How to Modify MailSite Express Active Content Filter

MailSite Express 7.0.3 and later contains active content filtering to protect against potentially harmful content from emails when they are opened in Express, to avoid damage to the users account or computer.

This document explains how you can turn off or adjust this feature.

You must be running cleanhtml.asp version 7.0.3.1 or later. If you have a previous version, please ensure you apply the hotfixes for Express which are available from the Rockliffe Customer Center

The MailSite Express file cleanhtml.asp has been introduced in version 7.0.3 to help safeguard users from image borne viruses, potentially harmful content on third party websites, and protect users against scripting attacks which can alter their account or steal information from their computer.

This file does NOT offer protection from all possible attacks but is designed to help reduce the possible vectors and methods available for such attacks. It does this by using Regular Expressions to remove any of the following from the message before display:

• <SCRIPT> sections
• ASP code
• External links and image references
• onEvent Javascript actions
• Javascript and expresion() instructions
• Encoded characters

If any of the above are removed from the email then a link is displayed above the filtered message allowing the user to view the message in its original form, if they trust the sender.

This file is editable to allow it to be fine tuned and new rules added to filter out as much, or as little as you want. (Requires knowledge of VBScript)

Filtering of long messages

The use of Regular Expressions to filter the content can cause a high CPU load on the server if a user is trying to open a very large email.

To alleviate this, a safeguard has been built in to stop the processing of messages over a certain number of characters in length. This is set by default to 50,000, but you can change this by altering the number found in line 12 of the file.

By default if the message is over the character limit it is returned unfiltered, however you can change this to display a security message by changing line 13 of the file thus:

MakeHTMLSafe = "The message is too large to safely check the contents for harmful content. Please either open the message from a Desktop E-mail Client or click the link above if you know the sender."

Express will then show this message instead of the email body and display the active content warning link to allow the user to view the full message if they trust the sender.

Turning off all filtering

If you decide that you want to disable the active content filtering entirely simply change line 12 of the file to read:

IF Len(strItem) > 0 Then