Dangerous browser extensions: how they steal data without malware

Browser extensions are often seen as harmless tools that add convenience, improve productivity, or customise everyday web use. In reality, some extensions operate far beyond their declared purpose, silently collecting personal data without triggering antivirus warnings or behaving like classic malware. In 2025, this threat remains underestimated, despite growing evidence from cybersecurity researchers and browser vendors.

Why browser extensions can be dangerous

Unlike traditional malicious software, browser extensions usually enter a device with the user’s consent. They are installed directly from official browser stores and often request broad permissions that appear reasonable at first glance. Access to browsing history, active tabs, cookies, or form data is frequently justified as necessary for functionality.

The problem arises when these permissions are abused. Once installed, an extension operates inside the browser environment with persistent access to user activity. It does not need to exploit system vulnerabilities or bypass security tools, because the browser itself grants the requested access. This makes detection significantly harder.

By 2025, security audits have shown that even extensions with millions of downloads may include hidden tracking components, third-party data brokers, or poorly documented data-sharing practices. In many cases, the risk is not obvious until detailed code analysis is performed.

Permission abuse and silent data collection

Most users approve permission requests without reviewing them carefully. Extensions often ask for access to “read and change all data on websites you visit”, a phrase that sounds technical but grants extensive control over browsing behaviour.

With this level of access, an extension can log visited URLs, search queries, login pages, and interaction patterns. Even if passwords are not directly captured, behavioural profiles can be built with high accuracy, allowing identification of interests, habits, and sometimes real-world identity.

Because this activity happens entirely within the browser, it does not trigger classic malware indicators. No suspicious files are downloaded, no system processes are altered, and antivirus software typically remains silent.

How data is stolen without traditional viruses

Modern data theft increasingly relies on legitimate technical mechanisms rather than malicious exploits. Browser extensions are a prime example of this shift. They use documented APIs provided by browser developers, which makes their behaviour appear compliant at a technical level.

Many extensions monetise collected data through partnerships with analytics firms or advertising networks. While some disclose this vaguely in privacy policies, the actual scope of data collection is rarely clear to the average user.

In 2025, investigations have confirmed that certain extensions transmit browsing data in real time to remote servers, where it is aggregated, enriched, and resold. This process often continues even after an extension is no longer actively used.

Updates that change behaviour after installation

A common tactic involves releasing a clean, useful extension initially, gaining trust and a large user base. After this, updates introduce new code that expands tracking capabilities or integrates external scripts.

Because browser updates are usually applied automatically, users may never notice these changes. The extension name, description, and ratings remain the same, creating a false sense of continuity and safety.

This delayed activation strategy has been documented repeatedly and remains effective, as most users do not re-evaluate permissions after an update.

Real risks for personal and professional data

The impact of unsafe browser extensions extends beyond targeted advertising. Access to corporate dashboards, cloud services, and internal tools through the browser means that professional data is also exposed.

In remote and hybrid work environments, browsers have become the primary interface for sensitive operations. An extension with access to session tokens or page content can unintentionally leak confidential information without breaching any server-side security.

By 2025, several data protection authorities have highlighted browser extensions as a growing compliance risk, particularly in sectors handling financial, medical, or legal information.

Why users rarely notice the damage

Data theft through extensions is typically passive and gradual. There are no visible signs such as system slowdowns, pop-ups, or crashes that would alert the user.

Consequences often appear much later, for example through account targeting, unexpected access attempts, or highly personalised phishing messages. At this stage, tracing the source back to a browser extension is extremely difficult.

This delayed effect contributes to low awareness and underreporting, allowing unsafe extensions to remain active for years before being removed from browser stores.