AntiVirus

This section describes how to configure the antivirus options. From an antivirus profile you can configure the FortiGate unit to apply antivirus protection to HTTP, FTP, IMAP, POP3, SMTP, IM, and NNTP sessions. If your FortiGate unit supports SSL content scanning and inspection, you can also configure antivirus protection for HTTPS, IMAPS, POP3S, SMTPS, and FTPS sessions. You can also apply flow-based antivirus projection to SMB or CIFS (Microsoft file sharing) traffic.

In many cases you can just customize the default antivirus profile and apply it to the security policy that accepts the traffic to be virus scanned. You can also create custom antivirus profiles if want to apply different types of virus protection to different traffic.

The following topics are included in this section:

• Antivirus concepts
• Enable antivirus scanning
• Grayware scanning
• Windows file sharing (CIFS) flow-based antivirus scanning
• Advanced Persistent Threat (APT) protection
• Testing your antivirus configuration
• Antivirus examples

Antivirus concepts

The word “antivirus” refers to a group of features that are designed to prevent unwanted and potentially malicious files from entering your network. These features all work in different ways, which include checking for a file size, name, or type, or for the presence of a virus or grayware signature.

The antivirus scanning routines your FortiGate unit uses are designed to share access to the network traffic. This way, each individual feature does not have to examine the network traffic as a separate operation, and the overhead is reduced significantly. For example, if you enable file filtering and virus scanning, the resources used to complete these tasks are only slightly greater than enabling virus scanning alone. Two features do not require twice the resources.

How antivirus scanning works

Antivirus scanning examines files for viruses, worms, trojans, and malware. The antivirus scan engine has a database of virus signatures it uses to identify infections. If the scanner finds a signature in a file, it determines that the file is infected and takes the appropriate action.

The most thorough scan requires that the FortiGate unit have the whole file for the scanning procedure. To achieve this, the antivirus proxy buffers the file as it arrives. Once the transmission is complete, the virus scanner examines the file. If no infection is present, it is sent to the destination. If an infection is present, a replacement message is set to the destination.

During the buffering and scanning procedure, the client must wait. With a default configuration, the file is released to the client only after it is scanned. You can enable client comforting in the Proxy Options profile to feed the client a trickle of data to prevent them from thinking the transfer is stalled, and possibly cancelling the download.

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Buffering the entire file allows the FortiGate unit to eliminate the danger of missing an infection due to fragmentation because the file is reassembled before examination. Archives can also be expanded and the contents scanned, even if archives are nested.

Since the FortiGate unit has a limited amount of memory, files larger than a certain size do not fit within the memory buffer. The default buffer size is 10 MB. You can use the uncompsizelimit CLI command to adjust the size of this memory buffer.

Files larger than the buffer are passed to the destination without scanning. You can use the Oversize File/Email setting to block files larger than the antivirus buffer if allowing files that are too large to be scanned is an unacceptable security risk.

Flow-based antivirus scanning

If your FortiGate unit supports flow-based antivirus scanning, you can choose to select it instead of proxy-based antivirus scanning. Flow-based antivirus scanning uses the FortiGate IPS engine to examine network traffic for viruses, worms, trojans, and malware, without the need to buffer the file being checked.

The advantages of flow-based scanning include faster scanning and no maximum file size. Flow-based scanning doesn’t require the file be buffered so it is scanned as it passes through the FortiGate unit, packet-by-packet. This eliminates the maximum file size limit and the client begins receiving the file data immediately. Also, flow-based scanning does not change packets as they pass through the FortiGate unit, while proxy-based scanning can change packet details such as sequence numbers. The changes made by proxy-based scanning do not affect most networks.

The trade-off for these advantages is that flow-based scans detect a smaller number of infections. Viruses in documents, packed files, and some archives are less likely to be detected because the scanner can only examine a small portion of the file at any moment. Also, the file archive formats flow-based scanning will examine are limited to ZIP and GZIP.

