browser, traditional logging misses it.
2. DOM-Based Cross-Site Scripting (XSS)
DOM-based XSS is a common and persistent attack. It happens when JavaScript reads untrusted input (like URL parameters for example) and writes it back into the page without sanitization. This form of injection doesn't rely on server responses, making it hard to catch with traditional web application firewalls.
Client side scanning tools are often the only way to detect this in real time. Without them, attackers can inject arbitrary scripts and fully compromise the user’s browser session.
3. Sensitive Data Leakage
Client-side data leakage is one of the most expensive risks in JavaScript security. It happens when scripts collect sensitive information (like names, emails, or credit card details for example) and transmit them to external domains. Sometimes it’s intentional. Often it isn’t.
When Kaiser Permanente leaked over 13.4 million records, it was because embedded client-side code sent data to third parties without user consent. Client side monitoring would have flagged the exfil attempts before the data ever left the page.
4. Vulnerable and Outdated Components
Outdated JavaScript libraries are one of the most exploited entry points in client side attacks. They’re often pulled from CDNs or third-party providers and never reviewed again. If that library has a known CVE and your site loads it, you're exposed.
This is exactly what made the Polyfill attack so effective. A widely used script was compromised at the source, and most businesses didn’t even know they were loading it. Client side protection needs to include library version tracking to stay ahead of these types of risks.
5. Lack of Third-Party Origin Control
Most client-side attacks don't start from your code. They come from a third-party origin you’ve trusted without verification. These scripts load with full permissions inside your browser environment, giving them access to everything the user sees and does.
When third-party JavaScript is allowed to execute freely, you give up control. Unless you’re using a strong Content Security Policy (CSP) and real-time client side scanning, you have no idea what that script is doing.
Without the real-time client side scanning, a CSP is more often than not inadequate of stopping it.
6. JavaScript Drift
JavaScript drift happens when a script’s content changes over time, but nobody notices. A script that was safe last week may now behave completely differently. Especially if it’s served from a remote source. Some attackers exploit this by introducing malicious behavior gradually to avoid detection.
At cside, we don’t just track the URL. We record and analyze the full payload of every script, every time. It’s how we detect new client side vulnerabilities before they become breaches.
7. Sensitive Data Stored Client-Side
When JavaScript stores data in localStorage, sessionStorare, or cookies, that data can be accessed by any script on the page, including third-party ones. This creates a massive attack surface for session hijacking, token theft, and cross-site leakage.
If you're storing anything sensitive on the client (see examples above), you need strict scoping, expiration logic, and real-time monitoring to catch abuse. Most websites skip this entirely.
8. Client-Side Logging and Monitoring Failures
You can’t secure what you don’t observe. Most organizations still treat logging as a server-side responsibility. But when scripts execute in the browser, and attacks happen in real time, you need client side monitoring too.
This includes visibility into form interactions, script behavior, unexpected outbound requests, and JavaScript errors. Without it, you’re flying blind.
9. Not Using Standard Browser Security Controls
Browser-native defenses exist. But many sites ignore them. Content Security Policy (CSP), Subresource Integrity (SRI), iframe sandboxing, ... these are all ways to reduce risk from rogue scripts and injected content. But most teams either misconfigure them or turn them off entirely to avoid breaking things.
If you want real client side security, start by enforcing the controls the browser already supports.
10. Including Proprietary Logic in the Client
Your frontend is visible to anyone. That includes business logic, pricing algorithms, internal routing rules, and anything else you ship to load in the browser. It’s common to find sensitive processes hardcoded into JavaScript where they can be reverse engineered or abused.
This isn’t just a privacy risk, it’s an IP risk too. If it runs client-side, it’s exposed. If it’s exposed, it should be monitored.
Why client side detections can’t rely on threat feeds alone
Most tools that claim to offer client side protection rely on two things:
- Allow-lists
- Precompiled threat intelligence feeds.
Attackers now design client-side payloads to evade these detection methods. They’ll change the script content dynamically. They’ll target only specific websites. They’ll serve malicious code only once.
If your detection method only watches for known domains or past attack signatures, you won’t see what’s coming. The script may already be compromised, but your tool doesn’t know because nobody reported it yet.
That’s why real-time JavaScript payload inspection is the only reliable approach to client side scanning.
Client side monitoring for compliance
Until recently, client-side security was mostly ignored by regulators. That’s changing.
PCI DSS 4.0.1 now requires businesses to monitor and authorize all client-side scripts by March 2025. If your site processes payments, that means real-time visibility into what runs in the browser. Not just a static list of approved domains.
GDPR and CCPA (and CPRA) also apply. If personal data is being collected or transmitted via third-party scripts without consent or oversight, you're exposed. It doesn’t matter if the script came from a trusted vendor. You’re still responsible.
HIPAA, SOC 2, and DORA are moving in the same direction. Real client side protection is no longer just a security issue. It’s a compliance one. And enforcement is ramping up.
How cside secures the client side
We built cside to provide real-time client side monitoring and JavaScript threat protection without slowing down your site.
- Payload Inspection: We capture and compare full script payloads on every fetch and load request, not just the domain or hash.
- Third-Party Script Management: Monitor, version, and lock third-party scripts from changing silently in production.
- Data Exposure Detection: We catch data being leaked to unexpected endpoints before it becomes a compliance issue.
- Compliance Readiness: Whether you're preparing for PCI DSS 4.0 or managing GDPR risk, we help you stay audit-ready.
FAQ
What is client-side security?
It’s the practice of protecting browser-based scripts and user interactions from being exploited by malicious JavaScript or unauthorized data collection.
Why is real-time client-side scanning important?
Because scripts can change without notice. Payloads served today might not match what was tested in staging.
What is JavaScript drift?
JavaScript drift refers to when a script changes behavior or content without detection. Often used by attackers to slowly introduce malicious logic.
What are the risks of using third-party scripts?
They can access sensitive data, execute DOM manipulations, and leak PII to external domains often without your knowledge.
Is client-side protection required for PCI DSS?
Yes. PCI DSS 4.0 requires businesses to monitor and authorize all client-side scripts by March 2025.
Do you know every domain your scripts are sending data to?
Most websites load third-party scripts that communicate with dozens of external endpoints. Without client-side monitoring in place, you have no idea where user data is actually going. If your scripts are silently exfiltrating information to unknown domains, you're already compromised.
Have you version-locked all third-party JavaScript?
Third-party libraries update in the background without your knowledge. If you’re not locking versions or monitoring for changes, a trusted script can turn malicious overnight. This is how attacks like Polyfill spread so widely. Real client side protection includes tracking and alerting on version drift.
Can you detect a script change within 30 seconds of it being pushed?
Most tools can’t. They rely on crawlers or threat feeds that check once every few hours or even days. That’s not fast enough. Modern attacks execute in real time. Our system scans every payload as it loads, which means you know the moment something changes.
Are you meeting PCI DSS 4.0 client-side script monitoring requirements?
PCI DSS v4.0.1 now requires active monitoring and authorization of all client-side scripts. That includes third-party dependencies. If you're not continuously tracking what runs in the browser, you're not compliant and you could be penalized.
Can you prove where sensitive data is flowing inside the browser?
This is the core of client side security. It’s not enough to secure the backend. You need visibility into how scripts interact with forms, cookies, and session storage. If you can’t prove where data is going, you can’t secure it.