Published: July 26, 2025
By: BreachFin Security Insights Team
When we talk about client-side threats, most attention is given to JavaScript supply chain attacks, third-party scripts, and DOM manipulation. But there’s another growing blind spot: browser extensions.
More than 70% of internet users have at least one extension installed. From password managers and coupon finders to dark-mode toggles and ad blockers, these tools operate inside the same environment as your critical web pages—including your checkout flows.
And while they’re installed by the user, they can compromise your site.
Why Browser Extensions Are a Security Threat
Extensions are granted broad permissions:
- Access to all page content (
read and change data on all websites) - Ability to intercept keystrokes and clipboard actions
- Script injection into any webpage, including payment and login portals
If a malicious or compromised extension is installed, it can:
- Harvest credit card data directly from your forms
- Bypass script integrity checks since it runs outside your delivery chain
- Deface your DOM or alter links/buttons
- Exfiltrate customer input before your site can encrypt it
These attacks don’t leave a trace in your server logs—they happen entirely in the user’s browser.
PCI DSS and the Problem of Local Compromise
PCI DSS 11.6.1 mandates monitoring of all scripts on payment pages, but browser extensions aren’t scripts you control. Still, their behavior can violate data integrity and privacy in ways that make merchants indirectly liable.
Regulators and auditors increasingly expect proactive controls—not just for what you serve, but for how your page behaves in the wild.
How BreachFin Addresses Extension-Based Risks
While you can’t uninstall extensions from a customer’s device, you can detect abnormal behavior at runtime. BreachFin’s browser-side integrity engine monitors:
- Unexpected DOM mutations originating outside your known script set
- New event listeners, especially on form and payment fields
- API calls initiated from injected code
- CSP violations triggered by unauthorized script sources
These signals often correlate with rogue browser activity, giving you visibility into real-world risk.
What You Can Do Today
- Encourage users to use secure, updated browsers during checkout
- Warn about unsafe extensions on high-risk actions (e.g., via banners or modals)
- Implement Content Security Policies to limit damage from unauthorized injections
- Use client-side monitoring (like BreachFin) to flag suspicious behavior in real time
Final Thought:
The line between browser and endpoint is fading. Attackers have moved into the client, and now so must your defenses. If your visibility ends at the server, you’re not just behind—you’re exposed.
With BreachFin, extend your security perimeter all the way to the user’s screen.
