SSR Cache Isolation Flaw in Popular React Data Fetching Library Leaks User Data

A security researcher auditing a widely used React data fetching library discovered a cache isolation vulnerability affecting server-side rendering environments. The library initializes its cache as a module-level variable — a single shared Map object created once and reused across all incoming server requests in Node.js. Because modules are loaded once and shared globally, concurrent users hitting the server could inadvertently receive another user's cached data under the same cache key. The flaw requires no malicious input; two legitimate users making simultaneous requests is enough to trigger cross-user data exposure. The researcher found no built-in mechanism such as AsyncLocalStorage or per-request cache factories to enforce request-scoped isolation by default.