In today’s digital business environment, data is both an asset and a target. Among emerging cyber threats, infostealers have become a prominent menace – often quietly hiding on computers and siphoning off sensitive information without immediate signs of trouble. Infostealer malware is designed to steal valuable data like login credentials, browser cookies (which keep users logged in), personal identifiable information, and other secrets from an infected device. Unlike loud ransomware attacks that announce themselves, infostealers operate silently in the background to exfiltrate as much data as possible without detection.
Why should business leaders care? Because a single stolen password or session cookie can snowball into a major breach. In fact, many high-profile cyber incidents start with an infostealer infection. One study found that one-third of companies hit by ransomware had an infostealer infection in the weeks prior – a warning sign that often goes unnoticed. And the scale of the threat is staggering: in just the first half of 2025, infostealers were responsible for stealing about 1.8 billion credentials (usernames, passwords, emails, etc.) across corporate and personal accounts. Criminal groups trade this stolen data on dark web marketplaces, where initial access brokers sell login details to the highest bidder. In other words, that quietly stolen password from an employee’s PC could be sold within hours and later used by another attacker to impersonate the employee or penetrate your network.
In short: Infostealers are a silent threat that bridge everyday security lapses (like an employee clicking a bad link) and large-scale attacks. This primer will explain what infostealers are, how they find their way into company systems, how you can proactively defend against them, and what to do if you discover one in your environment. The goal is to help both general business readers and security leaders build resilience against this stealthy menace.
What Exactly Is an Infostealer?
An infostealer is a type of malware built specifically to harvest information from infected devices. Once it sneaks onto a computer (be it an employee’s laptop or a server), it begins collecting data such as:
- Usernames and Passwords – e.g. those saved in web browsers or even in some password managers.
- Session Cookies – small files that keep you logged into websites, which attackers can use to bypass login prompts and multi-factor authentication (MFA).
- System Details – such as the device’s operating system, IP address, installed apps, and even hardware specs. This info helps the malware avoid detection (for example, it might self-terminate if it detects it’s in a security researcher’s test environment) and helps criminals assess the value of the target.
- Files and Screenshots – infostealers can grab files from specific folders (like your Desktop or Documents) or take screenshots of active sessions. For instance, some strains look for cryptocurrency wallets or confidential documents.
- Browser Autofill Data – personal data like names, addresses, phone numbers, and credit card details that users save in browsers.
- Email and App Credentials – logins for things like email accounts, VPN clients, remote desktops, or business applications if they’re stored on the machine.
The infostealer quietly bundles all this stolen data into an archive (often a **“.zip” log file) and sends it out to the attacker’s server. Many infostealers even erase their traces after stealing data (for example, some automatically delete themselves once their job is done), making them hard to notice. To a busy employee or IT admin, the system still appears to function normally while the theft is happening.
Once the data is exfiltrated, the real damage begins. Cybercriminals waste no time exploiting stolen data: they can use it themselves to break into accounts, or (very commonly) sell it on the dark web. In fact, there’s a whole criminal economy around infostealers. So-called “Initial Access Brokers” package and sell stolen login credentials to other attackers. For example, an infostealer might steal an employee’s VPN password; an access broker sells it; then a ransomware gang buys it and uses it to break into a company’s network months later. This handoff is one reason infostealer infections are so dangerous – the harm often comes later, when a different attacker uses the stolen data as a foothold.
To put it simply, an infostealer is like a silent data vacuum: it sucks out sensitive information from a device without alerting the victim. That information can then facilitate all sorts of attacks – from fraudulent money transfers to full-blown network breaches. No organisation is too small or too “non-tech” to be targeted. Infostealers cast a wide net, and any useful data they catch can be leveraged for profit. Businesses must therefore treat infostealer malware as a serious threat to corporate security and personal privacy alike.
How Do Infostealers Get Into Company Systems?
