FortiOS 6 – AntiVirus

Archive scan depth

The antivirus scanner will open archives and scan the files inside. Archives within other archives, or nested archives, are also scanned to a default depth of twelve nestings. You can adjust the number of nested archives the FortiGate unit will scan with the uncompressed-nest-limit CLI command. Further, the limit is configured separately for each traffic type.

Configuring archive scan depth

For example, this CLI command sets the archive scan depth for SMTP traffic to 5. That is, archives within archives will be scanned five levels deep.

config firewall profile-protocol-options edit “default” config http set uncompressed-nest-limit 5




You can set the nesting limit from 2 to 100.

Scan buffer size

When checking files for viruses, there is a maximum file size that can be buffered. Files larger than this size are passed without scanning. The default size for all FortiGate models is 10 megabytes.

Archived files are extracted and email attachments are decoded before the FortiGate unit determines if they can fit in the scan buffer. For example, a 7 megabyte ZIP file containing a 12 megabyte EXE file will be passed without scanning with the default buffer size. Although the archive would fit within the buffer, the uncompressed file size will not.

Configuring the uncompression buffer

In this example, the uncompressed-oversize-limit CLI command is used to change the scan buffer size to 20 megabytes for files found in HTTP traffic:

config firewall profile-protocol-options edit <profile_name> config http set uncompressed-oversize-limit 20




The maximum buffer size varies by model. Enter set uncompressed-oversize-limit ? to display the buffer size range for your FortiGate unit.

Windows file sharing (CIFS)

FortiOS supports virus scanning of Windows file sharing traffic. This includes CIFS, SMB, and SAMBA traffic. This feature is applied by enabling SMB scanning in an antivirus profile and then adding this profile to a security policy that accepts CIFS traffic. CIFS virus scanning is available only through flow-based antivirus scanning.

FortiOS flow-based virus scanning can detect the same number of viruses in CIFS/SMB/SAMBA traffic as it can for all supported content protocols.

Note the following about CIFS/SMB/SAMBA virus scanning:

  • Some newer version of SAMBA clients and SMB2 can spread one file across multiple sessions, preventing some viruses from being detected if this occurs.
  • Enabling CIFS/SMB/SAMBA virus scanning can affect FortiGate performance. l SMB2 is a new version of SMB that was first partially implemented in Windows Vista.
  • Currently SMB2 is supported by Windows Vista or later, and partly supported by Samba 3.5 and fully support by Samba 3.6.
  • The latest version of SMB2.2 will be introduced with Windows 8. l Most clients still use SMB as default setting.

Configuring CIFS/SMB/SAMBA virus scanning

Use the following command to enable CIFS/SMB/SAMBA virus scanning in an antivirus profile:

config antivirus profile edit <smb-profile> config smb set options scan


Then add this antivirus profile to a security policy that accepts the traffic to be virus scanned. In the security policy the service can be set to ANY, SAMBA, or SMB.

config firewall policy edit <policy-id-integer> set service ANY


set utm-status enable set av-profile <smb-profile>


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