Third-Party Compromise

1. Purpose & Scope

• Purpose:

  Provide a structured response for suspected or confirmed third-party compromise. Use this playbook to validate the report, assess exposure, restrict risky access, identify secondary impact, and restore trusted access safely.

• Scope:

  Applies to compromised vendors, suppliers, MSPs, SaaS providers, cloud providers, software dependencies, contractors, partners, and trusted integrations. Impact is not assumed; it must be confirmed through evidence.

2. Incident Identification & Criteria

Incident Type: Third-Party Compromise

Trigger Conditions:

Initiate this playbook when any of the following occur:

• A vendor, MSP, SaaS provider, cloud provider, supplier, or partner reports a breach or possible customer impact
• A third-party product, dependency, package, update, appliance, or managed service used by the organisation is compromised or exploited
• Third-party credentials, API keys, tokens, certificates, secrets, integrations, delegated access, or federation relationships are exposed or abused
• Threat intelligence or internal telemetry indicates trusted third-party access may have been used against organisational systems, data, or operations

Severity Levels:

3. Roles & Responsibilities

• Incident Commander: Sets severity, coordinates response, approves containment, and owns escalation decisions.
• Incident Responder / Forensic Analyst: Validates the report, scopes exposure, analyses activity, and preserves evidence.
• Communications Lead: Coordinates internal updates, vendor communications, and affected-user messaging.
• Other Roles:
  • Vendor / Third-Party Owner: Confirms business relationship, services used, access paths, and vendor contacts.
  • Identity & Access Team: Reviews SSO, federation, MFA, accounts, tokens, API keys, and privileged roles.
  • Cloud / SaaS / Platform Teams: Review integrations, audit logs, service accounts, and workload exposure.
  • Application / Service Owners: Validate affected dependencies, data access, business impact, and recovery needs.
  • Legal / Compliance / Vendor Risk: Assess contractual, regulatory, customer, partner, and vendor-risk obligations.

4. Initial Actions

• Immediate Steps:

  • Validate notification source, affected service, scope, and urgency before taking disruptive action.
    • 📘 RB-TRIAGE-004: Third-Party Notification Validation
  • Identify the affected vendor, product, service, integration, account, or dependency.
  • Confirm whether the organisation uses it and identify the internal business owner.
  • Engage IR leadership, vendor owner, identity/platform owners, and Legal as needed.
    • 📘 SOP-001: Paging an Engineering Team's On-Call

  Decision Point:

  • If the organisation does not use the affected vendor, product, service, or integration → document evidence and close or downgrade.
  • If the organisation uses it → continue to exposure assessment.
  • If active abuse is suspected or identified → begin containment in parallel.

5. Investigation & Analysis

Common Failure Modes

• Assuming third-party compromise automatically means internal compromise
• Assuming no internal impact exists because no initial indicators were found
• Missing delegated access, service accounts, OAuth apps, API keys, or stale vendor accounts
• Restoring trust before vendor remediation and internal exposure validation are complete
• Failing to assess downstream dependencies, customer impact, or delayed attacker activity

• Evidence Collection:

  • Collect vendor notices, advisories, IOCs, affected versions, timelines, and contacts.
  • Collect relevant logs and inventory for affected vendors, products, accounts, integrations, keys, data stores, and connected systems.

• Analysis Steps:

  • Determine what access the third party has to systems, data, identity, production, source code, CI/CD, or administration.
    • 📘 RB-ANALYSIS-027: Third-Party Exposure Assessment
  • Assess affected products, versions, dependencies, deployment footprint, downstream dependencies, and attack paths.
    • 📘 RB-ANALYSIS-028: Supply Chain Impact Analysis
  • Review vendor-related SSO, federation, VPN, API, service account, and privileged login activity.
    • 📘 RB-ANALYSIS-007: Authentication Log Analysis
  • Review privileged third-party accounts, delegated admin, OAuth apps, API keys, and managed service tooling.
    • 📘 RB-ANALYSIS-009: Privileged Access Assessment
  • Identify related accounts, integrations, service accounts, secrets, systems, and business units with shared exposure.
  • Review SaaS, cloud storage, file access, and integration activity for sensitive data access.
    • 📘 RB-ANALYSIS-020: Cloud Storage & SaaS Review
    • 📘 RB-ANALYSIS-021: Sensitive Data Impact Assessment

  Decision Point: Run the post-detection phases (Analysis → Recovery) of any relevant sibling playbook concurrently alongside this one based on what was observed:

  • If account compromise or credential abuse is suspected or identified → execute PB-004: Account Takeover concurrently.
  • If data exposure, staging, or exfiltration is suspected or identified → execute PB-005: Data Exfiltration concurrently.
  • If cloud compromise is suspected or identified → execute PB-007: Cloud Compromise concurrently.
  • If malware is suspected or identified → execute PB-003: Endpoint Malware Infection concurrently.
  • If privilege escalation is suspected or identified → execute PB-009: Privilege Escalation concurrently.
  • If lateral movement is suspected or identified → execute PB-010: Lateral Movement concurrently.

