Securing a FinTech Payment Platform Against Critical API Vulnerabilities
A Series A-stage FinTech startup engaged us to conduct a comprehensive API security assessment and cloud configuration review ahead of investor due diligence. We uncovered critical authorization flaws, exposed AWS credentials, and a range of high-severity issues that posed direct risk to their payment processing infrastructure.
Confidential engagement. NDA available upon request.
83%
Vulnerabilities Reduced
0
Critical Findings Remaining
89%
CIS Benchmark Score
14→3
Unprotected Endpoints
About the Client
Industry
FinTech
Company Size
45 to 60 employees, Series A stage
Background
A venture backed payment processing startup handling card transactions and ACH transfers for SMB merchants. The platform was preparing for a Series A funding round and required a clean security posture to satisfy investor due diligence requirements.
Security Challenges Identified
Broken Object-Level Authorization Across Payment APIs
Authenticated users could access other customers' transaction records by manipulating user ID parameters, exposing sensitive financial data across the entire user base.
Active AWS Credentials Exposed in Public JavaScript
Hardcoded AWS access keys with broad permissions were embedded in a public-facing JavaScript bundle, accessible to any visitor. This created immediate cloud infrastructure risk.
No Rate Limiting on Payment Initiation Endpoints
Payment processing endpoints accepted unlimited concurrent requests with no throttling, enabling brute-force enumeration, fraud automation, and denial-of-wallet attacks.
Overprivileged Lambda Functions and IAM Roles
Multiple AWS Lambda functions operated with administrator-level IAM permissions far exceeding their operational requirements, creating significant blast radius in a breach scenario.
The Mission
Identify and remediate all critical API security vulnerabilities and cloud misconfigurations before Series A investor due diligence, achieving measurable CIS benchmark compliance and eliminating all critical and high-severity findings.
How We Approached It
01. Scoping & Reconnaissance
Week 1- Authenticated and unauthenticated API enumeration via Burp Suite
- Cloud infrastructure asset discovery using AWS Inspector
- Authentication flow analysis and session management review
- Git repository scanning with Trufflehog and git-secrets
02. Automated Vulnerability Scanning
Week 1 to 2- Nessus credentialed scans across all identified assets
- OWASP ZAP automated API scanning with custom payloads
- AWS Inspector agentless vulnerability assessment
- AWS IAM Access Analyzer review for cross-account and external access
03. Manual API Penetration Testing
Week 2 to 3- BOLA/IDOR testing across all authenticated API endpoints
- Payment workflow business logic abuse scenarios
- Rate limit bypass and brute-force testing on sensitive endpoints
- Postman-based API fuzzing and parameter manipulation
04. Cloud Security Configuration Review
Week 3 to 4- CIS AWS Foundations Benchmark assessment
- IAM least privilege audit across all roles and policies
- Lambda function permission review and overprivilege identification
- S3 bucket access controls and public exposure testing
05. Reporting & Remediation Support
Week 4 to 5- Executive summary and technical findings report
- Remediation walkthrough sessions with development team
- Post-remediation verification testing
- Investor due diligence security summary letter
Vulnerabilities Discovered
2
CRITICAL
2
HIGH
2
MEDIUM
0
LOW
Broken Object Level Authorization (BOLA)
Authenticated users could access other users' transaction records by manipulating user ID parameters in API requests.
Authenticated users could access other users' transaction records by manipulating user ID parameters in API requests.
Hardcoded AWS Access Keys
Active AWS credentials were found embedded in a public-facing JavaScript file.
Active AWS credentials were found embedded in a public-facing JavaScript file.
Unrestricted API Rate Limiting
Payment initiation endpoints had no rate limiting, enabling brute-force and enumeration attacks.
Payment initiation endpoints had no rate limiting, enabling brute-force and enumeration attacks.
Insecure Direct Object Reference (IDOR)
Invoice download endpoints exposed sequential IDs, allowing unauthorized document access.
Invoice download endpoints exposed sequential IDs, allowing unauthorized document access.
Missing Security Headers
Absence of Content Security Policy (CSP) and X-Frame-Options headers increased XSS and clickjacking risk.
Absence of Content Security Policy (CSP) and X-Frame-Options headers increased XSS and clickjacking risk.
Overprivileged IAM Roles
Several Lambda functions operated with administrative-level IAM permissions far exceeding operational requirements.
Several Lambda functions operated with administrative-level IAM permissions far exceeding operational requirements.
How We Fixed It
BOLA Remediation
Implemented server-side authorization checks validating resource ownership on every API request, replacing client-supplied user IDs with server-validated session tokens.
Credential Rotation
Rotated all exposed AWS credentials, implemented AWS Secrets Manager for secure credential storage, and introduced pre-commit hooks to prevent future credential exposure.
Rate Limiting
Deployed API Gateway throttling policies with progressive rate limits and account lockout mechanisms on sensitive endpoints.
IDOR Fix
Replaced sequential numeric IDs with non-guessable UUIDs and added ownership validation on all document retrieval endpoints.
Security Headers
Configured strict security headers across all application responses including CSP, HSTS, X-Content-Type-Options, and X-Frame-Options.
IAM Hardening
Applied least privilege across IAM roles, reducing permissions to the minimum required for each function.
Measurable Outcomes
The client successfully completed investor security due diligence with no material findings raised, contributing to a successful funding round close.
83%
Vulnerability Reduction
0
Critical Remaining
89%
CIS Benchmark Score
79%
Attack Surface Reduction
Critical Vulnerabilities
2
0
High Vulnerabilities
4
0
Total Vulnerability Count
23
4
AWS Security Score (CIS)
41% compliance
89% compliance
Exposed Attack Surface
14 unprotected endpoints
3 low-risk endpoints remaining
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