In this blog, we will explain what changes when a payment gateway or third-party integration is added to a web application. We will look at payment requests, APIs, redirects, callbacks, sensitive data, security misconfigurations, business logic, and common payment-related risks.
We will also discuss how Web Application Penetration Testing can help identify weaknesses in payment and third-party integrations. The blog will cover practical security checks and ways businesses can protect their applications after adding new integrations.
How Can Payment Gateway Integration Create Security Weaknesses?
A payment gateway can introduce security weaknesses when the application trusts client-controlled values, accepts payment responses without proper verification, or does not correctly connect payment information with the related order.
1. Payment Amount Manipulation
The payment amount is one of the most sensitive values in a checkout process. If the browser sends the amount to the payment gateway and the server does not independently verify it, an attacker may try to change the value.
For example, an order worth ₹5,000 should not become a ₹500 payment simply because a client-side parameter was changed. The server should calculate or verify the payable amount using trusted order data before processing the transaction.
During VAPT, testers can compare the amount displayed in the application with the amount sent through payment requests and check whether changing client-controlled values affects the final transaction.
2. Order ID Manipulation
Payment requests often contain an order ID or transaction reference. If the application accepts this identifier without checking ownership, a user may try to replace it with another user’s order ID.
This can create problems such as viewing another customer’s payment information, paying for the wrong order, changing transaction details, or receiving goods associated with another order.
Every transaction-related API should verify that the authenticated user has permission to access the requested order.
3. Currency Parameter Manipulation
Payment requests may contain a currency value such as INR, USD, EUR, or GBP. If the application allows users to change this value without server-side verification, the final payment calculation may become incorrect.
For example, an attacker may attempt to change the currency associated with an order or combine a currency value with an amount that does not match the application’s pricing rules.
The application should determine the permitted currency from trusted order or account data and verify the value before sending the transaction to the payment provider.
4. Transaction Status Manipulation
A payment status tells the application what happened to a transaction. Values such as success, failed, pending, and cancelled should not be accepted simply because they appear in a browser request.
If a user can change a failed transaction to a successful status, the application may release an order without receiving payment.
Testing should therefore check whether payment status comes from a trusted source and whether the application verifies the transaction before changing the order or account state.
5. Payment Confirmation Abuse
Some applications use a separate endpoint to confirm or complete a payment after the initial transaction is created.
If this endpoint does not verify the transaction properly, an attacker may try to call it directly, use another transaction ID, or submit the same confirmation multiple times.
The application should confirm that the payment belongs to the correct order, has the expected amount and currency, is in an appropriate state, and has not already been completed.
6. Duplicate Payment Processing
Payment processing should normally happen once for a successful transaction. Problems can occur when the same confirmation or callback is accepted multiple times.
For example, if two requests arrive together and both are treated as new successful payments, the application could update an order, balance, or reward more than once.
7. Refund Parameter Manipulation
Refund functions can be particularly sensitive because they move money in the opposite direction. A weak refund API may allow users to change the transaction ID, refund amount, account reference, or other parameters.
An attacker may try to request a refund for another transaction or request an amount that exceeds the amount originally paid.
Refund operations should therefore verify the original transaction, permitted refund amount, user or administrator permissions, and current transaction state before processing the request.
What Happens If a Third-Party Integration Is Added Without Security Testing?
Adding an integration changes how the application handles data and business actions. Without Web Application Security Testing, weaknesses in the new connection may remain unnoticed until they are abused.
1. New Attack Paths May Remain Undetected
A new payment or SaaS integration can add APIs, webhooks, redirects, callbacks, tokens, and new parameters. If these components are not included in security testing, vulnerabilities in the new paths may remain outside the application’s security review.
2. Payment Logic May Be Misconfigured
Payment configuration can affect how amounts, currencies, transaction statuses, refunds, and callbacks are processed. A configuration mistake may cause the application to accept an incorrect payment result or process an action that should have been rejected.
3. Sensitive Data May Be Exposed
Integrations may exchange customer information, transaction details, tokens, API credentials, and other sensitive data. Poor API responses or incorrect access controls can expose information to users who should not receive it.
4. Authorization Checks May Be Missing
New endpoints sometimes receive less security review than older application functions. This can result in an API that allows users to access another customer’s order, transaction, refund, or account information by changing an identifier or calling a restricted function.
5. Third-Party Responses May Be Trusted Too Much
Applications may treat data received from an external provider as trustworthy and apply fewer validation checks than they would apply to user input.
