Infostealer Malware: How It Works and How to Protect Your Organization

Learn how infostealers steal credentials silently and what you can do to detect exposure before attackers strike.

Infostealer attacks increased 84% in 2024 according to the IBM X-Force Threat Intelligence Index 2025. That’s not a random spike. Attackers have figured out that stealing credentials is easier than breaking through firewalls.

Here’s what makes infostealers dangerous: they operate silently. No ransom notes. No encrypted files. Just credentials quietly exfiltrated to criminal infrastructure. By the time you notice anything, your VPN credentials are already for sale on dark web markets.

The gap between infection and exploitation creates an opportunity. Stolen credentials circulate on criminal marketplaces for days or weeks before attackers use them. If you’re monitoring those markets, you can reset credentials before the attack begins.

This guide covers how infostealers work, what they steal, and how to detect if your organization’s credentials are already exposed.

What Is Infostealer Malware?

Infostealer malware is the quiet threat most security teams underestimate.

Traditional malware announces its presence. Ransomware encrypts files. Banking trojans redirect transactions. Infostealers do neither. They grab what they need and disappear into normal network traffic.

This silent operation is why infostealers have become the preferred tool for initial access. According to the IBM X-Force Threat Intelligence Index 2025, phishing campaigns now deliver infostealers 84% more often than the previous year. Attackers have learned that credentials open more doors than exploits.

The business model is straightforward. Infostealer operators harvest credentials at scale, package them into logs, and sell them on criminal marketplaces. Buyers include ransomware affiliates looking for corporate network access. The entire pipeline runs on stolen credentials.

How Do Infostealers Infect Systems?

Infection usually starts with a click. Phishing remains the primary delivery method.

Phishing campaigns deliver infostealers through malicious attachments and links. Emails impersonate vendors, IT departments, or delivery services. The malware hides in Office documents with macros or PDF files with embedded scripts. Fake software installers are common too.

Malicious downloads target people searching for cracked software or free tools. Attackers poison search results with sites offering popular software downloads. The installer works as expected while quietly dropping infostealer malware in the background.

Malvertising pushes malicious ads through legitimate advertising networks. Clicking these ads leads to malicious downloads disguised as legitimate software.

Messaging platforms have become distribution channels. Attackers share malicious files through Discord and Telegram. The social context makes recipients more likely to open files from apparent community members.

The infection itself takes seconds. Once executed, infostealers immediately begin harvesting data. Most complete their primary mission within minutes of execution.

What Data Do Infostealers Steal?

Infostealers target anything that provides access or has resale value.

Browser-Stored Credentials

The primary target is browser password databases. Chrome, Firefox, Edge, and other browsers store credentials in local databases. Infostealers extract these databases entirely, capturing every saved username and password combination.

Beyond saved passwords, infostealers capture credentials in transit. They hook into browser processes to intercept credentials as users type them, before TLS encryption occurs. This captures passwords that users choose not to save.

Session Tokens and Cookies

Authentication cookies are often more valuable than passwords. These tokens represent active sessions that have already passed MFA verification.

According to the Identity Threat Report 2025, 39% of data breaches involve stolen session cookies or tokens. Attackers don’t need your password if they have your authenticated session.

Application Credentials

Infostealers also target standalone applications. VPN clients store credentials locally. Email clients cache authentication tokens. FTP programs save server passwords. Each of these becomes a target.

Cryptocurrency wallets receive special attention. Infostealers scan for wallet files and browser extensions, extracting private keys and seed phrases. The financial incentive makes crypto theft a priority.

System Information

Beyond credentials, infostealers collect device fingerprints. Hardware IDs, installed software, network configuration, and location data all get exfiltrated. This information helps buyers assess the value of the access and plan their attacks.

Which Infostealers Are Most Active?

The infostealer landscape changes constantly as operators evolve their malware.

LummaC2 dominates the current threat landscape. The Identity Threat Report 2025 recorded 23.3 million detections globally, making it the single most prevalent infostealer. LummaC2 targets Windows systems and exfiltrates credentials and cryptocurrency wallets.

RedLine Stealer remains widely deployed despite law enforcement attention. It spreads through phishing campaigns and malicious downloads, harvesting browser data and cryptocurrency wallets. RedLine logs appear frequently on dark web marketplaces.

Vidar Stealer operates as malware-as-a-service, letting less technical criminals deploy credential theft at scale. Vidar targets browsers and email clients. Its accessibility has made it popular among entry-level cybercriminals.

Raccoon Stealer returned after a brief hiatus following developer arrests. The updated version targets an expanded list of browsers and applications. Raccoon logs continue appearing on criminal forums.

The IBM X-Force report documented over 3.7 million Lumma credentials and 568,000 RedLine credentials advertised on dark web markets in 2024 alone. These numbers represent just the publicly advertised portion.

What Happens After Credentials Are Stolen?

The credential theft pipeline moves faster than most organizations realize.

Immediate Exfiltration

Infostealers send harvested data to command-and-control servers within minutes of infection. There’s no delay, no staging period. The moment credentials are collected, they’re transmitted to attacker infrastructure.

