CVE-2026-73316 in XenForo
Summary
by MITRE • 09/08/2026
XenForo before 2.3.13 contains a payment replay vulnerability in the PayPal REST payment provider that allows attackers to process the same webhook payload multiple times by exploiting a missing duplicate transaction ID check. Attackers can replay a valid webhook payload to trigger duplicate payment events, resulting in repeated subscription activations and unauthorized account upgrades.
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Analysis
by VulDB Data Team • 09/08/2026
The identified security flaw resides within the PayPal REST payment integration module of XenForo versions prior to 2.3.13, representing a critical failure in idempotency handling for financial transactions. This vulnerability is classified under CWE-345, which denotes Insufficient Verification of Data Authenticity, and specifically relates to CWE-840, the Performed Action with Incorrectly Specified Condition, as the system fails to validate that a transaction has already been processed before executing it again. The core technical deficiency lies in the absence of a robust duplicate detection mechanism for incoming webhook payloads from PayPal. When PayPal sends payment notifications or subscription status updates via REST API webhooks, XenForo is expected to process these events exactly once to maintain data consistency and financial integrity. However, due to the missing check against previously processed transaction identifiers, the application logic does not distinguish between a new legitimate event and a replayed copy of an existing one. This architectural oversight allows external actors to intercept valid webhook payloads and resend them repeatedly without triggering any rejection or error states from the server side.
From an operational perspective, this vulnerability enables attackers to execute payment replay attacks that directly compromise the financial stability and user account integrity of the platform. By capturing a legitimate webhook payload associated with a successful subscription activation or upgrade, an attacker can inject multiple copies of this same payload into the system. Each processed instance triggers the internal logic for activating subscriptions or upgrading accounts independently of actual monetary exchange. Consequently, users may receive unauthorized premium features, extended trial periods, or full membership privileges without having paid for them. This not only results in direct revenue loss for the platform administrators but also creates an uneven playing field where malicious actors can exploit free services intended for paying customers. The impact extends beyond immediate financial gain, as it undermines trust in the payment processing infrastructure and may lead to inventory depletion if limited-time offers or capped subscription slots are involved.
The exploitation of this flaw aligns with MITRE ATT&CK technique T1496, Resource Hijacking, where attackers leverage system resources for unauthorized purposes, specifically by consuming service capacity through fake premium status activations. Furthermore, it reflects principles associated with CWE-829, Inclusion of Functionality from Untrusted Control Sphere, as the application blindly trusts external webhook data without sufficient validation against a state database or transaction log. To mitigate this risk, immediate patching to XenForo version 2.3.13 or later is required, as these versions implement proper duplicate detection mechanisms. For environments where upgrading is not immediately feasible, administrators should enforce strict idempotency checks at the application level by maintaining a persistent record of processed transaction IDs and rejecting any subsequent requests that reference an already handled identifier. Additionally, implementing webhook signature verification using PayPal provided secrets ensures payload integrity, while rate limiting on payment endpoints can further reduce the blast radius of replay attempts by throttling excessive request volumes from single sources.