Tailoring Intrusion Protection to Your Network Assets

The following topics describe how to use Cisco recommended rules:

About Cisco Recommended Rules

You can use intrusion rule recommendations to associate the operating systems, servers, and client application protocols detected on your network with rules specifically written to protect those assets. This allows you to tailor your intrusion policy to the specific needs of your monitored network.

The system makes an individual set of recommendations for each intrusion policy. It typically recommends rule state changes for standard text rules and shared object rules. However, it can also recommend changes for preprocessor and decoder rules.

When you generate rule state recommendations, you can use the default settings or configure advanced settings. Advanced settings allow you to:

  • Redefine which hosts on your network the system monitors for vulnerabilities

  • Influence which rules the system recommends based on rule overhead

  • Specify whether to generate recommendations to disable rules

You can also choose either to use the recommendations immediately or to review the recommendations (and affected rules) before accepting them.

Choosing to use recommended rule states adds a read-only Cisco Recommendations layer to your intrusion policy, and subsequently choosing not to use recommended rule states removes the layer.

The system does not change rule states that you set manually:

  • Manually setting the states of specified rules before you generate recommendations prevents the system from modifying the states of those rules in the future.

  • Manually setting the states of specified rules after you generate recommendations overrides the recommended states of those rules.


    Tip

    The intrusion policy report can include a list of rules with rule states that differ from the recommended state.

While displaying the recommendation-filtered Rules page, or after accessing the Rules page directly from the navigation panel or the Policy Information page, you can manually set rule states, sort rules, and take any of the other actions available on the Rules page, such as suppressing rules, setting rule thresholds, and so on.


Note

The Talos Intelligence Group determines the appropriate state of each rule in the system-provided policies. If you use a system-provided policy as your base policy, and you allow the system to set your rules to the Cisco recommended rule state, the rules in your intrusion policy match the settings recommended by Cisco for your network assets.

Default Settings for Cisco Recommendations

When you generate Cisco recommendations, the system searches your base policy for rules that protect against vulnerabilities associated with your network assets, and identifies the current state of rules in your base policy. The system then recommends rule states and, if you choose to, sets the rules to the recommended states.

The system performs the following basic analysis to generate recommendations:

Table 1. Rule State Recommendations Based on Vulnerabilities
Rule Protects Discovered Assets? Base Policy Rule State Recommend Rule State

Yes

Disabled

Generate Events

Generate Events

Generate Events

Drop and Generate Events

Drop and Generate Events

No

Any

Disabled

Note the following in the table:

  • If a rule is disabled in the base policy, or set to Generate Events, the recommended state is always Generate Events.

    For example, if the base policy is No Rules Active, in which all rules are disabled, there will be no recommendations to Drop and Generate Events.

  • Recommendations to Drop and Generate Events are made only for rules already set to Drop and Generate Events in the base policy.

    If you want a rule to be set to Drop and Generate events and the rule was disabled or set to Generate Events in the base policy, you must manually reset the rule state.

When you generate recommendations without changing the advanced settings for Cisco recommended rules, the system recommends rule state changes for all hosts in your entire discovered network.

By default, the system generates recommendations only for rules with low or medium overhead, and generates recommendations to disable rules.

The system does not recommend a rule state for an intrusion rule that is based on a vulnerability that you disable using the Impact Qualification feature.

The system always recommends that you enable a local rule associated with a third-party vulnerability mapped to a host.

The system does not make state recommendations for unmapped local rules.

Advanced Settings for Cisco Recommendations

Include all differences between recommendations and rule states in policy reports

By default, an intrusion policy report lists the policy's enabled rules, that is, rules set to either Generate Events or Drop and Generate Events. Enabling the Include all differences option also lists the rules whose recommended states differ from their saved states. For information on policy reports, see About Configuration Deployment.

Networks to Examine

Specifies the monitored networks or individual hosts to examine for recommendations. You can specify a single IP address or address block, or a comma-separated list comprised of either or both.

