The hidden cybersecurity risk lurking in your browser extensions

How everyday add-ons can compromise your security without you knowing

Takeaways

• Browser extensions run with deep access inside the browser, making them an attractive target for cybercriminals.
• Many recent attacks involve supply-chain compromises, where trusted extensions turn malicious after months or years of normal use.
• Malicious extensions have been used for spying, data theft, browser hijacking, fraud, and corporate espionage, often at massive scale.
• Even extensions from official stores with good reviews and “featured” badges have been abused.
• Reducing extension sprawl, auditing permissions and treating extensions as software assets are critical to limiting risk.

Browser extensions are meant to make the web more useful. From ad blockers and password managers to AI sidebars and productivity tools, extensions promise convenience with just a click. But that convenience comes at the cost of cyber vulnerabilities. And attackers are increasingly exploiting those vulnerabilities.

Recent investigations show that malicious browser extensions are no longer edge‑case threats or low‑level nuisances. They are now a scalable, stealthy attack vector capable of spying on millions of users, stealing sensitive data and quietly undermining organizational security.

How browser extensions work — and why they’re vulnerable

At a technical level, browser extensions operate with elevated privileges. Depending on what a user approves during installation, an extension may be able to read and modify web pages, track activity across tabs, access session data, or interact directly with web-based applications.

That access is what makes extensions powerful. But it’s also what makes them potentially dangerous. Once installed, extensions typically run persistently in the background and update automatically. Users rarely revisit permission settings or scrutinize updates, creating a long‑lived trust relationship that attackers can abuse.

Unlike traditional malware, malicious extensions don’t need to exploit software flaws. They operate entirely within the rules of the browser, using permissions the user already granted.

What extension‑based attacks can enable

Malicious browser extensions can steal sensitive data, harvest credentials, track user behavior, and inject or manipulate content directly within the browser, effectively turning the browser into an access point for broader attacks.

Because extensions sit inside the browser — where users authenticate, access SaaS applications and handle sensitive workflows — attackers can use them for surveillance, session hijacking, fraud, and corporate espionage without deploying traditional malware.

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

Real‑world attacks show the scale of the problem

Recent reporting underscores just how widespread these threats have become.

In mid‑2025, Malwarebytes documented a campaign involving malicious extensions in the official Chrome and Edge stores that spied on millions of users. These extensions offered legitimate functionality, accumulated positive reviews and even received verification or featured placement. Only later did researchers discover that malicious code had been introduced through updates, turning trusted tools into surveillance malware.

More recently, researchers uncovered Chrome extensions posing as AI productivity tools that secretly harvested conversations from platforms like ChatGPT and DeepSeek, along with browsing activity. For consumers, that may mean privacy loss. For organizations, it can expose proprietary code, confidential research or sensitive business discussions.

Long‑running campaigns such as DarkSpectre push this model even further. In some cases, extensions remained benign for five years or more before being weaponized, allowing attackers to build massive install bases before flipping the switch.

A browser‑level supply‑chain attack

One consideration that makes these incidents especially troubling is their supply‑chain aspect.

Many of the extensions involved were not malicious at the outset. They became dangerous only after an update, often following a change in ownership or developer control. From the user’s perspective, nothing changed. Updates installed silently, just as they always had.

“When an extension has been available in the web store for a while, cybercriminals can insert malicious code through updates to the extension. Some researchers refer to the clean extensions as ‘sleeper agents.’ These sleeper agents are the bases for future malicious activity.”
— Malwarebytes [socradar.io]

This approach mirrors software supply‑chain attacks seen elsewhere, but with far less scrutiny and governance.

Trust signals such as download counts, ratings and longevity are no longer reliable indicators of safety.

How to protect yourself from malicious extensions

Eliminating extensions entirely isn’t realistic, but there are practical steps individuals and organizations can take to reduce risk:

• Install fewer extensions. Every extension expands the attack surface. Remove anything you don’t actively use.
• Review extensions regularly. Pay special attention to long‑standing extensions and recent updates or ownership changes.
• Scrutinize permissions. Be wary of tools that request broad access without a clear, compelling reason.
• Separate work and personal browsing. Limiting extensions on work browsers can significantly reduce organizational exposure.
• Treat extensions like software. For businesses, that means inventory, governance and ongoing review, not blind trust.

In addition, you can improve your capacity to detect and respond to incidents of all kinds — without additional in-house IT workload — with an AI-enhanced XDR solution like Barracuda Managed XDR.

Trust is earned — and it can be revoked

Browser extensions sit at the center of modern digital work, where authentication, collaboration and sensitive data all converge. That makes them an increasingly attractive target for attackers looking for quiet, durable access.

The lesson from recent attacks is clear: Just because an extension has been safe in the past doesn’t mean it’s safe today. In a threat landscape shaped by supply‑chain compromise, trust must be continuously reevaluated, or it will be exploited.