Which common warning signs indicate an email may contain a malicious attachment use email attachments as a primary attack vector to infect computer systems with malware. These emails can deliver a wide range of attacks including keyloggers that steal usernames and passwords, ransomware which encrypts files and demands payment for their return or even spyware that harvests sender information from email conversations and other personal data.

The good news is that there are several warning signs to look for which indicate an email may contain a malicious attachment. The most common is that the message is not written in the normal writing style of the sender or about topics they normally address. Spelling and grammar errors are another indicator. Another is if the email contains a link to an unexpected file type such as an MP3 or PDF. Finally, if the list of recipients includes undisclosed-recipients or unlisted-recipients this is often a sign that the email may be malicious.

Must-Reads for the Cyber-Savvy: Top Books on Cyber Security

A more sophisticated warning sign is when the email is sent to you from a public network or from a domain that is not yours. Additionally, the email should avoid asking for sensitive information such as bank accounts, social security numbers, credit card information or passwords as this is a clear indication of a phishing attempt.

Finally, the file extension is also a good indicator of possible danger. The most dangerous are executable files (.exe) that contain encoded instructions to set a function in motion such as installing software applications. Compressed files (.zip and.rar) are commonly used by attackers to hide malware from email security and other scanning systems. Even Microsoft Office files can be dangerous if they include macros that install spyware or other malware. If you receive a file from an unknown sender and it is one of these types, don’t open it and consider calling or texting them to confirm they did indeed send the email.