Infostealers may be insidious, but they still need an initial way into your computers. Understanding their common infiltration paths is crucial for prevention. Typically, infostealer infections start with human error or social engineering. Here are some common scenarios:
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Phishing Emails and Malicious Links: Many infostealers arrive via phishing. An employee might receive a convincing email – perhaps posing as an invoice, a file share, or even a security alert – urging them to click a link or open an attachment. If they take the bait, malware can be silently installed. For instance, a spreadsheet attachment laced with malicious macros could install an infostealer when opened. Unlike ransomware, which would announce itself, the infostealer will quietly begin plundering data. It only takes one errant click by an unsuspecting staff member for the organisation to be compromised.
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Trojanised Software and Downloads: Infostealers also hitch rides on fake or tampered software. Imagine an employee downloading what looks like a useful utility or a pirated application from an untrusted website – it might actually be bundled with an infostealer. (One real example: users installing a “cracked” game found their machine infected with a token-stealing malware soon after.) These malware often masquerade as legitimate programs or are hidden inside software cracks, browser plugins, or free downloads. Once executed, the infostealer payload deploys in the background.
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Drive-by Downloads and Exploit Kits: In some cases, simply visiting a compromised website (especially one laden with malicious ads) can result in an infostealer download if the site exploits a browser or plugin vulnerability. This is less common than phishing but still a risk – particularly if employees browse less-than-reputable sites on a work device without proper browser protection.
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Infected Personal Devices (BYOD) and Third-Party Connections: A growing weak point for businesses is the use of unmanaged devices – such as personal laptops or contractors’ machines – that connect to company systems. If such a device is infected with an infostealer (perhaps from the user’s own activities), it can act as a bridge into corporate data. Statistics indicate that as many as 90% of security compromises originate from unmanaged or third-party devices. For example, a staff member’s personal computer infected at home could compromise their credentials, which are later used to access the corporate VPN. Similarly, a vendor with an infected device could expose shared systems. Many recent breaches have been traced back to an infostealer on a personal or contractor’s device that had access to the company network. This risk is amplified in today’s hybrid work era, where the line between home and work devices blurs.
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Malware-as-a-Service Campaigns: Infostealers are widely available for sale in the cybercrime underground, some for as little as ~$50-$100 per month. This means even relatively low-skilled attackers can rent an infostealer service and spam it out using botnets or broad phishing campaigns. The malware spreads far and wide looking for any vulnerable machine. So, a small business might not be specifically targeted at first; they could simply be one hit among thousands in a mass-distribution campaign. If an infostealer “lands” on one of your company devices through such a campaign, it will quietly do its job and report back to its operator.
Regardless of the entry method, the key point is that infostealers rely on gaps in our defences – human or technical – to get in. Phishing preys on our trust and busy schedules; outdated software offers loopholes for silent exploits; and unmanaged devices extend the attack surface beyond IT’s visibility. By recognising these common vectors, organisations can start plugging the holes (as we’ll discuss in the next section).
Building a Proactive Defence Against Infostealers
When it comes to infostealers, the old saying “prevention is better than cure” certainly applies – but with a twist. We want to prevent infections and be ready to respond if one slips through. A resilient cybersecurity posture layers multiple defences to reduce the chance of an infostealer incident, and ensure that if one does occur, the damage is limited. Here are proactive measures both business leaders and security teams should consider:
1. Strengthen Endpoint Protection and Monitoring: Ensure all company devices have up-to-date anti-malware and Endpoint Detection & Response (EDR) tools. Modern security software can often spot suspicious behaviours that infostealers exhibit – like unusual processes trying to access password stores or browser data. EDR solutions can flag or block such behaviour in real time, and even isolate a machine if it acts maliciously. Additionally, maintain regular software updates and patches on all systems to close known vulnerabilities that malware might exploit. An infostealer might try to use old exploits to gain higher privileges; don’t give it the chance.
