Fiber is Vulnerable to Denial of Service via Flash Cookie Unbounded Allocation in github.com/gofiber/fiber/v3

Open Source Vulnerabilities CVE-2026-25899

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.

Open Source Vulnerabilities GO-2026-4534

Fiber is Vulnerable to Denial of Service via Flash Cookie Unbounded Allocation in github.com/gofiber/fiber/v3

Open Source Vulnerabilities GHSA-2mr3-m5q5-wgp6

### 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 messages before they are getting removed func (r *Redirect) parseAndClearFlashMessages() { // parse flash messages cookieValue, err := hex.DecodeString(r.c.Cookies(FlashCookieName)) if err != nil { return } _, err = r.c.flashMessages.UnmarshalMsg(cookieValue) if err != nil { return } r.c.Cookie(&Cookie{ Name: FlashCookieName, Value: "", Path: "/", MaxAge: -1, }) } ``` The auto-generated `tinylib/msgp` deserialization reads a `uint32` array header from the attacker-controlled byte stream and passes it directly to `make()` with no bounds check: https://github.com/gofiber/fiber/blob/f8f34f642fb3682c341ede7816e7cf861aa7df89/redirect_msgp.go#L242 ```go // UnmarshalMsg implements msgp.Unmarshaler func (z *redirectionMsgs) UnmarshalMsg(bts []byte) (o []byte, err error) { var zb0002 uint32 zb0002, bts, err = msgp.ReadArrayHeaderBytes(bts) if err != nil { err = msgp.WrapError(err) return o, err } if cap((*z)) >= int(zb0002) { (*z) = (*z)[:zb0002] } else { (*z) = make(redirectionMsgs, zb0002) } for zb0001 := range *z { bts, err = (*z)[zb0001].UnmarshalMsg(bts) if err != nil { err = msgp.WrapError(err, zb0001) return o, err } } o = bts return o, err } ``` where `zb0002, bts, err = msgp.ReadArrayHeaderBytes(bts)` translates the attacker-controlled value into the element count and `make(redirectionMsgs, zb0002)` performs the unbounded allocation So we can craft a gofiber cookie that will force a huge allocation: `curl -H "Cookie: fiber_flash=dd7fffffff" http://localhost:5000/hello` The cookie val is a hex-encoded msgpack array32 header: - `dd` = msgpack array32 marker - `7fffffff` = 2 147 483 647 elements ### Impact Unauthenticated remote Denial of Service (CWE-789). Anyone running a gofiber v3.0.0 or v3 server is affected. The flash cookie parsing is hardcoded.