FortiOS 6 – Custom Application & IPS Signatures

Custom Application & IPS Signatures

Creating a custom IPS signature

The FortiGate predefined signatures cover common attacks. If you use an unusual or specialized application or an uncommon platform, add custom signatures based on the security alerts released by the application and platform vendors.

You can add or edit custom signatures using the GUI or the CLI.

To create a custom signature

  1. Go to Security Profiles > Intrusion Prevention.
  2. Select [View IPS Signatures]
  3. Select Create New to add a new custom signature.
  4. Enter a Name for the custom signature.
  5. Enter the Signature. For information about completing this field, see Custom signature syntax and Custom signature keywords.
  6. Select OK.

Custom signature syntax

All custom signatures follow a particular syntax. Each begins with a header and is followed by one or more keywords. A custom signature definition is limited to a maximum length of 512 characters. A definition can be a single line or span multiple lines connected by a backslash (\) at the end of each line.

A custom signature definition begins with a header, followed by a set of keyword/value pairs enclosed by parenthesis [( )]. The keyword and value pairs are separated by a semi colon (;) and consist of a keyword and a value separated by a space. The basic format of a definition is HEADER (KEYWORD VALUE;)

You can use as many keyword/value pairs as required within the 512 character limit. To configure a custom signature, go to Security Profiles > Intrusion Prevention, select View IPS Signatures, select Create New, and enter the data directly into the Signature field, following the guidance in the next topics.

The table below shows the valid characters and basic structure. For details about each keyword and its associated values, see Custom signature keywords.

Valid syntax for custom signature fields

Field   Valid Characters     Usage
HEADER   F-SBID     The header for an attack definition signature. Each custom signature must begin with this header.
Field Valid Characters Usage
KEYWORD Each keyword must start with a pair of dashes (–), and consist of a string of 1 to 19 characters.

Normally, keywords are an English word or English words connected by an underscore (_). Keywords are case insensitive.

The keyword is used to identify a parameter.
VALUE Double quotes (“) must be used around the value if it contains a space and/or a semicolon (;).

If the value is NULL, the space between the KEYWORD and VALUE can be omitted.

Values are case sensitive.

Note: If double quotes are used for quoting the value, the double quotes are not considered as part of the value string.

The value is set specifically for a parameter identified by a keyword.

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One thought on “FortiOS 6 – Custom Application & IPS Signatures

  1. I need to look myself, but would be nice if there were some links to some examples of a signature to build off of. I’ve noticed a few customer apps that show up unknown, something I was interested in seeing here is more example of the syntax of even a pre-existing sig or few, but missing that, it’s a rather ambiguous article.

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