2. Enforce Strong Identity Security: Because infostealers target credentials and sessions, robust identity protections are crucial. Implement Multi-Factor Authentication (MFA) or better yet, Passkeys across all accounts (but especially critical ones) – that way, even if a password is stolen, the thief likely cannot use it without the second factor (and stolen cookies will expire eventually). Use a policy of strong, unique passwords (consider a corporate password manager to help employees manage them safely). Monitor for suspicious login attempts, such as impossible travel logins, which might indicate someone is trying to use stolen credentials. In short, assume some credentials will leak, and put safety nets (MFA, alerting, conditional access policies) to catch misuse early. Our earlier blog about Passkeys will help further with regards stolen passwords, as they cannot be stolen in the first place.
3. Limit Exposure from Unmanaged Devices: If possible, limit or control the use of personal devices for work purposes. Corporate Bring-Your-Own-Device (BYOD) policies should require security measures on personal devices that access company data – for example, installing company-approved security software or using virtual desktop environments. Better yet, provide employees with secured corporate devices for remote work. Likewise, extend security requirements to third parties and contractors: if they access your network or cloud apps, they should meet certain device security standards. By reducing how much an infected home computer can reach, you cut off a common infostealer pathway.
4. Employee Awareness and Training: Human vigilance is a powerful defence. Conduct regular security awareness training that includes the topic of infostealers and phishing. Employees should learn how to spot suspect emails or links, and understand that even something as innocuous as saving passwords in their browser can pose a risk if malware strikes. Emphasise a culture where employees double-check unexpected requests (like “urgent” file downloads or login pages) with IT. Consider running simulated phishing exercises to keep everyone on their toes. The aim is not to scold users, but to empower them as the first line of defence. When staff know about threats like infostealers, they are less likely to fall prey to the tricks that let these malware in.
5. Network and Cloud Protections: A strong network security posture helps too. Implement measures like web filtering and email filtering to block known malicious sites and attachments before they ever reach users. Use network monitoring to detect unusual outbound traffic – for example, large encrypted uploads from a user machine to an odd external server could indicate an infostealer exfiltrating data. In cloud environments, monitor for mass data downloads or API calls with stolen tokens. Basically, watch for the footprints an infostealer might leave as it moves stolen data out. Early detection can trigger an immediate response (cutting off that device’s network access) before more data is stolen.
6. Threat Intelligence and Dark Web Monitoring: Here’s a more advanced tactic that shows depth for security leaders: monitor for your organisation’s credentials appearing in data leaks or dark web markets. There are services that keep an eye on criminal forums and dumps for stolen logins, or you can engage a threat intelligence provider. This approach can alert you that an infostealer has hit one of your users even if you missed it internally. An infostealer log containing your @company.com accounts might surface on a dark web marketplace; if you find out, you can quickly reset those accounts and investigate. In fact, proactive threat intelligence is increasingly key for infostealer defence. It extends your visibility beyond your walls, catching problems at the earliest stage – when stolen data is being circulated. While this may be beyond the scope of very small businesses, larger organisations or those with high risks should strongly consider it.
7. Prepare an Incident Response Plan: Finally, being proactive means preparing for the worst. Have an incident response plan or playbook specifically for malware infections that steal data. This plan should outline quick steps to contain an incident (disconnect devices, revoke credentials, etc.) and assign responsibilities (who contacts IT, who resets accounts, who communicates to stakeholders). Conduct drills or tabletop exercises so that if an infostealer outbreak happens, your team can respond swiftly and confidently. Preparedness is a cornerstone of resilience – it turns a panicky scramble into a practiced routine, reducing mistakes and damage. Many organisations even keep an Incident Response Retainer with cybersecurity firms, so expert help is one call away. We’ll talk more about response next.
By implementing these measures, organisations build layers of defence. You’re making it harder for infostealers to get in, limiting what they can do if they do get in, and setting up tripwires to catch and contain them quickly. This multi-faceted approach is the essence of cyber resilience against threats like infostealers – you anticipate attacks and are ready to bounce back from them.
