Contents

Why is signature X retired?
What is the difference between disabled and retired?
Why are some new signatures disabled or retired by default?
Does Cisco IPS detect virus X?
How do I write custom signatures for Cisco IPS?
Can we have some Snort signatures ported to Cisco IPS?
Why is port 0 or address 0.0.0.0 displayed in alerts?
Why was signature X changed?
Does an obsolete signature need to be retired?
Why is an IPS not good at catching compressed malicious files?

Q: Why is signature X retired?

A: Signatures can be retired or disabled for a variety of reasons:

• The signature is “old” and of very little value.
• The vulnerability being detected is sufficiently old enough to be widely patched
• The vulnerability is unlikely to be exploited in the wild.
• The signature is more than 2 years old
• Specifications have changed and what was previously considered an indicator of malicious activity is now valid or no longer considered malicious. Any reporting of those signatures would essentially be false positives.
• The signature has a resources impact.
• The sensor resources are limited. Occasionally as new signatures are released, old signatures must be retired to ensure the sensor runs optimally.
• There is no way to run all signatures with the resources constraint, so the default shipping signature set must run a subset of all signatures.
• There have been reports of false positives and it is not possible to tune the signature to reduce false positives.
• The signature effectively detects a vulnerability potentially being exploited, but has the potential in many environments to produce false positive alerts. It is therefore disabled or retired to prevent “noise” in other customers' networks.

At any time, the end customer is still able to enable and unretire a signature if the customer is still running the affected/vulnerable software and needs the protection provided by the signature.

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Q: What is the difference between disabled and retired?

A: Disabled means that the signature does not produce an alert but is compiled into memory and inspection takes place. There are advantages of having signatures disabled, such as allowing the customer to quickly enable the signature without waiting for it to be loaded into memory and for inspection to take place.

Retired means that the signature is not loaded into memory at all and no inspection takes place.

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Q: Why are some new signatures disabled or retired by default?

A: New signatures may be disabled or retired by default in signature updates because the signature may

• Not be suitable for every customer
• Negatively affect customers' network traffic depending on where the sensor is deployed
• Be a policy signature that detects otherwise legitimate traffic that a customer may wish to block on the network
• Have concerns regarding memory or inspection time, but is otherwise suitable depending on network conditions

If you have a specific query about a specific signature, contact the Cisco TAC.

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Q: Does Cisco IPS detect virus X?

A: The IPS is not a suitable platform for antivirus because IPS units are generally placed at critical points in the network. Due to the network design, the IPS does not, or may not, see all the traffic to perform effective antivirus functions.

If a virus spreads using a vulnerability, signatures will cover the vulnerability being used to gain access to remote systems where possible. Because the IDS/IPS inspects network traffic, these systems cannot detect a virus that does not spread via the network.

Cisco may be able to provide a signature to help detect the effects of an infection to help contain infected workstations or devices.

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Q: How do I write custom signatures for Cisco IPS?

A: The white paper Writing Custom Signatures for the Cisco Intrusion Prevention System provides instruction in writing and testing signatures for Cisco IPS. The Cisco Intrusion Prevention System Engine Quick Reference describes methods for blocking certain types of traffic.

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Q: Can we have some Snort signatures ported to Cisco IPS?

A: Contact the Cisco TAC if you require Snort signatures to be ported to Cisco IPS. The TAC will be in the best position to determine how Cisco can help you complete this task.

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Q: Why is port 0 or address 0.0.0.0 displayed in alerts?

A: Summarization of events can cause an address of 0.0.0.0 to be displayed in alerts, and the majority of the time port 0 is shown. Sometimes attackers will use port 0 to try to bypass firewall port filtering rules.

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Q: Why was signature X changed?

A: Signatures may be changed for a variety of reasons:

• Signature or engine replacement: A new signature caused the previous signature to become obsolete, or the signature was moved to another engine.
• Cosmetic changes: Cosmetic changes occurred (for example, ensuring all regular expressions meet certain guidelines that do not affect how the signature operates).
• Signature fidelity: The signature fidelity rating has changed after actual field deployment has shown the signature is better or worse at detecting attacks than previously believed.
• Summary key: The summary key has changed (for example, Axxx may make more sense after signatures are deployed, but AxBx was used when the signature was first released).
• Memory/performance trade off: Based on detection history, the signature may be expanded or decreased in memory to increase coverage or improve performance.

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Q: Does an obsolete signature need to be retired?

A: The short answer is no. The longer answer is that any signature that is obsoleted by any another signature will be set to “enabled false, retired true” internally, regardless of the settings on the signature.

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Q: Why is an IPS not good at catching compressed malicious files?

An IPS as a network device would need to reassemble the packets to get the full file (no matter its size), then unpack it and scan with an antitvirus engine. If an IPS did that, customers would complain about the device being so slow. For detecting malicious files, an antivirus solution is still the tool of choice.

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This document is part of Cisco Security Intelligence Operations.

This document is provided on an "as is" basis and does not imply any kind of guarantee or warranty, including the warranties of merchantability or fitness for a particular use. Your use of the information on the document or materials linked from the document is at your own risk. Cisco reserves the right to change or update this document at any time.

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Cisco Security Intelligence Operations