Firewall & Email Attachments: Risks?

TL;DR

Yes, a firewall can tamper with email attachments, but it’s rare and usually happens due to misconfiguration or malicious software. Firewalls scan for threats, and sometimes that process can corrupt files. Modern firewalls are less likely to do this directly, but older systems or those with poorly written rules could cause problems. The best defence is a good antivirus program, keeping your firewall updated, and being careful about opening attachments from unknown senders.

How Firewalls Handle Email Attachments

Firewalls sit between your computer/network and the internet. They inspect incoming and outgoing data – including emails and their attachments – to block anything suspicious. Here’s how they can interact with attachments:

1. Scanning for Viruses & Malware

1. Attachment Analysis: When an email arrives with an attachment, the firewall often sends a copy of that attachment to its antivirus engine.
2. Signature Matching: The antivirus looks for known virus signatures within the file.
3. Heuristic Analysis: If no signature is found, it might use ‘heuristic analysis’ – looking at how the file behaves to see if it looks malicious.
4. Action Taken: If a threat is detected, the firewall can block the email, quarantine the attachment, or delete it entirely.

This scanning process itself *can* sometimes cause issues.

2. Content Filtering

1. File Type Blocking: Firewalls can be configured to block certain file types (e.g., .exe, .zip) that are often used for spreading malware.
2. Size Limits: They might also have limits on attachment sizes.
3. Content Inspection: Some firewalls attempt to inspect the contents of attachments – looking for keywords or patterns associated with threats. This is more common in enterprise-level systems.

Incorrectly configured content filters can block legitimate files.

3. Deep Packet Inspection (DPI) & Attachment Corruption

Some advanced firewalls use DPI to examine the data within packets, including attachments. While rare, this process could lead to corruption if there’s a bug in the firewall software or an incompatibility with the file format.

4. Older Firewalls & Misconfiguration

1. Limited Resources: Older firewalls might have less processing power and memory, making them more prone to errors during scanning.
2. Poorly Written Rules: Incorrect or overly aggressive firewall rules can accidentally block or corrupt legitimate attachments. For example, a rule that incorrectly identifies a common file extension as malicious.

Example of checking basic firewall rules (Windows Firewall):

netsh advfirewall show allprofiles

5. Malware on the Firewall Itself

If the firewall itself is infected with malware, it could tamper with attachments before they reach you.

How to Protect Yourself

1. Keep Your Firewall Updated: Updates often include bug fixes and improved threat detection.
2. Use a Good Antivirus Program: A strong antivirus provides an extra layer of protection.
3. Be Careful with Attachments: Don’t open attachments from unknown senders or if the email seems suspicious.
4. Scan Attachments Before Opening: Even if your firewall doesn’t flag something, scan it manually with your antivirus before opening.
5. Check File Extensions: Be wary of unexpected file extensions (e.g., a document that ends in .exe).
6. Regularly Review Firewall Logs: Look for blocked files or suspicious activity.

Example of checking firewall logs (Linux using iptables):

sudo iptables -L

What to Do If an Attachment is Corrupted

• Don’t Open It: A corrupted attachment could contain malware.
• Delete the Email: Get rid of it immediately.
• Run a Full System Scan: Check your computer for viruses and other threats.
• Contact Your IT Department (if applicable): If you’re using a work network, let them know about the issue.