[New] Potential cPanel WHM CRLF Authentication Bypass (CVE-2026-41940)

[eric-forte-elastic]

Pull Request

Issue link(s):

Summary - What I changed

Network detection rule based on info from https://www.unfold.ai/blog/cpanel-exploit-cve-2026-41940. The primary event we are detecting is a GET call to / using basic authentication. This should not typically occur as this endpoint does not require authentication to reach.

Given that this requires some specific network packet capture policy additions for this integration it is unclear if this should be included in our standard protections.

Note

Default network packet capture policy for HTTP will not catch this traffic. In order to use this rule one will need to follow the steps in the guide to capture headers and monitor the admin ports shown below.

How To Test

Use RTA from https://github.com/elastic/cortado/pull/34/changes#diff-fe0955063d14991a45b359397e8a394d07da51317dce2a03721202d00cabe034.

Result from 2 runs

Checklist

• Added a label for the type of pr: bug, enhancement, schema, maintenance, Rule: New, Rule: Deprecation, Rule: Tuning, Hunt: New, or Hunt: Tuning so guidelines can be generated
• Added the meta:rapid-merge label if planning to merge within 24 hours
• Secret and sensitive material has been managed correctly
• Automated testing was updated or added to match the most common scenarios
• Documentation and comments were added for features that require explanation

Contributor checklist

• Have you signed the contributor license agreement?
• Have you followed the contributor guidelines?

[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

• ❌ cPanel WHM CRLF Authentication Bypass (CVE-2026-41940) (lucene)
  • coverage_issue: no_rta
  • stack_validation_failed: no_rta

[tradebot-elastic]

⛔️ Test failed

Results

• ❌ cPanel WHM CRLF Authentication Bypass (CVE-2026-41940) (lucene)
  • coverage_issue: no_rta
  • stack_validation_failed: no_rta

[tradebot-elastic]

⛔️ Test failed

Results

• ❌ cPanel WHM CRLF Authentication Bypass (CVE-2026-41940) (lucene)
  • coverage_issue: no_rta
  • stack_validation_failed: no_rta

[tradebot-elastic]

⛔️ Test failed

Results

• ❌ cPanel WHM CRLF Authentication Bypass (CVE-2026-41940) (lucene)
  • coverage_issue: no_rta
  • stack_validation_failed: no_rta

[tradebot-elastic]

⛔️ Test failed

Results

• ❌ cPanel WHM CRLF Authentication Bypass (CVE-2026-41940) (lucene)
  • coverage_issue: no_rta
  • stack_validation_failed: no_rta

[tradebot-elastic]

⛔️ Test failed

Results

• ❌ Potential cPanel WHM CRLF Authentication Bypass (CVE-2026-41940) (lucene)
  • coverage_issue: no_rta
  • stack_validation_failed: no_rta

[terrancedejesus]

the pre-auth requirement worth adding an OR here?

ref: https://github.com/debugactiveprocess/cPanel-WHM-AuthBypass-Session-Checker/blob/d58fdeee4d3caa2ecd39e35cfa8da5efcfc8cb4b/check_session.py#L61

[eric-forte-elastic]

Taking a look 👀 Generally, that is looking for the stage 1 part of the exploit where this detection is focused on the stage 2 part.

[terrancedejesus]

rgr that!

[tradebot-elastic]

⛔️ Test failed

Results

• ❌ Potential cPanel WHM CRLF Authentication Bypass (CVE-2026-41940) (lucene)
  • coverage_issue: no_rta
  • stack_validation_failed: no_rta

[terrancedejesus]

Given that this requires some specific network packet capture policy additions for this integration it is unclear if this should be included in our standard protections.

Given the severity / risk and global adoption and we have the capability to detect, I don't see why not.
If we have 0 adoption, we can move to a hunting query / doc.
Network rules for vuln-exploit / probing are usually hit or miss with adoption.

[tradebot-elastic]

⛔️ Test failed

Results

• ❌ Potential cPanel WHM CRLF Authentication Bypass (CVE-2026-41940) (lucene)
  • coverage_issue: no_rta
  • stack_validation_failed: no_rta