Exchange Attachment filtering must remove undesirable attachments by file type.

Overview

Finding ID	Version	Rule ID	IA Controls	Severity
V-80559	EX16-ED-000400	SV-95269r1_rule		Medium

Description
By performing filtering at the perimeter, up to 90 percent of spam, malware, and other undesirable messages are eliminated from the message stream rather than admitting them into the mail server environment. Attachments are being used more frequently for different forms of attacks. By filtering undesirable attachments, a large percent of malicious code can be prevented from entering the system. Attachments must be controlled at the entry point into the email environment to prevent successful attachment-based attacks. The following is a basic list of known attachments that should be filtered from Internet mail attachments: .ade .crt .jse .msi .scr .wsh .dir .adp .csh .ksh .msp .sct .htm .dcr .app .exe .lnk .mst .shb .html .plg .asx .fxp .mda .ops .shs .htc .spl .bas .hlp .mdb .pcd .url .mht .swf .bat .hta .mde .pif .vb .mhtml .zip .chm .inf .mdt .prf .vbe .shtm .cmd .ins .mdw .prg .vbs .shtml .com .isp .mdz .reg .wsc .stm .cpl .js .msc .scf .wsf .xml

Description

By performing filtering at the perimeter, up to 90 percent of spam, malware, and other undesirable messages are eliminated from the message stream rather than admitting them into the mail server environment. Attachments are being used more frequently for different forms of attacks. By filtering undesirable attachments, a large percent of malicious code can be prevented from entering the system. Attachments must be controlled at the entry point into the email environment to prevent successful attachment-based attacks. The following is a basic list of known attachments that should be filtered from Internet mail attachments: *.ade *.crt *.jse *.msi *.scr *.wsh *.dir *.adp *.csh *.ksh *.msp *.sct *.htm *.dcr *.app *.exe *.lnk *.mst *.shb *.html *.plg *.asx *.fxp *.mda *.ops *.shs *.htc *.spl *.bas *.hlp *.mdb *.pcd *.url *.mht *.swf *.bat *.hta *.mde *.pif *.vb *.mhtml *.zip *.chm *.inf *.mdt *.prf *.vbe *.shtm *.cmd *.ins *.mdw *.prg *.vbs *.shtml *.com *.isp *.mdz *.reg *.wsc *.stm *.cpl *.js *.msc *.scf *.wsf *.xml

STIG	Date
Microsoft Exchange 2016 Edge Transport Server Security Technical Implementation Guide	2019-03-12

Details

Check Text ( C-80291r1_chk )
Review the Email Domain Security Plan (EDSP). Determine the list of undesirable attachment types that should be stripped. Open the Exchange Management Shell and enter the following command: Get-AttachmentFilterEntry For each attachment type, if the values returned are different from the EDSP documented attachment types, this is a finding.

Fix Text (F-87411r1_fix)
Update the EDSP to reflect the list of undesirable attachment types that should be stripped. Open the Exchange Management Shell and enter the following command: Add-AttachmentFilterEntry -Name <'*.FileExtension'> -Type FileName Repeat the procedure for each undesirable attachment type.