Fiber is an Express inspired web framework written in Go. In versions on the v3 branch prior to 3.1.0, the use of the `fiber_flash` cookie can force an unbounded allocation on any server. A crafted 10-character cookie value triggers an attempt to allocate up to 85GB of memory via unvalidated msgpack deserialization. No authentication is required. Every GoFiber v3 endpoint is affected regardless of whether the application uses flash messages. Version 3.1.0 fixes the issue.

Affected: Go: github.com/gofiber/fiber/v3

### Summary The use of the `fiber_flash` cookie can force an unbounded allocation on any server. A crafted 10-character cookie value triggers an attempt to allocate up to 85GB of memory via unvalidated msgpack deserialization. No authentication is required. Every GoFiber v3 endpoint is affected regardless of whether the application uses flash messages.

### Details Regardless of configuration, the flash cookie is checked:

```go func (app *App) requestHandler(rctx *fasthttp.RequestCtx) { // Acquire context from the pool ctx := app.AcquireCtx(rctx) defer app.ReleaseCtx(ctx)

// Optional: Check flash messages rawHeaders := d.Request().Header.RawHeaders() if len(rawHeaders) > 0 && bytes.Contains(rawHeaders, flashCookieNameBytes) { d.Redirect().parseAndClearFlashMessages() } _, err = app.next(d) } else { // Check if the HTTP method is valid if ctx.getMethodInt() == -1 { _ = ctx.SendStatus(StatusNotImplemented) //nolint:errcheck // Always return nil return }

// Optional: Check flash messages rawHeaders := ctx.Request().Header.RawHeaders() if len(rawHeaders) > 0 && bytes.Contains(rawHeaders, flashCookieNameBytes) { ctx.Redirect().parseAndClearFlashMessages() } } ```

The cookie value is hex-decoded and passed directly to msgpack deserialization with no size or content validation:

https://github.com/gofiber/fiber/blob/f8f34f642fb3682c341ede7816e7cf861aa7df89/redirect.go#L371

```go // parseAndClearFlashMessages is a method to get flash mess…