Browser extensions are the closest thing most users have to custom software on their laptops.

They automate boring tasks, fill gaps in SaaS products, and keep people productive.

They also sit in the perfect spot to steal the one thing that really matters in the cloud: session tokens.

A session token is what lets you stay logged in to your email, HR portal, CRM, or admin console.

It might live in a cookie, local storage, or an authorization header.

If an attacker gets that token, they do not need your password or your second factor.

From the SaaS app’s point of view, they are you.

Session token abuse through browser extensions is not a theoretical issue.

Real campaigns have used hijacked or fake extensions to lift Facebook cookies, steal Microsoft 365 sessions, and compromise corporate accounts at scale.

The impact is ordinary account takeover, but at enterprise scale and without tripping most of the controls we usually rely on.

Understanding the risk

In a typical enterprise, the browser is now the main client for critical systems.

Identity providers, email, source control, financial systems, all live there.

Extensions run inside the same browser, often with broad permissions, and they update automatically in the background.

Session token abuse fits into this picture as a quiet form of credential theft.

Instead of keylogging passwords, an extension reads or observes the token that proves you are authenticated, then sends it to an attacker.

The attacker replays that token from their own machine and walks into your SaaS tenant.

This threat is easy to overlook because the extension ecosystem looks curated and safe.

Extensions come from official stores, have thousands of users, and positive reviews.

The problem is that "trusted once" does not mean "trusted forever." Developer accounts get phished. Ownership of popular extensions changes hands. A benign tool can become a session stealer in a single update.

How attackers exploit the browser’s architecture

Browsers deliberately give extensions more power than normal web pages.

That is the point: extensions are supposed to do things pages cannot.

Those same powers are what make session tokens vulnerable.

With cookie permissions and host access, an extension can enumerate cookies for a domain, including HttpOnly cookies that page JavaScript cannot see.

For apps that keep session identifiers in cookies, that is game over. The extension simply reads the cookie value and posts it to a command and control server.

Other apps store tokens in local storage or JavaScript variables.

Content scripts, or injected scripts running in the page’s main world, can reach into that storage and pull out OAuth access tokens or JWTs.

Extensions can also watch traffic with the webRequest APIs and capture authorization headers as they pass through.

Recent proof of concept work has shown exactly this path against Azure Entra ID.

A malicious Manifest V3 extension watched for specific cookies that represent a logged in Microsoft 365 session, exfiltrated them, and let an attacker open Outlook and Teams as the victim without a new login.

Campaigns against consumer platforms have done the same thing with social media cookies for years.

Permissions and user behavior fill in the rest of the story.

Many extensions ask for "read and change all your data on the websites you visit." Users click accept without thinking. Once granted, that level of access is enough to see almost any token in the browser.

Why traditional defenses fall short

From the operating system’s perspective, a browser with an extension is still just a browser.

The extension’s code runs under the same user, making API calls that look like normal browsing.

It does not drop binaries on disk, scan the file system, or open odd ports.

Endpoint protection tools that focus on processes and files are mostly blind.

Network defenses do not help much either.

Exfiltration typically goes out over encrypted HTTPS to a cloud service.

To a firewall, this looks like every other outbound TLS connection a browser makes.

Even if you terminate TLS and inspect traffic, pulling out which cookies are legitimate and which are being stolen is hard and brittle.

Many security teams also overestimate what HttpOnly cookies buy them.

HttpOnly stops document.cookie from exposing values to page scripts.

It does not stop a privileged extension from reading these cookies through its own API.

So designs that assume "we are safe because tokens are HttpOnly" miss this entire threat path.

Finally, most organizations have little visibility into which extensions are installed where, what permissions they use, and when they change.

That lack of inventory and policy is exactly what attackers need.

Organizational blind spots and human factors

The human side is simple.

Users want productivity.

The extension store makes installing tools easy.

The permission prompts are vague and repetitive, so people learn to click through.

Ratings and download counts stand in for real security review.

Administrators face their own pressures.

Locking down all extensions would cause an immediate revolt.

So the default becomes "allow anything from the store" and hope curation is enough.

Developer mode and sideloading remain enabled for "flexibility." There is rarely a formal approval workflow or periodic review of extensions already in use.

This is how a compromised extension can sit inside a sales org, reading CRM cookies and browser storage for weeks, while everyone assumes their browser is just a browser.

Strategies for defense

Defending against session token abuse starts with treating extensions as software, not as harmless browser decorations.

At the policy layer, move from allow‑by‑default to deny‑by‑default.

Use Chrome and Edge policies that block all extensions by default, then maintain an allowlist of approved IDs.

Tie approval to a simple security review, where you check the publisher, requested permissions, and behavior on installation.

Apply least privilege to permissions.

For approved extensions, limit what they can do and where.

Use ExtensionSettings and similar controls to block sensitive permissions like cookie access and powerful scripting for anything that does not strictly require them.