6. Business Logic May Be Bypassed
An integration may introduce a new route to perform an existing business action. For example, an order may normally require successful payment before fulfillment, but a new callback or API may allow the order state to be changed without completing every required check.
Web Application Penetration Testing should therefore follow the entire business process and compare every route that can change an important transaction state.
What Sensitive Data Should Be Checked By Web Application Penetration Testing After an Integration?
Adding a payment gateway or third-party service can increase the amount of sensitive data moving between systems. During web application penetration testing, testers should check whether this information is properly protected, limited, and available only to authorized users.
1. Payment Information
Payment integrations may process transaction amounts, payment references, billing details, and other payment-related information. Testers should check whether this data is exposed in URLs, browser responses, API responses, logs, or error messages.
2. Customer Information
Customer details such as names, email addresses, phone numbers, billing information, and account details may be shared with an external service. Testing should verify that users cannot access another customer’s information through API parameters, transaction IDs, or integration functions.
3. API Keys and Secrets
Payment and SaaS integrations often require API keys, client secrets, access credentials, or signing keys. These values should not appear in frontend code, public repositories, browser responses, source maps, or error messages. Testers should also check whether exposed credentials can be used to access the connected service.
4. Transaction Identifiers
Transaction IDs, order IDs, payment references, and customer identifiers may appear in requests and responses.
These identifiers should not provide access by themselves. Web Application Penetration Testing should check whether changing an identifier allows access to another user’s transaction or sensitive payment information.
5. Authentication Tokens
Integrations may use OAuth tokens, session tokens, API tokens, or other credentials to identify users and services. Web Application Penetration Testing should check where these tokens are stored, whether they are exposed in responses or URLs, and whether expired or revoked tokens can still be used.
6. Error and Debug Information
Integration failures may produce detailed error messages containing API URLs, database information, service names, request data, or internal identifiers.
During VAPT, testers should review both successful and failed requests to determine whether excessive technical information is exposed to users.
7. Data Exposed Through API Responses
An API may return more information than the application interface displays. This can include internal identifiers, customer details, payment information, account fields, or integration data.
Web Application Penetration Testers should compare API responses with the information a user is expected to access and verify that sensitive fields are not returned unnecessarily.
Why Should Web Application Penetration Testing Include Third-Party Integrations?
A Web Application Penetration Testing that ignores newly added integrations may leave important application functions outside the assessment. Payment gateways, SaaS platforms, APIs, callbacks, and webhooks can all affect how users access data and how transactions are processed.
1. New Integrations Add New Entry Points
Every new API, webhook, callback, or redirect can create another place where requests enter the application. Testing these points helps identify authentication, authorization, input validation, and configuration weaknesses.
2. Payment Flows Handle Sensitive Transactions
Payment functions directly affect orders, account balances, refunds, and customer information. Web Application Penetration Testing should verify that users cannot manipulate payment values, transaction states, or confirmation processes.
3. APIs Can Expose Business Functions
An API may provide access to functions that are not visible through the normal website interface. Web Application Penetration Testing can test whether these functions enforce the same authorization and business rules as the application’s user interface.
4. Integration Logic May Introduce New Weaknesses
An application may be secure before an integration is added but become exposed through the new data flow or business process. For example, a callback may update an order without sufficient validation, or an API may accept a transaction ID without checking ownership.
5. Manual Testing Can Find Business Logic Issues
Automated tools can identify many common technical weaknesses, but payment and integration workflows often require testers to understand how different requests affect the business process. Manual testing can examine unusual request sequences, transaction states, parameter changes, repeated requests, and authorization boundaries.
6. Regular Testing Helps Identify Changes in Security Risk
Security testing should not stop after the first successful deployment. When payment gateways, SaaS platforms, APIs, or business workflows change, the application’s security exposure can also change. Regular Web Application Penetration Testing helps review these changes and verify that important security controls continue to work.
What Security Misconfigurations Should Be Checked While Conducting Web Application Penetration Testing?
New integrations can introduce configuration changes across the web server, APIs, browsers, cloud services, and third-party connections. Penetration testing should check whether these changes have created publicly accessible services or weakened existing security controls.
1. Exposed API Endpoints
New integrations may add API endpoints that were not present before. Testers should identify accessible endpoints and check whether authentication, authorization, rate limits, and input validation are applied correctly.
2. Test and Sandbox Endpoints
Development, testing, and sandbox endpoints may contain weaker controls or outdated configurations. Testers should check whether such endpoints are accessible from the public internet and whether they expose test credentials, customer data, internal functions, or production resources.
