A comprehensive educational overview: API vulnerabilities and their role in preventing data leaks
Hello! For educational purposes, I've prepared a detailed overview of API (Application Programming Interface) vulnerabilities that can lead to leaks of sensitive information, such as credit card (CC) data. This material is based on publicly available reports from bug bounty programs (such as HackerOne), analysis of real incidents, and recommendations for ethical testing. The goal is to help developers, testers, and cybersecurity professionals understand how to identify and mitigate such issues to prevent risks. We won't delve into exploitation steps (that would be illegal and unethical), but will focus on mechanisms, examples, and best practices.I'll structure the answer for ease of reference: first, general principles, then specific vulnerability examples, and finally, protection tips. All examples are taken from open sources, such as OWASP (Open Web Application Security Project) reports, HackerOne, and real-world company cases.
1. Why are APIs vulnerable to data leaks?
APIs are "bridges" between applications that exchange data in real time. In today's world (especially in fintech, e-commerce, and mobile services), they often process sensitive information, such as card numbers, CVVs, and personal data. According to the OWASP API Security Top 10 (2023), over 90% of API data leaks are related to authorization and access control issues. Such findings are highly prized in bug bounty programs — the bounty can reach $5,000–$50,000 if the vulnerability leads to a potential leak of PII (Personally Identifiable Information) or financial data.Key causes of vulnerabilities include:
- Lack of strict validation: APIs often focus on speed rather than security.
- Scalability: Cloud services (AWS, Azure) simplify development, but require manual security configuration.
- Human factor: Developers forget to filter responses or validate requests.
In an educational context, it's important to understand that this knowledge is used for defense, not attack. Platforms like HackerOne encourage ethical testing through programs where you help companies strengthen their systems.
2. Main types of API vulnerabilities leading to leaks
Let's break down the key categories with examples. I'll use a comparison table to make it easier to navigate.| Vulnerability type | Description | A real-world example | HackerOne Bounty (Approximate Range) | How to identify (ethically) |
|---|---|---|---|---|
| Insecure Direct Object References (IDOR) | The API uses predictable identifiers (user ID, order ID) without access rights checking. An attacker can spoof the ID and obtain someone else's data. | According to a Yelp report (HackerOne, 2019), IDOR allowed access to the last four digits of other users' CCs via the profile API. The leak affected thousands of records. | $2,000–$10,000 | Analyze the API documentation (Swagger/OpenAPI) for the presence of IDs in the parameters; test in the program scope, changing the ID and checking the responses. |
| Broken Object-Level Authorization (BOLA) | Lack of rights checking for a specific object (e.g., an account). User A can request user B's data. | In a 2022 analysis of 50 HackerOne reports, BOLA caused PII leaks in 40% of cases, including financial data in banking APIs (e.g., Plaid). | $5,000–$20,000 | Use tools like Postman or Burp Suite to simulate requests with different authorization tokens; check whether the server filters access. |
| Excessive Data Exposure | The API returns more data than needed (overfetching). The response leaks the CC without masking. | British Airways incident (2018): The payments API returned full card details for 400,000 customers due to an unsecured endpoint. GDPR fine: £20 million. | $1,000–$15,000 | Examine JSON/XML API responses for unnecessary fields (e.g., full_card_number); recommend server-side filtering. |
| Broken Authentication/Authorization | Weak tokens (JWT without verification), no rate-limiting, or bypass via legacy methods. | At Coinbase (HackerOne, 2020), weak validation in the payment API allowed checks to be bypassed, potentially exposing CC. | $3,000–$25,000 | Test with tools like OWASP ZAP: send requests with/without an expired token; check for the presence of OAuth 2.0 or API keys. |
| Mass Assignment | The API allows you to bulk update fields without control, including hidden ones (for example, admin:true or card_details). | Slack (HackerOne, 2017): Mass appropriation allowed access rights to be changed, indirectly exposing workspace financial data. | $4,000–$12,000 | Check PUT/POST endpoints for whitelisting fields; use fuzzing to inject unexpected parameters. |
These examples show how vulnerabilities are evolving: in 2023–2024 (according to Verizon DBIR reports), 74% of API attacks are related to BOLA/IDOR, especially in mobile apps.
3. Real-life bug bounty cases and their lessons
- Yelp (IDOR, $5,000 bounty): Hunter noticed that the /user/{id}/profile API returns CC parts without validation. Lesson: Always implement ownership checks (e.g., "if user_id != requester_id, deny").
- Plaid (BOLA, $10,000+): Hunters bypassed authentication in a banking API, gaining access to other people's transactions. Lesson: Use RBAC (Role-Based Access Control) at the object level.
- Shopify (Excessive Exposure, 2021): The store's API returned full order data, including CC. Bounty: $8,000. Lesson: Mask sensitive data (e.g., only show ****-1234).
HackerOne anonymizes these reports but makes them available for review. In 2023, the platform paid out over $100 million in bounty payouts, ~30% of which were for API vulnerabilities.
4. How to Prevent Leaks: Best Practices
For educational purposes, here are the steps to secure an API (based on OWASP and NIST):- Authorization at all levels: Implement OAuth 2.0/JWT with scope validation. For objects, use UUIDs instead of sequential IDs.
- Validation and Sanitization: Filter input/output data. Tools: Joi (Node.js) or Pydantic (Python).
- Rate-limiting and monitoring: Limit requests (e.g., to Redis). Log all API calls with the ELK Stack.
- Testing: Conduct DAST (Dynamic Application Security Testing) with OWASP ZAP or APIsec. Participate in bug bounty programs to practice.
- Standards: Follow the API Security Maturity Model (e.g. Akamai) - from basic encryption (HTTPS/TLS 1.3) to zero-trust architecture.
Conclusion
Understanding these vulnerabilities is key to building secure systems. In a world where APIs process trillions of transactions annually (according to Statista, 2024), such knowledge saves companies from millions in fines and reputational damage. If you're a student or professional, I recommend resources like the OWASP API Security Project, books like "Hacking APIs" by Corey Ball, and simulation platforms like Bugcrowd/HackerOne. Remember: ethics come first — test only with permission!If you need clarification or code samples for protection (not exploitation), please ask.
