Introduction to Carding and Payment Gateway API Vulnerabilities
Carding is a type of cybercrime in which attackers (carders) steal, test, and use credit card data for fraudulent transactions. Payment gateways such as Stripe, PayPal, and others are often integrated through APIs (Application Programming Interfaces), which allow apps and websites to process payments. These APIs are targeted because they handle sensitive information, such as card numbers, CVV codes, expiration dates, addresses, and personally identifiable information (PII). Vulnerabilities arise from poor configuration, coding errors, or insufficient monitoring, allowing automated attacks to bypass defenses.For educational purposes, it is important to understand that such attacks do not require physical access — they are carried out remotely over the network. According to reports such as those from OWASP (Open Web Application Security Project), API vulnerabilities are among the top 10 security risks. For example, in 2025, there were cases where outdated API versions were used to validate stolen cards, affecting dozens of websites. Below, I'll discuss these methods in more detail, focusing on the concepts without providing technical implementation instructions. This will help you understand the risks and security principles.
1. Testing and validation of stolen cards (carding and card cracking)
Carders often start with databases of stolen cards obtained from previous breaches (for example, via the dark web). Payment gateway APIs allow card validity checks without a full transaction — a legitimate feature for businesses, but attackers exploit it.- Carding: Attackers use bots to automatically test cards. The bots send API requests for small authorizations (e.g., 1 cent), checking whether the card is active. If the API doesn't rate limit requests or detect suspicious patterns (e.g., multiple requests from a single IP), the attack scales. Bots can impersonate legitimate devices, using proxy servers to rotate IP addresses to avoid blocking. This leads to "card stuffing" — mass testing where working cards are selected from thousands of cards for further use in purchases or sales.
- Card Cracking: If a carder has partial data (card number, but without CVV or expiration date), brute-force attacks are used. The API may allow multiple validation attempts if there is no CAPTCHA or behavioral analysis. Attackers generate combinations (for example, CVV values from 000 to 999) and test them through the API. In some cases, legacy API versions, like Stripe's, are used for this purpose because they have fewer security features and allow validation without full authorization. This is particularly dangerous because such attacks can occur undetected, without the cardholder's knowledge.
Educational aspect: These methods exploit the lack of a "zero-trust" model, where each request is independently verified. Statistics show that carding attacks constitute a significant portion of online fraud, resulting in billions of dollars in losses annually.
2. Stealing API keys and secrets
Payment gateway APIs require authentication, often through API keys or tokens. Carders focus on stealing these.- Access methods: Keys can be found in public GitHub repositories (if developers accidentally commit them), through phishing (deceiving employees), or through malware on servers. With the key, an attacker can log in to the API and extract data — for example, transaction lists or card details. In 2025, there were cases of keys being used to make unauthorized requests, masquerading as legitimate applications.
- Exploiting Weak Authentication: If an API uses basic authentication without multi-factor authentication (MFA) or OAuth, attacks are easier. For example, in open banking (open banking APIs), weak authentication allows unauthorized access to payment data.
Educational aspect: This highlights the importance of "secret management" — tools like Vault or AWS Secrets Manager for storing keys. Without them, a single compromised key can lead to a massive breach.
3. Manipulating business logic and queries
APIs not only transfer data but also implement business logic, such as calculating amounts or validating transactions. Vulnerabilities here arise due to design flaws.- Parameter Tampering: Attackers modify requests (e.g., POST parameters) to change prices, quantities, or transaction details. If the API doesn't validate input on the server (server-side validation), this allows them to bypass validation checks. For example, in a shopping cart, the "price" parameter could be modified, leading to a purchase at zero price.
- HTTP Parameter Pollution (HPP): If an API processes duplicate parameters (for example, customer_id twice), an attacker can insert their own, overwriting the original. This allows access to someone else's payment data, as in the cases of pentest reports.
- Broken Object Level Authorization (BOLA/IDOR): An API may allow access to objects (e.g., transactions) by ID if there is no permissions check. Carders guess the ID and extract data from other users.
Educational aspect: These vulnerabilities are part of the OWASP API Top 10. They demonstrate why business logic testing is critical: attacks are not always technical, but logical.
4. Data Interception (Man-in-the-Middle and Skimming)
- MitM attacks: If an API uses HTTP instead of HTTPS or weak encryption, traffic is intercepted (for example, on public Wi-Fi). Attackers extract data from requests and responses.
- Web Skimmers: Inject malicious JavaScript into a website (via supply chain attacks or CMS vulnerabilities). A skimmer replaces card entry forms with fake ones, collecting data before sending it to a legitimate API. In 2025, such skimmers used fake iframes to bypass Content Security Policy (CSP), affecting Stripe and similar companies.
- Injection Attacks: SQL or command injection in API requests allows for the extraction of databases containing cards, as in historical breaches (e.g. Heartland in 2009, but still relevant today).
Educational aspect: This demonstrates the evolution of attacks — from simple interceptions to complex overlays. Standards like PCI DSS require encryption and monitoring for prevention.
5. Scaling and combinations of methods
Carders combine approaches: stealing access keys, using bots for testing, and injecting code for scrimming. Tools like proxy farms and device emulators allow for scalable attacks. In 2025, attacks on legacy APIs and open banking will increase.Prevention: Educational Recommendations
For protection use:- Multi-level authentication: MFA, OAuth 2.0, rate limiting.
- Monitoring and ML: Bot detection through behavioral analysis (e.g. DataDome).
- Encryption and validation: HTTPS, server-side checks, tokenization (replacing data with tokens).
- Regular audits: Penetration testing, PCI DSS compliance.
- Zero-Trust: Validate every request, regardless of source.
Understanding these concepts helps developers and businesses minimize risks, making digital payments more secure.