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What is an infostealer? Stealer logs explained

Infostealers are malware that harvest saved passwords, cookies, and session tokens in seconds, then sell them as stealer logs. Learn how stealer logs work, why they threaten companies whose systems were never breached, and how to check your exposure.

An infostealer is malware with a narrow job: land on a computer, copy everything valuable the browser and operating system have saved, send it out, and often delete itself. The whole run can take under a minute. The output is a stealer log — a bundle containing saved passwords with the exact websites they belong to, browser cookies and active session tokens, autofill data, crypto wallet files, and a fingerprint of the infected machine.

Why this is a company problem, not just a personal one

The employee who saved their work email password in a browser on their personal laptop just extended your attack surface to that laptop. Infostealers do not distinguish personal from corporate credentials; the log contains both, each labelled with the exact login URL. Buyers filter logs by domain, which means a criminal can search specifically for credentials belonging to your company — even though your own systems were never touched.

Three things make stealer logs worse than ordinary breach dumps:

  • They are fresh. Breach compilations recirculate passwords that are often years old. A stealer log is typically sold within hours or days of infection, while every credential in it still works.
  • They carry the exact URL. Not “this email appeared in a breach” but “this password opens your admin console.” No guessing required.
  • They include session cookies. A live session token can let an attacker ride straight past your password and your MFA, because the session is already authenticated.

How stealer logs get used

The typical chain: infection (cracked software, malicious ads, fake browser updates) → the log is uploaded and sold in bulk on marketplaces and Telegram channels → an access broker filters for valuable domains → credentials and sessions are tested → the working ones become ransomware footholds, business email compromise, or quiet data theft. From the defender's side the login often looks legitimate, because technically it is one: right credentials, real session, wrong human. This is the same ecosystem that feeds leaked-credential attacks, but faster and more targeted, and much of the trading happens in the dark-web and Telegram channels that monitoring services watch.

What actually defends against infostealers

  • MFA that stolen sessions cannot ride through: shorter session lifetimes on sensitive apps, re-authentication for privileged actions, and revoking sessions on any exposure signal.
  • Password manager policy instead of browser-saved passwords for work accounts — saved browser passwords are the first thing stealers harvest.
  • Separation: work accounts never signed in on personal devices, or at minimum isolated in a dedicated browser profile with no saved passwords.
  • Exposure monitoring: watching breach corpora, marketplaces, and Telegram channels for your domain, so you reset credentials and kill sessions before they are used. Detection here is measured in hours; incident response after use is measured in weeks.

To see where you stand today, check your domain's exposure counts with the free credential exposure checker, or run a broader external exposure scan to see stealer-log signals alongside everything else an attacker can find from outside.

Frequently asked questions

Is an infostealer the same as a keylogger?

No. A keylogger records what a victim types over time. An infostealer grabs everything already saved on the machine — browser passwords, cookies, autofill data, wallet files — in a single pass that often takes under a minute, and frequently deletes itself afterwards. That speed and self-removal are why infections are rarely noticed.

Does antivirus stop infostealers?

Sometimes, briefly. Stealer builders repackage their malware constantly to evade signatures, and the theft often completes before any detection fires. The practical posture is to assume some infections succeed and monitor for the output — your domain appearing in stealer logs — rather than relying on endpoint detection alone.

We use MFA. Are we protected from stealer logs?

Safer, but not protected. Stealer logs include session cookies, and a stolen live session can ride straight past login MFA because the session is already authenticated. Stolen passwords also still work anywhere MFA is not enforced. MFA plus short session lifetimes plus exposure monitoring is the combination that maps to this threat.

How would we know if our company is in stealer logs?

Your domain appears in the credential records — often with the exact login URL saved by the victim's browser, which is the classic stealer-log signature. SCRYPEX checks for that signature as part of credential monitoring, and the free credential exposure checker shows exposure counts for any domain, with full detail gated behind DNS proof that you own the domain.