3. Weak CORS Configuration
Cross-Origin Resource Sharing (CORS) controls which websites can make browser-based requests to an application. An incorrect CORS policy may allow an unauthorized website to interact with sensitive APIs through a user’s browser. Testing should review allowed origins, credentials, methods, and exposed headers.
4. Debug Features
Debug modes and detailed diagnostic features can expose information that should not be available to normal users. Web Application Penetration Testing should check for debug pages, verbose API errors, stack traces, diagnostic endpoints, and development settings that may have remained active after deployment.
5. Incorrect TLS Configuration
Payment and third-party integrations commonly exchange sensitive information over HTTPS. Web Application Penetration Testing should verify that secure protocols are used and that insecure communication is not permitted for sensitive requests.
6. Exposed API Documentation
API documentation can reveal endpoint paths, parameters, request formats, authentication methods, and internal functions.
Public documentation is not automatically a vulnerability, but testers should check whether it exposes administrative or sensitive endpoints that should not be available to unauthorized users.
7. Unnecessary HTTP Methods
APIs may support methods such as GET, POST, PUT, PATCH, and DELETE. If unnecessary methods are enabled, users may gain access to functions that were not intended for their role. Testing should verify that each HTTP method is required and properly authorized.
How Should Web Application Penetration Testing Cover the New Integration?
Testing should cover the complete integration rather than focusing only on the new page or API. The goal is to understand how the application communicates with the external service and how those interactions affect users, transactions, and application data.
1. Mapping the New Data Flow
First, identify how information moves between the user, web application, API, database, and third-party service. This helps testers find where data enters the system, where it is stored, and where external responses can change application behavior.
2. Identifying New Entry Points
List new APIs, webhooks, callbacks, redirects, authentication endpoints, payment pages, and integration services. Each entry point should be reviewed for authentication, authorization, input validation, and access restrictions.
3. Testing User-Controlled Parameters
Test parameters such as order IDs, customer IDs, transaction references, amounts, currency values, return URLs, and status fields. The application should validate these values on the server instead of relying only on values supplied by the browser.
4. Testing API Requests
Review requests sent to both the application and the integrated service. Testing should check authentication, authorization, input validation, rate limits, HTTP methods, error handling, and whether sensitive information is unnecessarily included in requests.
5. Testing Authorization
Check whether users can access or modify another user’s orders, transactions, payment details, or integration data. Authorization should be enforced for every sensitive function, including newly added API endpoints.
6. Testing Business Logic
Follow the complete workflow from the first action to the final result. For payment integrations, this can include creating an order, starting payment, completing payment, receiving confirmation, updating the order, and processing refunds. Testers should check whether any step can be skipped, repeated, or performed in an incorrect sequence.
7. Testing Error Handling
Send invalid, incomplete, unexpected, and repeated requests to the integration. The application should reject invalid requests without exposing sensitive technical information or entering an incorrect transaction state.
8. Testing Integration Responses
Review responses received from the third-party service before the application uses them. The application should validate important fields such as transaction status, amount, currency, order reference, and user mapping before changing account or transaction data.
How Should Payment Gateway Integrations Be Tested for Fraud Risks?
Payment testing should focus on whether a user can manipulate the payment process to receive goods, services, credits, or refunds without following the required payment rules.
1. Testing Price Manipulation
Check whether the amount sent for payment can be changed through browser requests or API parameters. The server should calculate or verify the payable amount using trusted order information before accepting the transaction.
2. Testing Duplicate Transactions
Check whether the same payment can be processed more than once. This includes testing repeated requests, repeated callbacks, and concurrent requests. The application should prevent a single payment from creating multiple orders, credits, or other benefits.
3. Testing Payment Replay
A previously successful payment request or callback should not normally be usable to trigger the same action again. Testers should check whether old transaction messages, confirmation requests, or callbacks can be replayed after the transaction has already been completed.
4. Testing Refund Abuse
Refund functions should verify the original transaction, refund amount, user permissions, and current transaction state. Testing should check whether a user can request a refund for another transaction, request an excessive amount, or submit the same refund request multiple times.
5. Testing Order and Payment Mismatches
The order paid for should match the order that receives the successful payment status. Testers can check whether changing an order ID, amount, product, quantity, or transaction reference creates a mismatch between the payment and the final order.
