How I Identified a Potential Subscription Payment Enforcement Issue Despite Insufficient Funds

This research was conducted on a live target, and the issue was privately reported to the platform.

Summary

The application uses an automatic subscription renewal feature. On the 29th, the scheduled payment failed because my bank account had insufficient funds to complete the international transaction.

Following the failed payment, I observed that the application continued to provide access to premium features. Instead of restricting access, the application sent me an email requesting that I update my payment method. After updating the payment method, premium access remained available.

Seven days after the initial failed payment, the application automatically retried the payment. However, premium access remained available despite the initial payment failure.

This behaviour may indicated a potential inconsistency between payment status and subscription entitlement enforcement. Further investigation was conducted to determine whether this is an unintended vulnerability or an intentional billing grace period.

Proof of Concept

The initial payment attempt failed because my bank notified me that my account had insufficient funds to process the international payment.

Although the payment failed, I did not update my payment method. I observed that the application continued to grant access to premium features instead of restricting them due to the unsuccessful payment. The application only sent an email requesting that I update my payment method, while premium access remained available.

After the initial payment failure, I updated my payment method using invalid bank account details to determine whether the application properly validated the new payment information.

The application appeared to accept the updated payment method without immediately verifying whether the payment could be successfully processed. Despite using invalid payment details, I continued to have access to premium features.

When the application repeated its payment process, I observed the same behaviour: premium access remained available even though I had not established a valid, successful payment.

These observations suggest a potential weakness in payment validation and subscription entitlement enforcement.

Remediation

To prevent potential subscription payment bypasses, applications should enforce subscription entitlements based on verified payment and subscription states rather than simply accepting an updated payment method.

Recommended measures include:

1. Enforce server-side subscription validation: Verify the user's subscription status on the server before granting access to premium features.
2. Validate payment status: Treat pending, failed, and declined payments differently from confirmed successful transactions. Updating payment details must not automatically be treated as proof of payment.
3. Implement clear billing grace periods: If temporary premium access is permitted after a failed payment, define a specific grace period and enforce its expiration consistently.
4. Synchronize payment events: Ensure payment-provider notifications, retries, and subscription state transitions are processed reliably to prevent inconsistencies between billing records and access permissions.
5. Revoke expired entitlements: When the applicable grace period expires without a successful payment, restrict premium access according to the application's billing policy.
6. Test subscription lifecycle scenarios: Include failed payments, pending transactions, invalid payment details, expired subscriptions, payment retries, and successful payment recovery in security and regression testing.

These measures help ensure that premium access is granted according to verified subscription entitlements and that payment failures are handled consistently.