What To Do If You Discover an Infostealer
Despite our best efforts, let’s say an infostealer incident occurs – perhaps your security tools caught it, or you found out through a third-party alert. How you respond in the first hours is critical to minimising harm. An infostealer infection isn’t like a ransomware scream; it’s more like discovering a leak. You must act fast to plug it and deal with the spill. Here’s a step-by-step game plan:
Step 1: Isolate and Disable Access – “Stop the bleeding.”
The moment you confirm (or strongly suspect) an infostealer on a device, immediately isolate that system from the network. Unplug it or disable its network interface via your XDR if possible, to stop any further data exfiltration. At the same time, revoke the user’s access credentials to corporate systems. Why? Because if an infostealer stole their passwords or session tokens, attackers could already be using those to get into your systems. Disable the user’s accounts, at least temporarily, and force log-outs of any active sessions (VPN, email, cloud apps, etc.). Essentially, you’re locking the doors that the thief might sneak through using stolen keys. Be thorough: check not just the obvious accounts, but any external services that employee has access to (HR systems, finance platforms, developer tools – infostealers often grab those credentials too). It might feel extreme to shut an employee out, but it’s a vital containment step. Consider implementing SSO to automatically include these external systems when the account is disabled.
Step 2: Investigate for Any Breach Activity – “Have attackers used the stolen data?”
With the device isolated and accounts on hold, the security team should hunt for signs that the stolen information was used inside your environment. Look at authentication logs for that user (or other unusual logins) across systems: Are there login attempts from odd locations or devices? Any logins at strange times (e.g. wee hours) or multiple failed password attempts that could indicate credential stuffing? Also inspect for unusual access patterns – such as large data downloads, sensitive files accessed, or new user accounts created (an attacker might create a backdoor account). If any suspicious activity is found, it means the infostealer’s data has already been weaponised by an intruder. In that case, escalate to a full incident investigation: involve your Incident Response team or partner to contain and eradicate the threat from the network, not just the single device. If nothing obvious is found, remain cautious – stolen data might yet be used later, but at least you have no active intruder at the moment.
Step 3: Analyse the Infected Machine – “Clean Patient Zero.”
Next, focus on the infected machine itself. Remove the infostealer malware and any other malicious code it might have dropped. For company-managed devices, your IT or security team should take control of the device and perform a deep clean: run EDR/XDR scans, check for unusual start-up entries or scheduled tasks (infostealers sometimes add persistence mechanisms), and possibly reimage the system to be safe. On an unmanaged device (say an employee’s personal laptop), guide the user through the clean-up: they may need to run trusted anti-malware tools or even do a full factory reset of their machine. This can be challenging for a non-technical user, so provide support and clear instructions – after all, the infostealer likely stole both company and personal data from them, a stressful situation. The key is not to restore the device’s access until you’re confident the malware is gone. If the infostealer isn’t completely removed, any new passwords you enter or sessions you start on that machine could be stolen again. In some cases, wiping and rebuilding the system is the surest solution. It’s also wise to scan other devices, in case the malware spread (though most infostealers don’t self-propagate extensively).
Step 4: Reset Credentials and Monitor Closely – “Neutralise the stolen data.”
Now that the device is clean, you can begin restoring the user’s access – but do so with fresh credentials and enhanced security. All passwords that were stored or entered on the infected device should be considered compromised and reset immediately. That likely means the user’s Windows/network login, email password, VPN password, and any other credentials they use (including personal ones if personal device – they’ll need to change those too, starting with important accounts like email, banking, etc.). In addition, invalidate any active session tokens that might still be valid (for example, force re-login on email, cloud apps, etc. for that user, which you likely did in Step 1). This ensures that even if attackers have session cookies, those are no longer useful.