Lists of addresses within the hosts that you specify are linked with an OR operation except for negations, which are linked with an AND operation after all OR operations are calculated.

If you want to dynamically adapt active rule processing for specific packets based on host information, you can also enable adaptive profile updates.

Recommendation Threshold (By Rule Overhead)

Prevents the system from recommending or automatically enabling intrusion rules with a higher overhead than the threshold you choose.

Overhead is based on the rule’s potential impact on system performance and the likelihood that the rule may generate false positives. Permitting rules with higher overhead usually results in more recommendations, but can affect system performance. You can view the overhead rating for a rule in the rule detail view on the intrusion Rules page.

Note that the system does not factor rule overhead into recommendations to disable rules. Also, local rules are considered to have no overhead, unless they are mapped to a third-party vulnerability.

Generating recommendations for rules with the overhead rating at a particular setting does not preclude you from generating recommendations with different overhead, then generating recommendations again for the original overhead setting. You get the same rule state recommendations for each overhead setting each time you generate recommendations for the same rule set, regardless of the number of times you generate recommendations or how many different overhead settings you generate with. For example, you can generate recommendations with overhead set to medium, then to high, then finally to medium again; if the hosts and applications on your network have not changed, both sets of recommendations with overhead set to medium are then the same for that rule set.

Accept Recommendations to Disable Rules

Specifies whether the system disables intrusion rules based on Cisco recommendations.

Accepting recommendations to disable rules restricts your rule coverage. Omitting recommendations to disable rules augments your rule coverage.

Generating and Applying Cisco Recommendations

Starting or stopping use of Cisco recommendations may take several minutes, depending on the size of your network and intrusion rule set.

Before you begin

  • Cisco recommendations have the following requirements:

    • Threat Defense License—IPS

    • Classic License—Protection

    • User Roles—Admin or Intrusion Admin

  • Configure a network discovery policy before you begin with the steps. Configure the network discovery policy to define internal hosts so that the Cisco recommendations are suitable. See, Network Discovery Customization.

Procedure

Step 1

In the Snort 2 intrusion policy editor's navigation pane, click Cisco Recommendations.

Step 2

(Optional) Configure advanced settings; see Advanced Settings for Cisco Recommendations.

Step 3

Generate and apply recommendations.

  • Generate and Use Recommendations—Generates recommendations and changes rule states to match. Only available if you have never generated recommendations.
  • Generate Recommendations—Regardless of whether you are using recommendations, generates new recommendations but does not change rule states to match.
  • Update Recommendations—If you are using recommendations, generates recommendations and changes rule states to match. Otherwise, generates new recommendations without changing rule states.
  • Use Recommendations—Changes rule states to match any unimplemented recommendations.
  • Do Not Use Recommendations—Stops use of recommendations. If you manually changed a rule's state before you applied recommendations, the rule state returns to the value you gave it. Otherwise, the rule state returns to its default value.

When you generate recommendations, the system displays a summary of the recommended changes. To view a list of rules where the system recommends a state change, click View next to the newly proposed rule state.

Step 4

Evaluate and adjust the recommendations you implemented.

Even if you accept most Cisco recommendations, you can override individual recommendations by setting rule states manually; see Setting Intrusion Rule States.

Step 5

To save changes you made in this policy since the last policy commit, click Policy Information, then click Commit Changes.

If you leave the policy without committing changes, changes since the last commit are discarded if you edit a different policy.

What to do next

  • Deploy configuration changes.

Script Detection

The script detection prevents the Snort blocks-too-late intrusion failures with a partial inspection. When HTML files are transferred between a client and a server, these files can contain malicious scripts, such as JavaScript, to initiate an attack. When such malicious scripts are found, the partial inspection allows any IPS rule to match on the malicious script, and the inspector flushes that data segment through inspection and detection. The malicious file never reaches its destination. This feature supports both HTTP/1 and HTTP/2 traffic.

This feature is always enabled by default. To turn it off, set http_inspect.script_detection=true to false.