Dark Web Distribution

Stolen credentials get packaged into stealer logs and distributed through criminal channels. Some operators sell directly on dark web marketplaces. Others distribute through private Telegram channels. The darknet markets where this data trades operate as sophisticated commercial platforms.

Pricing depends on the target. Corporate VPN credentials command premium prices. Domain admin accounts sell for thousands of dollars. Consumer credentials trade in bulk at pennies per record.

Initial Access Broker Pipeline

Initial access brokers specialize in verifying and reselling network access. They purchase credentials in bulk, test which ones still work, and sell verified access to ransomware affiliates.

This middleman layer adds days to weeks between initial theft and exploitation. That window is your detection opportunity. Dark web monitoring can catch credentials during this period, before ransomware operators use them.

Ransomware Connection

The Mandiant M-Trends 2025 report found stolen credentials tied for second place as ransomware initial access vectors at 21%. VPN credentials are especially valuable because they provide direct network access without triggering perimeter defenses.

The infostealer-to-ransomware pipeline is well documented. Credentials stolen today may enable ransomware attacks weeks later. Breaking this chain at the credential detection stage prevents the entire attack.

How Can You Detect Infostealer Infections?

Detection requires looking at both endpoints and external exposure.

Endpoint Indicators

Infostealers aren’t invisible. They leave traces. Watch for unusual process behavior, particularly processes accessing browser credential stores. Registry modifications that establish persistence are another indicator. Network connections to suspicious infrastructure can reveal active exfiltration.

However, endpoint detection has limits. According to the Identity Threat Report 2025, 66% of malware infections occur on devices with endpoint security or antivirus solutions installed. Infostealers are designed to evade these tools.

Why Endpoint Detection Isn’t Enough

Signature-based detection struggles against constantly evolving malware. By the time signatures exist for a new variant, it has already infected thousands of devices. Behavioral detection helps but generates false positives that overwhelm security teams.

The fundamental problem is timing. Even when endpoint tools detect an infostealer, the damage is done. Credentials were exfiltrated in the first minutes of infection. Detection after exfiltration is too late to prevent credential theft.

External Exposure Monitoring

The more reliable detection approach monitors where stolen credentials appear. Infostealer channel monitoring watches criminal marketplaces and Telegram channels where logs are sold.

When your organization’s credentials appear in infostealer logs, you get alerted. You can then reset those specific credentials before attackers purchase and use them. This detection happens regardless of whether endpoint tools caught the initial infection.

This approach inverts the detection model. Instead of hoping to catch malware on endpoints, you monitor the criminal marketplace where the impact becomes visible.

How Do You Protect Against Infostealers?

Protection requires multiple layers since no single control stops all infostealers.

Technical Controls

Browser security policies limit what browsers can store. Disabling password saving in managed browsers forces users toward password managers that infostealers can’t access. Enterprise browser configurations can restrict extensions and enforce security settings.

Application allowlisting prevents unauthorized executables from running. This blocks infostealers delivered through downloads and some phishing campaigns. The operational overhead is significant but effective for high-security environments.

Network segmentation limits lateral movement if credentials are stolen. Attackers with stolen VPN credentials still face internal barriers. Segmentation won’t prevent credential theft but reduces what attackers can do with stolen access.

Operational Security

Password rotation after infections is critical but poorly implemented. Only 54% of organizations routinely reset passwords after malware infections according to the Identity Threat Report 2025. Every confirmed or suspected infection should trigger credential resets for affected accounts.

Session termination is equally important. Resetting a password doesn’t invalidate existing session tokens. Only 33% of organizations terminate active sessions after credential theft detection. Implement forced session termination as part of incident response.

Security awareness training reduces initial infection rates. Users who recognize phishing attempts don’t click malicious links. Training should cover current infostealer delivery methods, not just generic phishing awareness.

External Monitoring

Credential monitoring services provide the earliest warning of exposure. When employee credentials appear in dark web marketplaces, you’re alerted. This detection happens during the window between theft and exploitation.

Third-party breach monitoring catches credentials exposed through vendor compromises. Your employees may use the same passwords across personal and professional accounts. When a vendor breach exposes those credentials, your corporate access is at risk.

The goal is detection before exploitation. Every credential reset that happens before attackers use the stolen access is a prevented breach.

Conclusion

Infostealers have become the preferred entry point for serious attacks. The 84% increase in infostealer delivery reflects attackers optimizing their methods. Credentials provide easier access than exploits.

Key takeaways:

• Infostealers operate silently and exfiltrate credentials within minutes of infection
• Endpoint protection catches only a fraction of infections since most occur on protected devices
• Stolen credentials circulate on dark web markets for days or weeks before use
• Session token theft bypasses MFA, making password resets alone insufficient
• External monitoring catches exposure during the window between theft and exploitation

The detection window between credential theft and exploitation is your opportunity. Credential monitoring catches stolen credentials while they’re being sold, before attackers use them.

Ready to see if your credentials are already exposed? Use our dark web scanner to check your organization’s exposure in infostealer logs and criminal marketplaces.