This article applies to:

• Trustwave MailMarshal
• External malware scanning products
• External file backup processes

Question:

• Why does MailMarshal unpack potential malware files to disk?
• Why is it necessary to exclude certain folders on a server from resident malware scanning?

Information:

MailMarshal unpacks the components of email messages to disk so that they can be evaluated by malware scanners and other technologies under MailMarshal control.

MailMarshal does not attempt execution or provide any environment for execution of any code that may be present in the unpacked files.

This is a standard procedure used by many gateway anti malware products to ensure full examination of the content of email messages. MailMarshal has used this method for over 20 years and is recognized as a technology leader in this area.

Other possible methods would introduce other risks and would not permit full examination of the material. 

• Email messages with attachments have a complex structure, and only a specialist product can fully unpack and examine the material.
• Message attachments in themselves can be complex (for example, zip archives). 
• Some external scanners (such as Crowdstrike) do not check the content of an email message or archive until another process unpacks it. 

Trustwave recommended best practices, such as the exclusion of specific folders from interference by uncontrolled anti-malware scanning, are the best assurance of safety. When uncontrolled scanners access the folders there is a risk that the original messages will be passed as clean without full examination. 

MailMarshal uses best efforts to determine the presence of uncontrolled scanners and will stop operations if any are detected. However, some scanners do not trigger on the industry standard EICAR virus test file, so MailMarshal is not able to determine the presence of these uncontrolled scanners automatically. One example of of a scanner that does not trigger on the presence of EICAR is Crowdstrike. 

Backup software can also lock files and must have the same exclusions.

It is essential that server administrators are aware of these issues and configure the servers correctly to ensure MailMarshal operation.

For details of the required exclusions see Trustwave Knowledgebase article Q10850.