Antivirus scanning order

The antivirus scanning function includes various modules and engines that perform separate tasks.

Proxy-based antivirus scanning order

Figur e 3 on page 28 illustrates the antivirus scanning order when using proxy-based scanning. The first check for oversized files/email is to determine whether the file exceeds the configured size threshold. The uncompsizelimit check is to determine if the file can be buffered for file type and antivirus scanning. If the file is too large for the buffer, it is allowed to pass without being scanned. For more information, see the config antivirus service command. The antivirus scan includes scanning for viruses, as well as for grayware and heuristics if they are enabled.

Figure 3: Antivirus scanning order when using the normal, extended, or extreme database

If a file fails any of the tasks of the antivirus scan, no further scans are performed. For example, if the file fakefile.EXE is recognized as a blocked file pattern, the FortiGate unit will send the end user a replacement message, and delete or quarantine the file. The unit will not perform virus scan, grayware, heuristics, and file type scans because the previous checks have already determined that the file is a threat and have dealt with it.

Flow-based antivirus scanning order

Figur e 4 on page 29 illustrates the antivirus scanning order when using flow-based scanning

(i.e. the flow-based database). The antivirus scan takes place before any other antivirus-related scan. If file filter is not enabled, the file is not buffered. The antivirus scan includes scanning for viruses, as well as for grayware and heuristics if they are enabled.

Figure 4: Antivirus scanning order when using flow-based scanning

 

Antivirus databases

The antivirus scanning engine relies on a database of virus signatures to detail the unique attributes of each infection. The antivirus scan searches for these signatures, and when one is discovered, the FortiGate unit determines the file is infected and takes action.

All FortiGate units have the normal antivirus signature database but some models have additional databases you can select for use. Which you choose depends on your network and security needs.

Normal	Includes viruses currently spreading as determined by the FortiGuard Global Security Research Team. These viruses are the greatest threat. The Normal database is the default selection and it is available on every FortiGate unit.
Extended	Includes the normal database in addition to recent viruses that are no-longer active. These viruses may have been spreading within the last year but have since nearly or completely disappeared.
Extreme	Includes the extended database in addition to a large collection of ‘zoo’ viruses. These are viruses that have not spread in a long time and are largely dormant today. Some zoo viruses may rely on operating systems and hardware that are no longer widely used.

Antivirus techniques

The antivirus features work in sequence to efficiently scan incoming files and offer your network optimum antivirus protection. The first four features have specific functions, the fifth, heuristics, protects against new, or previously unknown virus threats. To ensure that your system is providing the most protection available, all virus definitions and signatures are updated regularly through the FortiGuard antivirus services. The features are discussed in the order that they are applied, followed by FortiGuard antivirus.

Virus scan

If the file passes the file pattern scan, the FortiGate unit applies a virus scan to it. The virus definitions are kept up-to-date through the FortiGuard Distribution Network (FDN). For more information, see “FortiGuar d Antivirus” on page 30.

Grayware

If the file passes the virus scan, it will be checked for grayware. Grayware configurations can be turned on and off as required and are kept up to date in the same manner as the antivirus definitions. For more information, see “Graywar e scanning” on page 35.

Heuristics

After an incoming file has passed the grayware scan, it is subjected to the heuristics scan. The FortiGate heuristic antivirus engine, if enabled, performs tests on the file to detect virus-like behavior or known virus indicators. In this way, heuristic scanning may detect new viruses, but may also produce some false positive results. You configure heuristics from the CLI.

FortiGuard Antivirus

FortiGuard Antivirus services are an excellent resource which includes automatic updates of virus and IPS (attack) engines and definitions, as well as the local spam DNS black list (DNSBL), through the FDN. The FortiGuard Center web site also provides the FortiGuard Antivirus virus and attack encyclopedia.

The connection between the FortiGate unit and FortiGuard Center is configured in  System > Config > FortiGuard.

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