Securing the Browser

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Securing the Browser: Eliminating Blind Spots in Extension Security 

By: Almas Raza, Principal Product Manager  | Tom Goldberg, Sr. Product Manager 

 

Why Extensions Are a Growing Security Gap

Browser extensions are widely adopted because they make the web more useful. They help users manage passwords, summarize content, automate workflows, and interact with AI tools directly inside the browser. Industry research shows that 99% of enterprise users have browser extensions installed, and more than half run over 10 extensions, creating a large, trusted attack surface that is difficult for security teams to monitor fully. 

At a technical level, extensions operate with elevated privileges inside the browser. Depending on the permissions granted, they can access cookies, credentials, session activity, page content, and other sensitive data, and can also read or modify web pages and interact directly with web applications.

Once installed, extensions often run persistently and update automatically, creating a long-lived trust relationship that users rarely review. Attackers exploit this trust to steal credentials, session tokens, cookies, and sensitive data, including LLM chat history. They can also redirect users, monitor activity, communicate with command-and-control servers, download malware, or inject malicious scripts into trusted pages. 

AI adoption is accelerating this risk by making extension-based productivity tools more common and trusted. Malicious extensions now impersonate writing assistants, search copilots, summarizers, and productivity agents, requesting broad permissions that appear legitimate.

In many cases, the victim never sees a warning. The extension continues to work while quietly feeding data to attacker-controlled infrastructure.

 

How Attackers Turn Trusted Extensions Into Threats 

Malicious extension attacks often begin through channels that users and organizations already trust. Attackers do not always need to exploit the browser directly. Instead, they exploit the extension ecosystem and the trust users place in it.

• Marketplace-delivered malicious extensions: Attackers publish extensions through official marketplaces and drive installs via phishing, fake AI websites, search ads, or social-engineering pages.
• Weaponized version updates: Extensions may start benign, gain user trust, and later push malicious updates. This turns the browser’s auto-update model into a trusted delivery path for malicious code. 
• Sideloaded and off-store extensions
  Attackers can bypass marketplace vetting by pushing users to install extensions outside official stores. Industry research found that 26% of enterprise extensions are sideloaded, reducing visibility and control.  
• Trusted extensions turned malicious: Legitimate extensions can be compromised through developer account takeover or ownership changes, allowing attackers to weaponize tools users already trust. The Cyberhaven incident is a well-known example in which attackers pushed a malicious update to a trusted extension, affecting millions of users.

The key point is that extension attacks do not rely on a single path. They exploit the full ecosystem, from discovery and installation to updates, runtime execution, and built-in persistence.  

 

A Two-Pronged Approach to Extension Security 

Traditional controls were not designed for this threat model. URL filtering can block known malicious destinations and help prevent attacks that begin on websites promoting or distributing malicious extensions. But a different challenge emerges when extensions are downloaded directly from trusted marketplaces, appear benign at first, gain user trust, and later become malicious through updates or runtime behavior. In these cases, the activity is executed inside the browser after installation, where URL-based controls may lack the runtime context needed to understand extension behavior. Endpoint tools often lack visibility into browser-native activity, leaving gaps across the extension lifecycle. Closing this gap requires a two-pronged approach. 

Palo Alto Networks Cloud-Delivered Security Services (CDSS) combines Advanced URL Filtering (AURL) with Advanced Extension Security in Prisma Browser to protect users before installation and after an extension begins operating in the browser.

 

1. Blocking Malicious Extension Distribution with Advanced URL Filtering

Advanced URL Filtering helps stop attacks earlier by identifying and blocking websites that distribute or promote malicious extensions. This includes fake AI websites, phishing pages, search scams, and social engineering pages that trick users into installing malicious extensions.

By blocking distribution before installation, AURL reduces the likelihood that a risky extension ever reaches the browser.

2. Detecting Malicious Extensions with Advanced Extension Security

Advanced Extension Security provides the second layer of protection by analyzing extensions during installation, update, and runtime, especially when an extension was not previously known to be malicious or becomes risky after an update. 

Delivered through Prisma Browser, Advanced Extension Security analyzes extension code, permissions, publisher reputation, runtime behavior, network activity, script injection, data exfiltration, and remote code execution to detect both known and unknown threats before they impact users.

This enables real-time enforcement, including blocking malicious installations, preventing risky updates, monitoring runtime behavior, and removing malicious extensions when needed.

Together, AURL and Advanced Extension Security provide end-to-end coverage across the extension lifecycle. 

Turning Detection into Global Protection

One of the most powerful aspects of this approach is the continuous feedback loop between runtime detection and cloud-delivered threat intelligence. When Advanced Extension Security identifies malicious behavior, key artifacts such as command-and-control domains, malicious URLs, supporting infrastructure, and indicators tied to data exfiltration or script injection can be extracted and fed back into the Advanced URL Filtering solution.

This turns individual extension detections into broader protection, helping block related distribution sites, malicious infrastructure, and communication paths across the customer base.

Securing the Future of the Browser

As the browser becomes the primary interface for work, it will remain a prime target for attackers. Browser extensions, once considered low risk, are now part of the modern attack surface.

Securing this layer requires more than isolated controls. It demands a connected approach that spans access, execution, and intelligence. By combining browser-level protection with AURL, organizations gain visibility throughout the extension lifecycle, reduce risk, and block the distribution of malicious extensions before they impact users.

With Advanced Extension Security within Prisma browser and enhanced Advanced URL filtering detection, Palo Alto Networks is helping close this critical gap. Security teams can move from reactive response to coordinated prevention, detection, and response.

To see Advanced Extension Security in action, schedule a Prisma Browser demo with your Palo Alto Networks team.