6. Containment, Eradication & Recovery

• Containment Actions:

  • Restrict exposed or unnecessary third-party accounts, delegated roles, service accounts, OAuth apps, API keys, tokens, VPN access, remote support tooling, and integrations.
    • 📘 RB-CONTAIN-008: Third-Party Access Restriction
  • Lock compromised accounts and revoke exposed sessions or tokens.
    • 📘 RB-CONTAIN-001: Account Lockdown
    • 📘 RB-CONTAIN-005: Session & Token Revocation
  • Block outbound transfers, attacker infrastructure, or malicious update paths when active abuse is observed.
    • 📘 RB-CONTAIN-007: Outbound Transfer Blocking
  • Reduce exposure for affected third-party software, appliances, integrations, or public-facing services.
    • 📘 RB-CONTAIN-014: Vulnerability Exposure Reduction

• Eradication Steps:

  • Remove stale vendor accounts, unused integrations, exposed secrets, overprivileged roles, and untrusted packages.
  • Rotate credentials, API keys, certificates, service account secrets, signing keys, webhook secrets, and tokens.
  • Patch, upgrade, rebuild, or replace affected third-party software and dependencies.

• Recovery Steps:

  • Restore third-party access only after remediation is validated, exposure is resolved, credentials are rotated, and monitoring is active.
    • 📘 RB-RECOVERY-005: Third-Party Trust Restoration
  • Validate business functionality after restoring approved integrations.
  • Monitor for delayed activity, credential reuse, malicious updates, API abuse, and abnormal vendor account activity.

  Decision Point: Do not restore trust because the vendor says the incident is closed. Restore only after internal exposure is assessed, remediation evidence is sufficient, and business owners approve remaining risk.

7. Communication & Escalation

• Internal Communication:

  • Notify security leadership, vendor owners, service owners, identity/platform teams, vendor risk, and affected stakeholders.
  • Use geo handoff when the incident spans response shifts.
    • 📘 SOP-002: Incident Handoff Between Geos
  • Notify affected users if credentials, MFA, access, device posture, or availability is impacted.
    • 📘 SOP-004: User Notification

• External Communication:

  • Engage Legal/Privacy for contractual, customer, regulator, partner, law enforcement, and executive communication decisions.
    • 📘 SOP-003: Escalation, Legal & Executive Communications
  • Route vendor communications through the Incident Commander, vendor owner, Legal, and Communications Lead.
    • 📘 SOP-005: Vendor Communication During a Security Incident

• Escalation Criteria:

  Condition	Escalate To
  Account compromise or credential abuse suspected or identified	PB-004: Account Takeover
  Data exposure, staging, or exfiltration suspected or identified	PB-005: Data Exfiltration
  Cloud compromise suspected or identified	PB-007: Cloud Compromise
  Malware suspected or identified	PB-003: Endpoint Malware Infection
  Privilege escalation suspected or identified	PB-009: Privilege Escalation
  Lateral movement suspected or identified	PB-010: Lateral Movement
  Widespread supply chain compromise, critical provider compromise, or major business impact	PB-019: Major Security Incident Management

8. Post-Incident Activities

• Lessons Learned:

  • Conduct a PIR covering notification handling, exposure, integrations, access restrictions, trust restoration, communications, and evidence gaps.
  • Review vendor risk posture, contractual requirements, access review cadence, monitoring, integration inventory, and dependency management.

• Documentation Updates:

  • Update this playbook, vendor inventory, integration inventory, access review procedures, detections, and trust restoration criteria.

9. References & Linked Resources

• Playbooks:

  • PB-003: Endpoint Malware Infection
  • PB-004: Account Takeover
  • PB-005: Data Exfiltration
  • PB-007: Cloud Compromise
  • PB-009: Privilege Escalation
  • PB-010: Lateral Movement
  • PB-019: Major Security Incident Management

• Runbooks:

  • RB-TRIAGE-004: Third-Party Notification Validation
  • RB-EVIDENCE-003: Identity & Authentication Log Acquisition
  • RB-EVIDENCE-004: Cloud Control Plane Log Acquisition
  • RB-ANALYSIS-007: Authentication Log Analysis
  • RB-ANALYSIS-009: Privileged Access Assessment
  • RB-ANALYSIS-011: Identify Additional Compromised Accounts
  • RB-ANALYSIS-020: Cloud Storage & SaaS Review
  • RB-ANALYSIS-021: Sensitive Data Impact Assessment
  • RB-ANALYSIS-027: Third-Party Exposure Assessment
  • RB-ANALYSIS-028: Supply Chain Impact Analysis
  • RB-CONTAIN-001: Account Lockdown
  • RB-CONTAIN-005: Session & Token Revocation
  • RB-CONTAIN-007: Outbound Transfer Blocking
  • RB-CONTAIN-008: Third-Party Access Restriction
  • RB-CONTAIN-014: Vulnerability Exposure Reduction
  • RB-RECOVERY-005: Third-Party Trust Restoration

• SOPs:

  • SOP-001: Paging an Engineering Team's On-Call
  • SOP-002: Incident Handoff Between Geos
  • SOP-003: Escalation, Legal & Executive Communications
  • SOP-004: User Notification
  • SOP-005: Vendor Communication During a Security Incident

10. Appendices

Contributor

Vishal Thakur GitHub: https://github.com/malienist

Contributed to the Arcana Incident Response Documentation Framework.