Anthropic Excessive Chat Access Failures

AnalysisType: rule
RuleID: Anthropic.Activity.Excessive.Chat.Access.Failures
DisplayName: "Anthropic Excessive Chat Access Failures"
Enabled: true
Filename: anthropic_excessive_chat_access_failures.py
LogTypes:
  - Anthropic.Activity
Severity: Medium
Threshold: 50
DedupPeriodMinutes: 10
Description: >
  Detects when a single actor generates more than 50 chat access failures
  within a 10-minute window. Could indicate automated chat enumeration
  or unauthorized bulk access attempts. The claude_chat_id field identifies
  which chats were targeted — sequential or patterned IDs suggest scripted
  enumeration, while scattered IDs suggest shared-link browsing.
Runbook: |
  1. Find all Anthropic.Activity events by actor:email_address in the 1 hour before and after the alert to establish whether this is part of normal browsing or an isolated burst
  2. Compare the actor's claude_chat_access_failed count to their claude_chat_viewed count in the same window — a high failure-to-success ratio suggests enumeration rather than shared-link browsing
  3. Check if actor:ip_address appears in threat intelligence feeds or is associated with known VPN/proxy services
Tags:
  - Anthropic
  - Access Control
  - Enumeration
Reports:
  MITRE ATT&CK:
    - TA0007:T1087  # Account Discovery
Tests:
  - Name: Chat access failure event
    ExpectedResult: true
    Log:
      {
        "id": "evt_01ABC123",
        "created_at": "2026-04-28T08:00:00Z",
        "organization_id": "org_123",
        "type": "claude_chat_access_failed",
        "claude_chat_id": "chat_01ABC",
        "actor": {
          "type": "user_actor",
          "email_address": "user@example.com",
          "user_id": "user_01XYZ",
          "ip_address": "192.168.1.1",
          "user_agent": "Mozilla/5.0"
        }
      }
  - Name: Non-matching event type
    ExpectedResult: false
    Log:
      {
        "id": "evt_01DEF456",
        "created_at": "2026-04-28T08:00:00Z",
        "organization_id": "org_123",
        "type": "claude_chat_viewed",
        "claude_chat_id": "chat_01DEF",
        "actor": {
          "type": "user_actor",
          "email_address": "user@example.com",
          "user_id": "user_01XYZ",
          "ip_address": "192.168.1.1"
        }
      }
  - Name: API actor access failure
    ExpectedResult: true
    Log:
      {
        "id": "evt_01GHI789",
        "created_at": "2026-04-28T08:00:00Z",
        "organization_id": "org_123",
        "type": "claude_chat_access_failed",
        "claude_chat_id": "chat_01GHI",
        "actor": {
          "type": "api_actor",
          "api_key_id": "key_01ABC",
          "ip_address": "10.0.0.1"
        }
      }