Restrict which domains extensions may run on, and explicitly block identity providers, admin consoles, and financial systems from all but the most vetted tools.

On the application side, harden sessions.

Follow standard guidance for secure cookies and token lifetimes.

Shorten session duration for critical systems, rotate tokens, and require re‑authentication for high risk actions.

Where possible, adopt mechanisms that bind session tokens to a specific device so that replay from a different machine fails even if the cookie is stolen.

Finally, view this through a zero trust lens.

Do not assume that a valid session token is always proof of a legitimate user in front of a trusted browser.

Use device posture, behavioral signals, and conditional access policies to question suspicious session reuse.

The role of enterprise browsers

Consumer browsers are designed to keep end users happy.

Enterprise browsers and extension management platforms are built to keep organizations safe while people do their work.

These tools sit between the user and the open extension ecosystem.

They provide a central catalog where only approved extensions appear.

They enforce allowlists, block risky permissions globally, and can disable or uninstall an extension across the fleet if its behavior or publisher changes.

More advanced platforms score extensions based on permissions, network behavior, and update history.

They can block an extension from running on sensitive sites, even if it is allowed elsewhere, or strip specific capabilities like cookie access from anything that does not need it.

The key benefit is visibility.

Security teams get a live inventory of who has which extension, which versions are deployed, what permissions they hold, and when updates occur.

That is the foundation for any serious governance story around extension risk.

Practical steps for immediate risk reduction

There are actions most organizations can take in weeks, not years.

First, configure your browser management to block all unapproved extensions and build a small allowlist for core tools.

Turn off developer mode and sideloading on managed endpoints.

Restrict installs to official stores or your own enterprise catalog.

Second, identify your highest value web apps: identity provider, email, admin consoles, finance, HR.

Use runtime blocked host settings so that no extension can access these domains at all, except for a very narrow set that you explicitly bless.

Third, adjust your identity and logging.

Make sure your SaaS and identity providers log session reuse and anomalous sign‑ins, and that someone is looking at those logs.

Create an incident playbook for "browser extension steals session token" that covers disabling the extension, revoking tokens, and forcing re‑auth.

Fourth, start educating users.

Keep it simple. Explain that extensions are software with access to work accounts.

Ask them to install only from your catalog and to treat broad permission prompts as red flags.

The path forward

Most of the important work in a company now happens through a browser tab.

Extensions are part of how teams get that work done, but they also open a straight line into live, authenticated sessions.

Defending against session token abuse is about accepting that reality and adjusting the architecture.

Enterprise‑grade browsers, strong extension governance, and hardened session handling let you keep the convenience of extensions without handing attackers a master key to your SaaS estate.

Attackers will keep evolving, and they will follow users into whatever new browser capabilities appear.

The organizations that do best will be the ones that treat the browser as a critical security boundary and manage extensions with the same seriousness they bring to any other code running on their endpoints.

FAQ

What are session tokens and why are they valuable to attackers?

Session tokens are what allow you to stay logged in to applications like email, HR portals, CRMs, or admin consoles.

They might exist in cookies, local storage, or authorization headers.

If an attacker obtains your session token, they don't need your password or second factor authentication - from the application's perspective, they are you and can access your accounts without triggering most security controls.

How do malicious browser extensions steal session tokens?

Browser extensions have broad permissions that allow them to read cookies (including HttpOnly cookies), access local storage containing OAuth tokens or JWTs, and monitor web traffic to capture authorization headers.

Extensions can use content scripts to extract tokens from JavaScript variables or watch network requests as authentication data passes through the browser.

Once obtained, these tokens are sent to command and control servers for attackers to replay.

Why don't traditional security tools detect this type of attack?

From the operating system's perspective, a browser with an extension looks like normal browsing activity.

Extensions don't drop files, scan systems, or open suspicious ports, so endpoint protection tools often miss them.

Network defenses also struggle because token exfiltration typically occurs over encrypted HTTPS connections to cloud services, appearing identical to legitimate web traffic.

Additionally, most organizations lack visibility into which extensions are installed and what permissions they have.

What makes Island different in addressing browser extension risks?

Island provides enterprise-grade browser security with centralized extension management and governance.

It offers a controlled catalog where only approved extensions appear, enforces allowlists, blocks risky permissions globally, and can disable extensions across the entire organization if threats are detected.

Island also provides real-time visibility into extension inventory, permissions, versions, and updates, along with behavioral scoring and the ability to restrict extensions from accessing sensitive domains.

What immediate steps can organizations take to reduce extension-related risks?

Organizations should configure browser management to block all unapproved extensions and maintain a small allowlist of essential tools.

Disable developer mode and sideloading on managed devices.

Identify critical web applications like identity providers and admin consoles, then use blocked host settings to prevent any extensions from accessing these domains except for explicitly approved ones.

Additionally, ensure your identity providers log session reuse and anomalous sign-ins, and create incident response procedures for extension-based token theft.