How to find out if a file is dangerous before opening it

You received a file via email and something seems off. Or maybe you downloaded a program from the internet and you suspect it might contain a virus. Before opening it (and risking compromising your system), there are several ways to understand if a file is potentially harmful. In this guide, I explain how to do it, even without technical skills, using simple and free tools.

1. Check the file extension

Malicious files often disguise themselves with deceptive extensions. Here’s what to check:

• Double files (e.g.: documento.pdf.exe): these are executables disguised as PDFs.
• Rare or strange extensions (.scr, .bat, .js, .vbs, .msi): better not to open them unless you’re sure of the source.
• ZIP or RAR files: they aren’t always dangerous, but they can contain infected files. Never extract them lightly.

Tip: enable file extension viewing in Windows to clearly see what you are opening. Go to File Explorer > View > Show > File name extensions.

2. Verify the digital signature

Safe files, especially those distributed by well-known companies, are digitally signed. Just right-click > Properties > Digital Signature to see if the file is signed and by whom.

• If the signature is missing or the sender is unknown: better investigate before opening it.
• If the author’s name doesn’t match the person who sent you the file: it could be a sign of compromise.

3. Analyze the file with VirusTotal

VirusTotal is one of the best online tools to check if a file contains malware. Here’s how it works:

1. Go to virustotal.com.
2. Upload the suspicious file.
3. Wait for the analysis (uses over 70 antivirus engines simultaneously).
4. Read the result: if even a single antivirus detects it as suspicious, avoid opening it.

You can also use VirusTotal to check suspicious links before clicking on them.

4. Use an online sandbox service

A sandbox is an isolated environment where you can test a file without harming your PC. Some services offer this for free:

• Any.Run
• Hybrid Analysis
• Joe Sandbox

Basically, the file runs in a virtual environment and its behavior is recorded: if it tries to access the registry, open suspicious internet connections, or install strange services, you’ll know.

5. Analyze the file with your antivirus

It seems obvious, but we often forget. If you have an updated antivirus:

• Right-click on the file > Scan with [antivirus name].
• Check if the file is flagged.

Warning: a dangerous file can go unnoticed if the antivirus is not updated. That’s why using VirusTotal is also a smart move.

6. Check the file’s reputation (hash, name, author)

Every file has a hash (a unique fingerprint). Once you have the hash, you can search for it on Google or VirusTotal to see if it has already been reported by other users or security systems.

7. Trust your common sense (and the golden rule)

If the file came from an unknown sender, contains strange names, is attached to an email in English, or urges you to click quickly… don’t open it. Even if it looks like a PDF or a Word document, it may contain infected macros or exploits.

Golden rule: if you have a doubt, it’s better to spend 5 minutes checking than to risk compromising the entire system (especially if you use your PC for work).

What to do if you’ve already opened a suspicious file

If you already clicked (it happens to everyone), here are the steps to follow:

1. Disconnect your PC from the internet immediately.
2. Run a full scan with your antivirus.
3. Download Malwarebytes and perform a second scan.
4. Check what starts with Windows (use Autoruns o Task Manager).
5. Change your passwords immediately if you suspect they have been stolen.

Conclusions

Opening a file without checking it is a risk we can no longer afford today. With the increase in attacks via email, WhatsApp, Telegram, or scam websites, it is essential to learn to read the warning signs, use free and reliable tools, and keep a cool head. In a few minutes, you can protect yourself from viruses, ransomware, and data theft.

Did you find this guide useful? On our website, you will find many other practical tutorials to solve everyday computer problems. If you are interested in a specific topic, write to us and we will add it to our list!