6. Testing Failed Payment Workflows
A failed or cancelled payment should not result in a successful order or account credit. Testing should follow failed, cancelled, pending, and interrupted payment flows to verify that the application keeps the correct transaction state.
What Should Be Tested When Adding a Third-Party SaaS Integration?
SaaS integrations may connect the application to services for communication, analytics, customer management, storage, authentication, or other functions. Web Application Penetration Testing should cover both the connection itself, and the permissions given to the external service.
1. OAuth and Access Tokens
Check how OAuth authorization is implemented and how access and refresh tokens are handled. Web Application Penetration Testing should verify that tokens are protected, expire as expected, and cannot be used by unauthorized users or for unintended accounts.
2. API Permissions
Review the permissions granted to the third-party application. The integration should receive only the access required for its function. Excessive permissions can increase the impact of a compromised integration account.
3. User Account Mapping
The application may need to connect a local account with an account in the SaaS platform. Web Application Penetration Testing should verify that users cannot change account identifiers and gain access to another person’s external account or data.
4. Webhooks
Webhooks allow an external service to send events to the application. Testers should verify webhook authentication or signatures, event validation, replay protection, and authorization before an incoming event changes application data.
5. Data Sharing
Identify what customer and application information is sent to the third-party service. Web Application Penetration Testing should verify that only required data is shared and that sensitive information is not included unnecessarily in API requests or integration records.
6. Callback Validation
Callbacks may notify the application about events such as completed payments, account changes, or subscription updates. The application should verify the source, signature where applicable, event type, object identifier, and expected application state before accepting the callback.
7. Third-Party Error Handling
External services may return errors, unexpected responses, or incomplete information. The application should handle these responses safely without exposing sensitive information, creating incorrect account states, or allowing users to bypass required checks.
How Can Businesses Secure New Payment and Third-Party Integrations?
Security controls should be considered during integration development and checked again after deployment. Payment and third-party services should follow the same security requirements as other sensitive parts of the application.
1. Retest After Significant Changes
Payment providers and third-party services may change their APIs, authentication methods, or integration workflows. After major changes, Web Application Penetration Testing should be repeated to check whether the updated integration has introduced new security weaknesses.
2. Verify Payment Status on the Server
Payment success should be confirmed through trusted server-side checks. The application should not treat a browser redirect or client-controlled status value as sufficient proof that payment was completed.
3. Protect API Credentials
Store API keys, secrets, and other credentials securely. They should not be included in frontend code, public files, URLs, or application responses where unauthorized users can access them.
4. Apply Strong Authorization
Every sensitive integration function should verify whether the current user or service has permission to perform the requested action. This includes viewing transactions, changing payment information, requesting refunds, and accessing customer data.
5. Validate Webhooks and Callbacks
Incoming events should be authenticated and checked before they can change application data. The application should also prevent old or duplicate events from being processed as new transactions.
6. Use Secure API Authentication
Use suitable authentication methods for communication between the application and external services. Credentials and tokens should be protected, rotated when appropriate, and restricted to the permissions required by the integration.
7. Protect Sensitive Transaction Data
Limit the amount of payment and customer information included in requests, responses, logs, and stored records. Sensitive data should be protected during transmission and storage, with access limited to authorized users and services.
8. Monitor Integration Activity
Track important integration events such as failed authentication, unusual payment activity, repeated callbacks, refund requests, and unexpected API errors. Monitoring can help security teams investigate suspicious activity and identify problems after deployment.
9. Maintain an Updated API Inventory
Keep track of production APIs, third-party connections, webhooks, callback endpoints, and older API versions.
10. Validate All External Data
Do not automatically trust information received from browsers, payment gateways, webhooks, or SaaS APIs. Validate important fields before using them to make security or business decisions.
Conclusion
Adding a payment gateway or third-party service can change how a web application handles data, APIs, authentication, and business transactions. New endpoints, callbacks, webhooks, redirects, and external data can create security issues if they are not properly tested.
Web Application Penetration Testing helps examine these new components and identify weaknesses such as payment manipulation, authorization problems, sensitive data exposure, insecure API configurations, and business logic issues. Testing should cover the complete integration flow rather than only the visible payment or integration page.
Security testing should also continue after major changes to payment providers, APIs, SaaS services, or transaction workflows. Regular Web Application Penetration Testing can help verify that security controls continue to work as the application and its integrations change.
Peneto Labs can help assess web applications, APIs, payment workflows, and third-party integrations through professional VAPT services. Contact Peneto Labs to identify security weaknesses and improve the security of your application’s new integrations.