Defense Evasion: Modify VPC Service Control

This document describes a threat finding type in Security Command Center. Threat findings are generated by threat detectors when they detect a potential threat in your cloud resources. For a full list of available threat findings, see Threat findings index.

Finding description

This finding isn't available for project-level activations.

Audit logs are examined to detect changes to VPC Service Controls perimeters that would lead to a reduction in the protection offered by that perimeter. The following are some examples:

  • A project is removed from a perimeter
  • An access level policy is added to an existing perimeter
  • One or more services are added to the list of accessible services

To respond to this finding, do the following:

Step 1: Review finding details

  1. Open the Defense Evasion: Modify VPC Service Control finding, as directed in Reviewing findings. The panel for the finding details opens, displaying the Summary tab.

  2. On the Summary tab, review the information in the following sections:

    • What was detected, especially the following field:
      • Principal email: the account that performed the modification.
    • Affected resource, especially the following field:
      • Resource full name: the name of the VPC Service Controls perimeter that was modified.
    • Related links:
      • Cloud Logging URI: link to Logging entries.
      • MITRE ATT&CK method: link to the MITRE ATT&CK documentation.
      • Related findings: links to any related findings.

  3. Click the JSON tab.

  4. In the JSON, note the following fields.

    • sourceProperties
      • properties
        • name: the name of the VPC Service Controls perimeter that was modified
        • policyLink: the link to the access policy that controls the perimeter
        • delta: the changes, either REMOVE or ADD, to a perimeter that reduced its protection
        • restricted_resources: the projects that follow the restrictions of this perimeter. Protection is reduced if you remove a project
        • restricted_services: the services that are forbidden from running by the restrictions of this perimeter. Protection is reduced if you remove a restricted service
        • allowed_services: the services that are allowed to run under this perimeter's restrictions. Protection is reduced if you add an allowed service
        • access_levels: the access levels that are configured to allow access to resources under the perimeter. Protection is reduced if you add more access levels

Step 2: Check logs

  1. On the Summary tab of the finding details panel, click the Cloud Logging URI link to open the Logs Explorer.
  2. Find admin activity logs related to VPC Service Controls changes by using the following filters:
    • protoPayload.methodName:"AccessContextManager.UpdateServicePerimeter"
    • protoPayload.methodName:"AccessContextManager.ReplaceServicePerimeters"

Step 3: Research attack and response methods

  1. Review the MITRE ATT&CK framework entry for this finding type: Defense Evasion: Modify Authentication Process.
  2. Review related findings by clicking the link on the Related findings on the Related findings row in the Summary tab of the finding details.
  3. To develop a response plan, combine your investigation results with MITRE research.

Step 4: Implement your response

The following response plan might be appropriate for this finding, but might also impact operations. Carefully evaluate the information you gather in your investigation to determine the best way to resolve findings.

  • Contact the owner of the VPC Service Controls policy and perimeter.
  • Consider reverting the changes for the perimeter until the investigation is completed.
  • Consider revoking Access Context Manager roles on the principal that modified the perimeter until the investigation is completed.
  • Investigate how the reduced protections have been used. For example, if "BigQuery Data Transfer Service API" is enabled, or added as allowed service, check who started using that service and what they are transferring.

What's next