Readest is a modern, feature-rich ebook reader designed for seamless cross-platform access and an intuitive reading experience. Prior to 0.11.10, POST /api/stripe/check in apps/readest-app/src/app/api/stripe/check/route.ts accepted a client-controlled sessionId, retrieved the corresponding Stripe Checkout Session, and granted the paid plan to the authenticated caller without verifying session.metadata.userId. The createOrUpdateSubscription path then updated the plans row keyed by the caller's user identifier rather than the Checkout Session owner. Any authenticated free user with a paid active Checkout Session identifier, including one from the user's own single legitimate purchase, could replay that identifier to upgrade additional accounts without payment. This issue is fixed in version 0.11.10.
CWE-639