Infostealers: The Malware Behind Modern Credential Theft

A single malware infection on one employee's laptop can hand attackers the keys to your entire organization. Not just a password, but everything the browser remembered: saved logins, active session cookies, autofill data and more.

The malware behind this is known as an infostealer, and the data it exfiltrates is packaged into what threat actors call stealer logs. These logs are traded and shared at massive scale, and they have become one of the most damaging sources of corporate compromise today.

Understanding what stealer logs are, what they contain and how attackers use them is essential for any organization that wants to defend against account takeover and session hijacking.

In this article, we’ll break down how infostealers work, what ends up in a stealer log and how these logs can be leveraged defensively to protect your business.

What are they?

An infostealer (or simply stealer) is a type of malware designed to silently harvest sensitive data from an infected device and send it back to the attacker. Unlike ransomware, it is built to stay quiet and unnoticed: it runs once, exfiltrates everything it can and often deletes itself.

The exfiltrated data is bundled into a stealer log: a folder of files describing a single infected machine. These logs are then collected, repackaged and distributed through private Telegram channels and underground forums, either sold in bulk or shared for free to build reputation.

Infostealers are typically distributed through cracked software, malicious ads, phishing attachments and fake downloads. Because they target the individual device rather than a company's servers, a personal computer used for work can expose corporate credentials without the organization ever being directly breached.

The infection-to-exposure timeline

The most misunderstood thing about infostealers is how fast the damaging part is over. There is no dwell time to detect, no lateral movement to spot. By the time anything looks wrong, the data left the building long ago and the malware may no longer be on disk.

1. Infection. The user runs something they trusted: a cracked application, a fake installer from a search ad, a “download” from a phishing email. No exploit or admin rights are needed, the user simply executes it.

2. Exfiltration, in seconds. The stealer copies the browser credential stores, cookie databases, autofill tables, crypto wallets and a screenshot, then ships them out. This is a single burst of activity, not a persistent presence, and it is frequently done before any scheduled scan would ever run.

3. Packaging. The operator's panel bundles the haul into a per-machine folder: Passwords.txt, cookie files, system info, screenshot. Many stealers then delete themselves, leaving the victim with no symptom to notice.

4. Distribution. Logs are sold in bulk or dumped free into Telegram channels to build reputation. One infection is now in the hands of hundreds of unrelated actors — this is the point of no return, and it is why deleting the malware later changes nothing.

5. Exploitation, whenever they get to it. Someone eventually searches the corpus for vpn, sso or your domain name. That may be days or months after infection, long after the incident would have been considered closed.

Cleaning the laptop removes the malware. It does not un-share the data. Every credential and cookie that was on that machine is already somebody else's property.

Why are stealer logs important for enterprise cybersecurity?

Stealer logs are especially dangerous because they go beyond passwords. Even an organization with strong password policies and MFA can be compromised through the data in a single log. A stealer log can enable:

The session cookies are what make these logs so valuable. When an attacker imports a stolen cookie into their own browser, the service treats them as the already-authenticated user. There is no login prompt and no MFA challenge to trigger, which is why strong passwords alone are not enough.

From a defensive perspective, monitoring stealer logs lets security teams detect infected employees and exposed credentials early, so they can force password resets, revoke active sessions and clean up the affected device before the access is abused.

Why cookies beat MFA

This is the part that surprises most teams, so it is worth being precise about it. MFA protects the act of logging in. It does not protect a session that is already logged in.

When you authenticate, the service hands your browser a session cookie: a token that says “this browser already proved who it is, don't ask again.” Every subsequent request carries that cookie instead of your password. That is the entire mechanism that keeps you from re-entering an MFA code on every page load.

An infostealer copies that cookie. The attacker imports it into their own browser and loads your webmail or VPN portal, and the service checks the cookie, finds it valid, and serves the account. The login flow never runs. There is no password prompt to fail and no MFA challenge to trigger, because from the application's point of view nobody is logging in — an already-authenticated user is simply continuing their session.

The practical consequence: after a stealer log exposure, revoking active sessions is not optional cleanup, it is the actual remediation. A password reset without a session revocation leaves the door open.

Infostealer families compared

Infostealers are overwhelmingly sold as malware-as-a-service: the developer maintains the malware and a web panel, and affiliates pay a subscription to distribute it and collect the logs. That business model is why so many unrelated actors produce logs with a near-identical structure, and why the ecosystem reshuffles so quickly when one family is disrupted — affiliates simply move to a competitor.

The practical takeaway for a defender is that the family name barely matters. Whichever one infected the machine, the output is the same: credentials, cookies and autofill data in somebody else's hands. The ranking of families changes every time one is taken down; the exposure it creates does not.

Stealer logs in SynScan's data

When we group everything we've indexed by the type of source it came from, stealer logs are the second largest category of exposed credentials we hold, behind only the combo lists that are frequently built from them. Between them, those two categories dwarf every individual company breach in our database.

Credentials indexed by SynScan, by source category. Top 8 of 47 categories.

It is even starker at the level of individual datasets. Ranking every source we index by the number of credentials in it, the largest single source in our entire database is a stealer log collection — bigger than any breach of any company, by a wide margin:

The largest credential sources indexed by SynScan. Stealer-log collections highlighted.

Two things are worth reading out of that. First, malware-sourced credentials are not a niche next to conventional breaches, they are the main event. Second, the volume arrives from a small number of never-ending collections rather than discrete incidents, so there is no “breach announcement” to react to. Nobody will email you to say your employee got infected. Either you are watching this data, or the first sign is the intrusion itself.

How we approach this at SynScan

At SynScan, we treat stealer logs as a critical early-warning signal for our clients, so we built tooling to collect and analyze them at scale. This is our process:

Automated source monitoring. Our tooling continuously watches the Telegram channels and forums where threat actors publish stealer logs, collecting new posts and their attachments automatically as they appear.

Download and extraction. The shared files, whether direct attachments or links to external hosting, are downloaded and unpacked. Logs arrive in every archive format, so extraction is handled automatically before anything else.

Classification. Each extracted folder is inspected to tell a real stealer log (identified by files such as Passwords.txt) apart from plain combo lists and junk, discarding duplicates along the way.

Per-machine analysis. For every log we parse the information files to extract IP, country, operating system, hardware ID and infection date, count the exposed credentials, cookies and autofills, and even fingerprint the stealer family that produced it (RedLine, Lumma, Vidar, StealC, Raccoon and others).

Indexing and alerting. The normalized records are indexed into our search infrastructure alongside billions of other exposed credentials. Exposures matching our clients' domains are then surfaced in the platform, so security teams can act before the stolen data is used.

The result is a continuously updated view of infostealer exposure that turns the same logs attackers trade into an early warning system for our clients.

Stealer logs are only one half of the credential exposure picture. For the other half, the aggregated lists these logs so often feed into, read our companion article on combo lists.

How to protect your organization

Because infostealers target the device rather than your servers, protecting against them combines endpoint hygiene with fast reaction when something is exposed:

Deep Security Visibility Without the Complexity

SynScan provides clear, real-time security insights so you can monitor your attack surface, spot risks early, and act fast—without extra complexity.

• No setup fees
• 5-min deployment
• Cancel anytime

Book a Demo

Frequently Asked Questions