[New Rule] File Creation with Curly Braces or Command Substitution

[Aegrah]

Summary

This rule detects the creation of files with names containing curly braces "{}" or command substitution "$()". Such naming conventions are often used in scripts to dynamically generate file names or execute commands, which can be exploited by attackers for obfuscation or evasion techniques. The presence of these characters in file names may indicate an attempt to create files in a non-standard manner, potentially as part of a malicious activity.

[github-actions]

Rule: New - Guidelines

These guidelines serve as a reminder set of considerations when proposing a new rule.

Documentation and Context

• Detailed description of the rule.
• List any new fields required in ECS/data sources.
• Link related issues or PRs.
• Include references.

Rule Metadata Checks

• creation_date matches the date of creation PR initially merged.
• min_stack_version should support the widest stack versions.
• name and description should be descriptive and not include typos.
• query should be inclusive, not overly exclusive, considering performance for diverse environments. Non ecs fields should be added to non-ecs-schema.json if not available in an integration.
• min_stack_comments and min_stack_version should be included if the rule is only compatible starting from a specific stack version.
• index pattern should be neither too specific nor too vague, ensuring it accurately matches the relevant data stream (e.g., use logs-endpoint.process-* for process data).
• integration should align with the index. If the integration is newly introduced, ensure the manifest, schemas, and new_rule.yaml template are updated.
• setup should include the necessary steps to configure the integration.
• note should include any additional information (e.g. Triage and analysis investigation guides, timeline templates).
• tags should be relevant to the threat and align/added to the EXPECTED_RULE_TAGS in the definitions.py file.
• threat, techniques, and subtechniques should map to ATT&CK always if possible.

New BBR Rules

• building_block_type should be included if the rule is a building block and the rule should be located in the rules_building_block folder.
• bypass_bbr_timing should be included if adding custom lookback timing to the rule.

Testing and Validation

• Provide evidence of testing and detecting the expected threat.
• Check for existence of coverage to prevent duplication.

[tradebot-elastic]

⛔️ Test failed

Results

• ❌ File Creation with Curly Braces or Command Substitution (eql)
  • coverage_issue: no_rta
  • stack_validation_failed: no_rta

[tradebot-elastic]

⛔️ Test failed

Results

• ❌ File Creation with Curly Braces or Command Substitution (eql)
  • coverage_issue: no_rta
  • stack_validation_failed: no_rta

[Samirbous]

this looks like an expensive query - match on any file creation with regex or wildcards, can't you limit to some processes or file paths ? its OK to push expensive queries if there are no other alternatives and the scenario is so common (top TTPs)