Email attachments are one of the most common ways malware is distributed. Blocking high-risk file types can help reduce the risk of malicious software reaching users’ inboxes and improve the overall security of your email environment.
This article explains which attachment types are commonly considered risky and why organizations may choose to block them.
Table of Contents
Why block specific attachments?
Some file types can execute code directly on a computer or trigger automated actions when opened. If a malicious file reaches a user’s inbox and is opened, it may install malware, steal data, or compromise the device.
Blocking these file types before delivery reduces the likelihood of accidental execution and helps protect users from common email-based threats.
Common high-risk attachment types
While any file can potentially be harmful, certain attachment types are more frequently used in phishing and malware campaigns.
Executable files
Examples:
- .exe
- .com
- .bat
- .msi
These files are designed to run programs directly on a computer. For this reason, they are often blocked by email providers and security systems.
Script files
Examples:
- .vbs
- .js
- .ps1
Script files can automate actions on a device and may be used to download or execute malicious software.
Macro-enabled documents
Examples:
- .docm
- .xlsm
These file types can contain embedded macros that perform automated actions when opened. In some cases, attackers use malicious macros to compromise systems.
Compressed archives
Examples:
- .zip
- .rar
- .7z
Archives are commonly used for legitimate file sharing, but they can also be used to hide malicious files from basic filtering mechanisms.
Additional considerations
Password-protected archives
Encrypted archives cannot always be inspected by email security systems because their contents are protected by a password.
If you receive an unexpected password-protected archive, verify its source before opening it.
Large attachments
Unusually large attachments may not necessarily be malicious, but they can create storage and delivery issues. Many organizations choose to enforce attachment size limits as part of their email security policies.
Best practices for handling attachments
To improve email security:
- Do not open unexpected attachments from unknown senders.
- Verify suspicious files through a separate communication channel.
- Scan downloaded files with security software before opening them.
- Keep operating systems and antivirus software up to date.
- Use cloud storage links when sharing large files instead of email attachments.
Practical implications
Blocking high-risk attachment types helps reduce the likelihood of malware reaching users through email. However, attachment filtering should be combined with other security measures such as spam filtering, antivirus protection, user awareness training, and email authentication technologies.
No single security measure eliminates all threats, but a layered approach can significantly reduce risk.
Summary
Certain attachment types are commonly associated with malware distribution and phishing campaigns. By identifying and restricting high-risk file formats, organizations can strengthen email security and reduce the risk of malicious files being opened by users.
Attachment filtering is most effective when combined with broader email security practices and user awareness.