Malware often uses file extensions that exploit automatic execution features in operating systems or applications. These extensions represent executable file types which can run code when opened, differing from harmless document formats like .txt or .jpg. Malicious files may disguise themselves using double extensions (e.g., "report.pdf.exe") or abuse trusted formats associated with scripts, macros, or installers to trick users into launching them.

Common malicious extensions include .exe (Windows executables), .vbs and .js (script files), .docm/.xlsm (macro-enabled Office documents), .ps1 (PowerShell scripts), and .jar (Java archives). Attackers frequently employ these in phishing emails (delivering .exe or .js ransomware) or compromised websites pushing fake installers (.exe/.msi). Ransomware like Locky often arrives via macro-enabled Office documents.

While blocking specific extensions offers basic protection, attackers can simply rename files. Effective defense requires layered security: enabling "show file extensions" in Windows, applying email attachment filtering to block dangerous types, disabling macros by default in Office, and maintaining robust endpoint security software. User education remains critical to prevent execution, as malware relies heavily on deception. Security teams continuously update filters to counter new obfuscation techniques.