Description

The Simple Membership WordPress plugin before 4.7.8 does not validate that the membership level supplied in a PayPal payment notification matches the level configured for the paid payment button, allowing members to pay for a lower-priced membership while being granted a higher, more privileged membership level.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "5.4",
  "pubDate": "2026-09-13T06:16:25.350Z",
  "pubdate": "2026-09-13T06:16:25.350Z",
  "executiveSummary": "The Simple Membership plugin for WordPress, in versions prior to 4.7.8, contains a critical improper authorization vulnerability within its payment processing logic.\nThe flaw stems from a failure to perform server-side validation of the membership level identifier received via PayPal Instant Payment Notification (IPN).\nAn unauthenticated remote attacker can manipulate the payment payload to substitute a higher-priced, more privileged membership level identifier for the lower-priced level intended for the specific payment button.\nSuccessful exploitation results in unauthorized privilege escalation, as the application grants the elevated membership status without verifying that the payment corresponds to the assigned tier.\nThis vulnerability compromises the integrity of the membership access control system and allows attackers to bypass subscription pricing, resulting in potential financial loss for the site administrator and unauthorized access to restricted content or functionality.",
  "technicalDetails": "The vulnerability resides in the payment handling logic responsible for processing PayPal IPN callbacks within the Simple Membership plugin. Specifically, the application fails to perform a cross-reference validation between the incoming membership level identifier ('subsc_ref' or equivalent parameter) and the membership level associated with the specific payment button utilized by the user.\nThe root cause is an insecure assumption that the membership level relayed in the PayPal notification is implicitly authorized by the fact that it was included in the IPN data. The application fails to implement an integrity check to ensure that the payment amount and the resulting membership level upgrade strictly conform to the parameters defined in the original payment button configuration.\nThe attack flow proceeds as follows: First, an attacker initiates a checkout process for a low-tier membership. Second, before the transaction is completed, the attacker intercepts or modifies the outgoing payment request, specifically targeting the membership level identifier parameters. Alternatively, if the backend logic relies on trust in the IPN data, an attacker can craft a legitimate payment for a low-cost item but modify the backend notification payload to signal a high-tier membership level completion.\nBecause the plugin lacks server-side verification to confirm that the membership level requested in the notification corresponds to the level authorized by the specific button ID, the application automatically upgrades the user's account permissions to the high-privileged tier once the payment for the low-tier price is confirmed by the PayPal IPN listener.\nThis flaw effectively bypasses the pricing model of the platform, as the application logic trusts the user-supplied or potentially forged membership level identifier rather than querying the internal database to ensure the payment amount matches the membership level price. This exposure is critical as it requires no prior authentication or administrative privileges to execute. Once the malicious payload is accepted by the payment listener, the post-exploitation impact includes the granting of elevated permissions, allowing unauthorized access to protected member areas, digital assets, or specific administrative features typically reserved for high-paying subscribers."
}