Is it safe to open an email attachment from an unknown sender?

Malicious email attachments can deliver a wide range of harmful software: ransomware that encrypts your files and demands payment, banking trojans that monitor your keystrokes, remote-access tools that give criminals control of your device, or simply data-harvesting software. The file formats vary — common malicious formats include Word documents (.doc, .docx) with malicious macros, PDF files exploiting reader vulnerabilities, executable files (.exe), and compressed archives (.zip, .rar) containing any of the above.

Many effective attacks come not from completely unknown senders but from compromised accounts belonging to someone you know — a colleague, friend, or supplier whose email account was hacked. The message may use a familiar name and include some contextual detail to appear plausible. The attachment name is typically generic: 'Invoice', 'Document', 'Receipt'.

Before opening any unexpected attachment, verify the sender through a separate channel — a phone call or a new email asking if they sent you a file. Never reply to the suspicious email itself, as you may be replying to the attacker who controls the compromised account.

Many business email attacks specifically target invoices and payment files, inserting malicious content into what appears to be a routine business document. Organisations should train all staff in this specific threat, as the consequences of ransomware infection can be severe.