When restoring accounts, enable MFA if it wasn’t already on, or consider adding extra verification steps for the first few logins. You may also choose to elevate monitoring on this user’s accounts for a period of time – flag any new access attempts immediately. Essentially, we want to make sure that whatever data the infostealer stole (passwords, tokens, etc.) is now worthless to the thieves because we’ve changed the locks. This step is crucial; as one security expert put it, simply cleaning the malware isn’t enough if the stolen keys remain in enemy hands. Think of it as both sanitising the wound and also inoculating the patient against the stolen virus.
Step 5: Learn and Strengthen (Post-Incident Review) – “Prevent the next one.”
With the immediate crisis handled, take a breath – then take lessons from the incident. Conduct a post-incident review to understand how the infostealer got in and what could have been done to detect or stop it sooner. Questions to ask: Was the infection traced back to a particular email or download? If so, how did it bypass filters or employee caution? Are there security control gaps that need closing (e.g. missing patches, no EDR on that device, misconfigured permissions)? Also evaluate the impact: What data was likely stolen? Do you need to notify any partners or clients of a potential data exposure? In some cases, if sensitive personal data or large-scale credentials were taken, there might be regulatory or contractual breach notification requirements. Engage your privacy/legal team if necessary. Look to replace MFA with Passkeys.
Use this analysis to harden your environment against future infostealer attacks. This could mean updating policies (perhaps disallowing unmanaged devices unless certain criteria are met), deploying new security tools or settings (shorter session timeouts, stricter email filtering), or improving monitoring (e.g. implement that dark web credential monitoring if you didn’t have it). It’s also beneficial to run an organisation-wide password reset for any credentials that the infostealer might have stolen – for example, if one employee’s device was hit, and it had saved passwords for a shared admin account (not a good practice, but it happens), you’d want to change that shared password everywhere. Furthermore, re-educate employees if needed: without blaming, communicate that this incident happened and highlight the safety practices that can help avoid the next one. Often an incident close to home makes lessons stick better.
By following these steps, an organisation can effectively contain the infostealer incident, eradicate the threat, and mitigate the fallout (especially the potential data misuse). It’s never pleasant to deal with a malware infection, but a swift, structured response can turn a potential disaster into a manageable event. The emphasis should always be on both technical recovery (devices and accounts) and addressing the information exposure (the stolen data) to truly neutralise the risk.
Conclusion: Embracing Resilience Against Stealthy Threats
Infostealers might operate in the shadows, but they have forced cybersecurity into the spotlight. These silent data thieves teach us that protecting an organisation isn’t just about keeping intruders out – it’s also about quickly detecting and ejecting those who slip through, and safeguarding what matters most (our data and identities) so that even a breach doesn’t spell catastrophe. In an era where a single stolen login can escalate into a full-scale business crisis, building resilience is key. That means investing in preventive measures, fostering a security-aware culture, and having robust response plans at the ready.
From a general business perspective, infostealers underscore the importance of basic cyber hygiene and vigilance across all staff and systems. For security leaders, they exemplify why an “assume breach” mindset is practical – assume some credentials could be compromised at any time and design your defences accordingly (with layers, monitoring, and intelligence). By balancing strong preventive controls with prepared incident response, organisations can minimise the impact of even these stealthy threats and continue operating with confidence.
In summary, infostealers are a formidable foe but not an unbeatable one. A combination of awareness, good security practices, and prompt action can keep your company’s sensitive information out of criminals’ hands. In the face of ever-evolving cyber threats, such resilience – the ability to absorb shocks and bounce back – is what will set successful, secure organisations apart.
Ready to bolster your defences against stealthy threats like infostealers? Cyber & Data Protection can help your organisation build a stronger cybersecurity posture, from prevention and Cyber Essentials certification to incident response. Get in touch with our experts to learn how we can help safeguard your business’s critical data and keep you a step ahead of cybercriminals. Contact CDP today to start a proactive defence strategy for your organisation and fortify your cyber resilience today.
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