What Is Infostealer Malware? How Info-Stealers Work & Spread

Infostealer malware (also written "info-stealer" or just "stealer") is a category of malware built for one job: rapidly harvesting sensitive data from an infected device and exfiltrating it to the attacker. In a single run lasting seconds, a modern stealer can scrape saved passwords, browser session cookies, autofill data, credit-card details, and cryptocurrency wallets, then quietly send it all off and often delete itself. Infostealers are a specialized, highly automated form of spyware, and they have become one of the most significant threats in cybercrime.

In short: an infostealer is a smash-and-grab for your digital credentials. It doesn't stick around to spy — it grabs everything valuable at once and leaves, and what it steals frequently becomes the entry point for a much larger attack.

What infostealers steal

• Saved passwords from browsers and password stores.
• Session cookies and tokens — arguably the most dangerous prize, because a valid session cookie can let an attacker hijack an already-logged-in account and bypass multi-factor authentication entirely.
• Autofill data — names, addresses, and saved form details.
• Credit-card and payment information.
• Cryptocurrency wallets and their keys.
• System and application data — sometimes including messaging apps, VPN configs, and more.

How infostealers spread

• Cracked / pirated software and game cheats — a hugely common vector, since users run them voluntarily with high privileges.
• Malvertising and fake software downloads — malicious ads and lookalike sites pushing trojanized installers.
• Phishing emails and messages with malicious attachments or links.
• Fake updates and "fix-it" prompts, including deceptive browser-update and CAPTCHA-style lures.
• YouTube and social media posts linking to "free" tools that are actually stealers.

Many stealers are sold as malware-as-a-service, with subscriptions and support, which has lowered the barrier to entry and driven an explosion in stealer activity.

The stealer economy: from logs to breaches

The data an infostealer collects from one device is bundled into a package called a "log." These logs are sold in bulk on criminal marketplaces and Telegram channels, often for just a few dollars each. Specialized buyers — including initial access brokers — comb through logs for corporate credentials and active session tokens, then sell that access to ransomware crews and other attackers. This is the critical link: a single employee running a cracked app at home can hand criminals a valid corporate login that becomes the on-ramp to an enterprise ransomware attack or data breach. Stolen credentials surfacing in these markets are a major focus of dark web monitoring.

Why infostealers are so dangerous

• They bypass MFA. Stolen session cookies let attackers resume an authenticated session without needing the password or the second factor.
• They fuel everything else. Stealer logs are now a leading source of the initial access behind major breaches and ransomware.
• They're fast and cheap. Malware-as-a-service makes large-scale credential theft accessible to low-skill criminals.
• They blur home and work. A personal-device infection can compromise corporate accounts, sidestepping enterprise defenses entirely.

Notable infostealer families

The ecosystem shifts constantly as families rise, get disrupted, and are replaced, but names like RedLine, Raccoon, Vidar, Lumma, and Raccoon-style successors have dominated recent years. The specific brand matters less than the model: cheap, subscription-based, and engineered to grab credentials and session tokens at scale.

How to defend against infostealers

• Never run pirated or "cracked" software. It's the single most common stealer delivery method.
• Use phishing-resistant MFA (such as passkeys or hardware keys), and where possible bind sessions to a device so a stolen cookie is less useful.
• Use a dedicated password manager rather than saving credentials in the browser, where stealers look first.
• Deploy behavior-based EDR to catch the rapid, anomalous data-collection behavior stealers exhibit.
• Monitor for exposed credentials via dark web monitoring and respond fast — forcing password and session resets when an employee's data appears in a log.
• Separate work and personal devices, and keep everything patched.

Infostealers and the rise of session hijacking

The single most important shift infostealers have driven is the move from stealing passwords to stealing sessions. For years, defenders treated multi-factor authentication as the answer to credential theft: even if an attacker had your password, they couldn't pass the second factor. Infostealers broke that assumption. By grabbing the session cookies your browser stores after you log in, an attacker can import those cookies into their own browser and step straight into your already-authenticated session — no password, no MFA prompt, nothing to stop them. This is why session-cookie theft is now considered one of the most dangerous outcomes of an infostealer infection, and why simply resetting a password after exposure is no longer enough. To fully cut off an attacker, organizations must also invalidate active sessions, forcing every device to re-authenticate.

What to do if your data is in a stealer log

1. Assume everything entered on the device is compromised — passwords, cookies, and any data the browser stored.
2. Change passwords from a different, clean device, prioritizing email, banking, and any reused credentials.
3. Invalidate active sessions by signing out of all devices on each important account, so stolen cookies stop working.
4. Enable phishing-resistant MFA such as passkeys or hardware keys.
5. Clean or rebuild the infected device — a stealer may have been bundled with other malware.
6. Monitor financial and crypto accounts closely for unauthorized activity.

Where threat intelligence fits

Infostealers sit at the very start of the modern breach chain, which makes intelligence about them extremely high-value. Threat intelligence tracks active stealer campaigns, the marketplaces where logs are sold, and — crucially — when your organization's credentials appear in those logs. Catching a compromised session token in a stealer log before an attacker uses it can prevent the entire breach that would otherwise follow.

The bottom line

Infostealer malware is a fast, automated credential thief that grabs saved passwords, session cookies, payment data, and crypto wallets, then sells them as "logs" on criminal markets. Because stolen session cookies can bypass multi-factor authentication and stealer logs feed initial access brokers, infostealers have become a primary on-ramp to ransomware and major breaches. Defense centers on avoiding pirated software, phishing-resistant MFA, EDR, and monitoring for exposed credentials. To track stealer activity and exposed-credential reporting, follow our live threat intelligence feed, aggregated from dozens of authoritative sources.