Event severity scoring

• Event severity scoring
  • Customizing event severity scoring

As Zeek logs are parsed and enriched prior to indexing, a severity score up to 100 (a higher score indicating a more severe event) can be assigned when one or more of the following conditions are met:

• cross-segment network traffic (if network subnets were defined)
• connection origination and destination (e.g., inbound, outbound, external, internal)
• traffic to or from sensitive countries
  • The comma-separated list of countries (by ISO 3166-1 alpha-2 code) can be customized by setting the SENSITIVE_COUNTRY_CODES environment variable in lookup-common.env
• domain names (from DNS queries and SSL server names) with high entropy as calculated by freq
  • The entropy threshold for this condition to trigger can be adjusted by setting the FREQ_SEVERITY_THRESHOLD environment variable in lookup-common.env. A lower value will only assign severity scores to fewer domain names with higher entropy (e.g., 2.0 for NQZHTFHRMYMTVBQJE.COM), while a higher value will assign severity scores to more domain names with lower entropy (e.g., 7.5 for naturallanguagedomain.example.org)
• file transfers (categorized by mime type)
• notice.log, intel.log and weird.log entries, including those generated by Zeek plugins detecting vulnerabilities (see the list of Zeek plugins under Components)
• detection of cleartext passwords
• use of insecure or outdated protocols
• tunneled traffic or use of VPN protocols
• rejected or aborted connections
• common network services communicating over non-standard ports
• file scanning engine hits on extracted files
• large connection or file transfer
  • The size (in megabytes) threshold for this condition to trigger can be adjusted by setting the TOTAL_MEGABYTES_SEVERITY_THRESHOLD environment variable in lookup-common.env
• long connection duration
  • The duration (in seconds) threshold for this condition to trigger can be adjusted by setting the CONNECTION_SECONDS_SEVERITY_THRESHOLD environment variable in lookup-common.env

As this feature is improved, it is expected additional severity scoring categories will be identified and implemented.

When a Zeek log satisfies more than one of these conditions its severity scores will be summed, with a maximum score of 100. A Zeek log’s severity score is indexed in the event.severity field and the conditions that contributed to its score are indexed in event.severity_tags.

Customizing event severity scoring

The category severity scores can be customized by editing logstash/maps/malcolm_severity.yaml:

• Each category can be assigned a number between 1 and 100 for severity scoring.
• Any category may be disabled by assigning it a score of 0.
• A severity score can be assigned for any supported protocol by adding an entry with the key formatted like "PROTOCOL_XYZ", where XYZ is the uppercased value of the protocol as stored in the network.protocol field. For example, to assign a score of 40 to Zeek logs generated for SSH traffic, you could add the following line to malcolm_severity.yaml:

"PROTOCOL_SSH": 40

Restart Logstash after modifying malcolm_severity.yaml for the changes to take effect.

Severity scoring can be disabled globally by setting the LOGSTASH_SEVERITY_SCORING environment variable to false in the logstash.env file and restarting Malcolm.