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Malware reaches devices through several routes: a person may open a malicious attachment or download, a vulnerable application may be exploited, or attackers may misuse credentials, third-party access, or removable media. Security researchers examine suspicious files using methods such as automated scanning, sandbox observation, and—in deeper investigations—manual reverse engineering. These are specialist activities; do not run an unknown file on your everyday device to see what it does.
How does malware infect a device?
Infection usually involves both a route onto a device and an opportunity for the code to run or gain access. The route varies by campaign: malware does not always arrive as an email attachment, and simply encountering a suspicious message or website does not mean a device is infected.
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Phishing messages, attachments, and links
An attacker may impersonate a trusted organization or person and persuade someone to click a link, open an attachment, or download a file. CISA’s Emotet advisory describes malicious Word attachments that relied on a recipient opening the document and enabling execution. In some campaigns, an initial foothold can also fetch more malware: CISA describes Emotet as a Trojan that commonly served as a downloader or dropper.
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A malicious or compromised website can expose a visitor to an exploit, or a page can use misleading prompts to persuade someone to download and run software. CISA’s #StopRansomware Guide discusses drive-by downloads and sandboxed browsers as a protective measure. Visiting a website is not, by itself, proof of infection: the outcome depends on factors such as the device’s software, the vulnerability involved, and whether the exploit succeeds. A download prompt is also not evidence that a file is safe or legitimate.
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Vulnerable public-facing applications
Attackers may exploit a weakness in an internet-facing application to gain initial access without relying on someone opening an email attachment. For example, CISA and partner agencies documented Truebot activity exploiting CVE-2022-31199 in Netwrix Auditor, alongside other delivery methods, in their Truebot advisory. A vulnerability creates an opportunity, not an automatic infection: the affected software and exploit conditions matter.
Stolen credentials and third-party access
Malware or other malicious activity may follow an attacker’s use of stolen account credentials or access obtained through a supplier or managed service provider. These routes do not necessarily begin with a file arriving directly on the affected device. CISA’s ransomware guidance recommends phishing-resistant multi-factor authentication (MFA) and careful assessment of third-party access as controls addressing these risks.
Removable media
USB drives and other removable storage can carry malware between devices. CISA’s Truebot advisory and Emotet material document removable media as a delivery route. Treat unfamiliar storage devices cautiously, especially when there is no clear reason to trust their origin.
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How can you avoid downloading malware?
No single precaution blocks every route. Match the measure to the risk: updates address some software weaknesses, while careful message handling reduces the chance of being tricked into opening or downloading malicious content.
- Keep operating systems and applications updated. Updates can address known vulnerabilities that attackers may target. CISA recommends routine software and operating-system updates in its guidance on protecting data stored on devices.
- Use a standard user account for everyday work. CISA recommends avoiding routine use of an administrator account. This can limit what some malicious code can do with the account’s permissions; it is not a guarantee against infection.
- Be cautious with unexpected messages and downloads. Check whether a message is genuinely expected before opening an attachment or following a link. Organizations can reduce email-borne risk with filtering and phishing-awareness measures, as described in CISA’s ransomware guidance.
- Use phishing-resistant MFA where available, especially for organizational access. MFA helps protect accounts when credentials are compromised; phishing-resistant methods provide a stronger defense against credential theft than relying on a password alone.
- Limit and review third-party access. Organizations should assess the access granted to suppliers and managed service providers, rather than treating trusted business relationships as risk-free.
- Handle removable media carefully. Avoid connecting storage devices from unknown or untrusted sources.
- Keep backups. CISA recommends maintaining backups as part of protecting data. Backups support recovery if data is lost or damaged, but they do not prevent an initial infection.
For organizations, application allowlisting, endpoint detection, and sandboxed browsers are additional controls CISA discusses. They address different risks and should be treated as layers, not as a promise that every malicious file or behavior will be caught.
How do security researchers analyze malware?
Analysis can range from a timely summary of initial findings to deeper examination of a sample’s code. Researchers may also observe how a file or URL behaves in a sandbox, an isolated analysis environment. This is specialist work: an ordinary personal device is not a safe test environment for an unknown file.
Initial findings and indicators
CISA describes a Malware Initial Findings Report (MIFR) as a way to provide analysis to organizations in a timely manner. An initial report can communicate useful findings and indicators without claiming to explain every capability of a malware family or every stage of an incident.
Behavior observed in a sandbox
CISA and MS-ISAC’s Ransomware Guide describes sandbox analysis services that can scan a file or URL, observe it in a sandbox, and summarize activity such as files accessed, tasks created, and outbound connections. These are observations made for a particular sample in a particular environment; the guide describes service capabilities, not a guarantee that every behavior will appear or be detected.
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Deeper analysis and reverse engineering
A more detailed Malware Analysis Report (MAR) can include findings from manual reverse engineering, as illustrated by CISA’s MIFR-10050855-1.v2 report materials. This deeper work can help explain how code functions beyond what a quick scan or observed run reveals. The cited CISA materials describe report types and examples; they do not establish one universal laboratory procedure.
What an analysis result can—and cannot—show
A detection result, report, or sandbox summary is evidence about the sample and conditions examined, not a universal verdict about every file that looks similar. The result depends on what was submitted, what the analysis method can observe, and the environment in which behavior was assessed. No observed malicious activity is not proof that a file is safe.
Submitting a file or URL to an analysis service also raises handling and privacy questions. Before sharing a sample, consider whether it contains sensitive information, what the service’s sharing and confidentiality terms say, and whether you are authorized to submit it. Do not upload confidential work or personal files to a public scanning service without checking those conditions.
What should you do if you suspect an infection?
Treat a credible suspicion as a security incident rather than trying to confirm it by opening or running the suspicious file again. If the device belongs to an organization, contact its IT or security team. For a personal device, seek help from a qualified support professional when you are unsure how to proceed. The CISA guidance cited here supports seeking appropriate help but does not prescribe a universal step-by-step response for every device or incident.
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