Infostealers: How Stolen Browser Credentials Fuel the Breach Economy

Behind a great many modern breaches sits a quiet, unglamorous piece of malware: the infostealer. It does not encrypt your files or announce itself. It simply rifles through an infected device, scoops up every credential it can find, and sends them off to be sold. One careless download can arm attackers with the keys to dozens of accounts.

The short version: infostealer malware arrives via a phishing link or a cracked download, harvests saved passwords, cookies and session tokens from the browser, and bundles them for sale on criminal markets. A buyer then logs straight into corporate accounts, and because stolen session tokens can bypass MFA, a valid password is not even always required.

How infostealers fuel breaches

The chain is efficient. A user is tricked into running the malware, often through a fake tool, a pirated application, or a phishing link. It quietly extracts saved logins, authentication cookies and session tokens from browsers and apps, then exfiltrates them. Those credentials are packaged into “logs” and sold, sometimes for a few pounds, to other criminals who use them for the actual break-in. The person who steals the credentials is rarely the person who exploits them.

Why stolen tokens are so dangerous

The real prize is often not the password but the session token, a small file that tells a service the user is already logged in. With a valid token, an attacker can resume a session without a password and, in many cases, without triggering multi-factor authentication at all. That is what makes infostealers such a potent, under-appreciated engine of account takeover.

How to defend

Stop the infection

Block risky and unauthorised downloads, discourage pirated software, and keep endpoint protection current so the malware is caught before it runs.

Prefer phishing-resistant MFA

Passkeys and FIDO2 keys are far more resilient than codes, and modern controls can bind sessions to a device so a stolen token is useless elsewhere.

Assume compromise and rotate

If a device may have been infected, treat its saved credentials as already stolen: reset passwords and revoke active sessions rather than hoping for the best.

Cut off the delivery route

Since many infostealers arrive by phishing, run realistic simulations and give staff a quick way to report suspicious emails before anyone runs the file.

The bottom line

Infostealers turn a single infected device into a wholesale supply of working credentials, tokens included, that quietly powers a huge share of intrusions. Blocking the infection, moving to phishing-resistant authentication, and treating any exposed credential as compromised break that supply chain at its source.

Phishing Tackle offers the tools businesses need to strengthen their human risk strategies, with multi-platform testing, real-time behavioural insights, and actionable data to keep your organisation ahead of modern cyber threats.

Contact us today to learn how Phishing Tackle can help safeguard your organisation from the growing array of cyber risks.