Block Screenshot Traffic with a Firewall

TL;DR

Firewalls generally can’t directly identify screenshots as they look like normal image traffic (JPG, PNG). However, you can block common screenshot tools and their network activity using application control features, URL filtering, or by inspecting traffic patterns. This isn’t foolproof but significantly reduces the risk.

How to Block Screenshot Traffic

1. Understand the Challenge

• Screenshots are typically sent as image data (JPG, PNG, GIF) over standard protocols like HTTP/HTTPS. Firewalls see this as regular web traffic.
• Most screenshot tools don’t have unique network signatures that a firewall can easily identify.
• Users could bypass restrictions by using built-in OS tools or less common software.

Block Common Screenshot Applications

Many firewalls offer application control features. Use these to block known screenshot applications.

• Windows: Block Snagit, Greenshot, Lightshot, and the built-in Snipping Tool (though blocking the Snipping Tool is often impractical as it’s core OS functionality).
• macOS: Block Skitch, CleanShot X, and the macOS screenshot utility. (Blocking the macOS utility is generally not recommended.)
• Linux: Block Flameshot, Spectacle (KDE), or other common screenshot tools.

The exact method for blocking applications varies depending on your firewall vendor (e.g., Palo Alto Networks, Fortinet, Cisco). Consult your firewall documentation.

URL Filtering

Some screenshot tools upload images to cloud services like Imgur or Dropbox. Block access to these sites:

• Create URL filtering rules to block imgur.com, dropbox.com, and other image hosting/sharing websites.
• Be careful not to block legitimate business use of these services. Consider creating exceptions for specific users or groups.

Traffic Pattern Analysis (Advanced)

This is more complex but can be effective.

• Identify Screenshot Tool Network Behaviour: When a screenshot tool captures an image and uploads it, it often creates a specific pattern of network requests. For example, a small initial request followed by a larger upload.
• Create Firewall Rules Based on Patterns: Configure your firewall to look for these patterns. This might involve inspecting packet sizes, timing, or destination ports.
• Example (Conceptual): If a screenshot tool uploads images via HTTPS with a specific User-Agent string and a characteristic file size range, you could create a rule like this (syntax will vary by firewall):

  
  # Example - Replace with your firewall's syntax!
  rule block_screenshot_upload {
    protocol https;
    user_agent "ScreenshotTool/1.0";
    file_size > 10KB and file_size < 500KB;
    destination port 443;
  }
  

• Caution: This method can lead to false positives (blocking legitimate traffic) if not carefully configured.

Data Loss Prevention (DLP) Solutions

Dedicated DLP solutions are the most effective way to prevent sensitive data from being captured in screenshots.

• DLP tools can inspect screen content and block screenshot attempts if they detect confidential information.
• These solutions typically require more investment and configuration than firewall-based methods.

Regularly Update Rules

New screenshot tools are constantly being developed. Keep your application control and URL filtering rules up to date.