API Security Vendor Assessment Template

API integrations represent your highest concentration of third-party risk. Every vendor API endpoint creates a potential attack vector into your infrastructure. Traditional security questionnaires miss API-specific vulnerabilities—authentication bypass, injection attacks, excessive data exposure, and broken object-level authorization.

This template addresses the gap between general security assessments and API-specific risks. Built from 500+ vendor assessments across financial services, healthcare, and SaaS platforms, it captures the controls that actually prevent breaches. Each question maps to specific regulatory requirements and includes evidence collection guidance.

Your vendors won't have perfect answers. That's expected. The template helps you identify compensating controls and document accepted risks for your risk register. Use it during vendor onboarding, annual reviews, or when adding new API integrations to existing vendors.

Core Template Sections

1. API Inventory and Documentation

Start with visibility. You can't secure what you don't know exists.

Control questions:

• Complete API endpoint inventory with version numbers
• API documentation currency and accessibility
• Deprecated endpoint retirement process
• Change management procedures for API modifications

Evidence requirements:

• API documentation portal screenshots
• Sample endpoint inventory spreadsheet
• Change control tickets from last 90 days
• Deprecation notices and timelines

Red flags:

• No centralized API inventory
• Documentation older than 6 months
• Active use of deprecated endpoints
• No versioning strategy

2. Authentication and Authorization

Authentication failures cause a significant number of API breaches. This section validates implementation of OAuth 2.0, API keys, JWT tokens, and multi-factor authentication.

Technical controls assessed:

• Token expiration and rotation policies
• API key generation and distribution methods
• Role-based access control (RBAC) implementation
• Service account management procedures
• Session timeout configurations

Compliance mapping:

• SOC 2 CC6.1: Logical access controls
• ISO 27001 A.9.4.2: Secure authentication procedures
• PCI DSS 8.2: Strong authentication requirements
• HIPAA 164.312(a)(2)(i): Unique user identification

3. Data Protection and Encryption

Every API call transmits sensitive data. This section verifies encryption in transit and at rest, data classification, and field-level protections.

Assessment areas:

Control	Requirement	Evidence Type
TLS Version	1.2 minimum, 1.3 preferred	SSL Labs scan results
Certificate Management	Valid certificates, proper chain	Certificate details screenshot
Encryption at Rest	AES-256 or equivalent	Encryption policy document
Data Masking	PII/PHI field protection	Sample API responses
Key Management	HSM or KMS usage	Key rotation procedures

4. Rate Limiting and Resource Management

Unprotected APIs face denial-of-service attacks and resource exhaustion. Verify implementation of:

• Request rate limits per endpoint
• Concurrent connection limits
• Payload size restrictions
• Timeout configurations
• DDoS protection mechanisms

Scoring methodology:

High Risk (0-3 points): No rate limiting implemented
Medium Risk (4-6 points): Basic rate limiting, no monitoring
Low Risk (7-10 points): Advanced rate limiting with alerting

5. Monitoring and Incident Response

Real-time visibility prevents minor issues from becoming breaches. Evaluate:

Logging requirements:

• All API calls logged with timestamp, user, endpoint, response code
• Log retention period (minimum 90 days, 1 year for regulated industries)
• SIEM integration for automated alerting
• Anomaly detection capabilities

Incident response validation:

• API-specific incident response procedures
• Breach notification timelines
• Forensic capability for API traffic
• Regular tabletop exercises including API scenarios

Industry-Specific Applications

Financial Services

Additional controls for Open Banking and PSD2 compliance:

• Strong Customer Authentication (SCA) implementation
• Transaction signing mechanisms
• Consent management APIs
• Daily transaction limits
• FAPI compliance certification

Healthcare

HIPAA and HITRUST-specific requirements:

• Minimum necessary data exposure
• Audit controls for PHI access
• Break-glass procedures
• Patient consent verification
• FHIR conformance statements

Technology/SaaS

Focus on developer experience and scalability:

• API versioning strategy
• Backward compatibility commitments
• Developer portal security
• Sandbox environment isolation
• CI/CD pipeline security

Implementation Best Practices

1. Pre-Assessment Preparation

Send vendors the template 2 weeks before the assessment call. Include:

• Specific examples of acceptable evidence
• Your risk rating methodology
• Non-negotiable requirements
• Remediation timeline expectations

2. Evidence Collection Strategy

Week 1: Initial questionnaire submission
Week 2: Evidence review and follow-up questions
Week 3: Technical validation calls
Week 4: Risk scoring and remediation planning

3. Risk Scoring Framework

Weight responses based on data sensitivity and integration depth:

Factor	Weight	Justification
Data Classification	40%	PII/PHI exposure increases breach impact
Integration Type	30%	Write access poses higher risk than read-only
Authentication Method	20%	Weak auth enables unauthorized access
Monitoring Maturity	10%	Detection capability affects incident response

4. Continuous Monitoring

Static assessments become outdated within 90 days. Implement:

• Quarterly control attestations
• Annual full reassessment
• Automated API security scanning
• Change notification requirements

Common Implementation Mistakes

1. Accepting Generic Responses

Mistake: "We use industry-standard encryption" Fix: Require specific algorithms, key lengths, and certificate details

2. Ignoring Compensating Controls

Mistake: Failing vendor for missing one control Fix: Document alternative controls that achieve the same risk reduction

3. One-Size-Fits-All Scoring

Mistake: Same requirements for read-only vs. write-access APIs Fix: Create risk-tiered assessment tracks based on data sensitivity

4. Missing Business Context

Mistake: Pure technical assessment without business impact analysis Fix: Include questions about data volume, user count, and criticality

5. No Remediation Tracking

Mistake: Identifying gaps without follow-up Fix: Create time-bound remediation plans with monthly check-ins

Frequently Asked Questions

How long should vendors take to complete the API security assessment?

Experienced vendors complete initial responses within 3-5 business days. Evidence collection adds another week. Total assessment cycle runs 2-3 weeks including validation calls.

Which API security frameworks should we reference beyond OWASP API Top 10?

Include NIST 800-204 (Microservices Security), ISO 27034 (Application Security), and industry-specific standards like FAPI for financial services or FHIR for healthcare.

Should we assess internal APIs with the same rigor as external vendor APIs?

Yes. Internal APIs often have weaker security controls due to assumed trust. Use the same template but adjust risk scoring based on network segmentation.

How do we handle vendors who claim proprietary API security methods?

Require demonstration in a controlled environment. Proprietary doesn't mean unverifiable. If they can't prove security effectiveness, score as high risk.

What's the minimum acceptable TLS version for API connections in 2024?

TLS 1.2 remains the baseline requirement. TLS 1.3 is preferred for organizations handling sensitive data. Anything below TLS 1.2 is an automatic failure.

How often should we revalidate API security controls for critical vendors?

Critical vendors need quarterly attestations and annual full reassessments. Medium-risk vendors can move to semi-annual reviews after establishing 12 months of clean assessments.

Can we automate parts of the API security assessment process?

Yes. Automated API scanning tools can validate TLS configuration, test rate limits, and check authentication endpoints. Manual review remains necessary for architecture decisions and incident response procedures.