Friday, October 11, 2013

Snort VRT Default Ruleset Rebalancing

In an upcoming Rule Update, the VRT will be shipping updated base policies for use in your Snort installation.

To help customers understand these changes, we are taking this opportunity to explain the process used by the VRT for deciding how rules are assigned to each policy.

The main metric used is the CVSS score assigned to each vulnerability that might be covered by a rule. For more information on CVSS please visit http://www.first.org/cvss. The second criteria is temporal based and concerns the age of a particular vulnerability. The final criteria is the particular area of coverage for the rule. So for example, SQL Injection rules are considered to be important enough to have influence when being considered for policy inclusion. Note that, the vulnerabilities covered by the rules in these categories are considered important regardless of age.

The considerations for each policy are described below.


Connectivity over Security Base Policy:

1. CVSS Score must be 10
2. Age of the vulnerability:

  • Current year (2013 for example)
  • Last year (2012 in this example)
  • Year before last (2011 in this example)

3. Rule Category

  • Not used for this policy


Balanced Base Policy:

(As a reminder, the "Balanced" policy is the default shipping state of the VRT Ruleset for Open Source Snort)

1. CVSS Score 9 or greater
2. Age of the vulnerability:

  • Current year (2013 for example)
  • Last year (2012 in this example)
  • Year before last (2011 in this example)

3. Rule Category

  • Malware-Cnc
  • Blacklist
  • SQL Injection
  • Exploit-kit

Security over Connectivity Base Policy:

1. CVSS Score 8 or greater
2. Age of the vulnerability:

  • Current year (2013 for example)
  • Last year (2012 in this example)
  • Year before last (2011 in this example)
  • Year prior (2010 in this example)

3. Rule Category

  • Malware-Cnc
  • Blacklist
  • SQL Injection
  • Exploit-kit
  • App-detect


All new rules are placed into the policies based on these criteria. Every year during the third quarter of the year, the policies will be re-assessed and rules from previous years, as the vulnerabilities age, will be removed from the policy to keep the policy compliant with our temporal selection criteria. Thus, in the third quarter of 2014, the rules from 2011 will be removed from the “Connectivity over Security” and “Balanced” policies while the rules from 2010 will be removed from the “Security over Connectivity” policy. If rules move between categories, their presence in policies will also be decided based on the category selection process. Likewise, should the CVSS score change for a particular vulnerability that is covered by a rule, it’s presence in a policy based on the CVSS metric is also re-assessed.

Rules in the listed policies are evaluated on a rule by rule basis. There will be some rules that are older and not in the criteria above that will be in the default policies. The above is the selection criteria for default rules, and is always subject to change based upon the threat landscape.

If there are any questions, feel free to email me @ joel [at] sourcefire [dot] com, or use the Snort-Sigs mailing list:

http://www.snort.org/community/mailing-lists