The vulnerability arises from the use of Ruby's String#== method to compare a computed 64-character SHA-256 HMAC with the Paddle-Signature header's attacker-controlled token. Because String#== is not constant-time, an attacker can perform a timing attack by repeatedly submitting requests and measuring response times to recover a valid signature prefix. This allows forging of accepted webhooks that enqueue jobs to update billing state or trigger other sensitive actions in the host application. The flaw affects pay-rails pay versions before 11.6.2 and is resolved in 11.6.2.

An unauthenticated remote attacker can exploit this timing discrepancy to forge valid webhook signatures, leading to unauthorized billing updates, provisioning of paid features, recording of refunds, or triggering customer notifications. This compromises the integrity of payment processing and billing state management in affected applications.

Upgrade to pay-rails pay version 11.6.2 or later, where the timing discrepancy vulnerability in PaddleBillingController#valid_signature? is fixed. No other mitigations are indicated by the vendor advisory.