The 10-Byte Killer: How a Tiny Cookie Crushed GoFiber v3

GoFiber loves speed. It's built on top of fasthttp, bypassing Go's standard library to squeeze every microsecond out of request processing. But in the race for performance, safety sometimes gets left in the dust. Enter the fiber_flash cookie, a mechanism designed to pass temporary messages between requests (like "Login Successful").

Here is the kicker: in GoFiber v3, this logic was baked into the core requestHandler. It didn't matter if your application actually used flash messages. It didn't matter if you disabled every optional middleware. Before your router even looked at the URL, before your auth middleware checked the headers, the framework was eagerly digging through cookies looking for fiber_flash.

It’s the digital equivalent of a stranger walking into your house and opening your fridge before you’ve even unlocked the front door. If that stranger brings a specific, malformed packet of data, they don't just eat your food—they burn the house down.

The vulnerability (CVE-2026-25899) is a textbook case of CWE-789: Memory Allocation with Excessive Size Value. It stems from how GoFiber handles—or rather, mishandles—deserialization using the tinylib/msgp library.

MessagePack (MsgPack) is a binary serialization format that is faster and smaller than JSON. When encoding an array, MsgPack uses a header to declare the number of elements. For example, 0x93 means "Array with 3 elements." The deserializer reads this header and prepares to read 3 objects.

The flaw lay in the auto-generated code's blind trust. It read the array size header and immediately allocated memory to hold that many elements, without checking if the input buffer actually contained enough data to fill them. It’s like a contractor ordering materials to build a skyscraper because a client wrote "100 floors" on a napkin, without checking if the client actually had the budget (or the land) for it.

Let's look at the generated code in redirect_msgp.go. This is where the magic (and the crash) happens. The UnmarshalMsg method is responsible for turning bytes back into a struct.

// The Vulnerable Logic in GoFiber v3 < 3.1.0
func (z *redirectionMsgs) UnmarshalMsg(bts []byte) (o []byte, err error) {
    var zb0002 uint32
    // 1. Read the array size from the wire (attacker controlled!)
    zb0002, bts, err = msgp.ReadArrayHeaderBytes(bts)
    if err != nil {
        return
    }
    
    // 2. THE BUG: Blind allocation
    // If the attacker says zb0002 is 2 billion, we allocate 2 billion structs.
    if cap((*z)) >= int(zb0002) {
        (*z) = (*z)[:zb0002]
    } else {
        (*z) = make(redirectionMsgs, zb0002) // <--- CRASH HERE
    }
    
    // ... loop to read elements ...
}

The variable zb0002 is a uint32 taken directly from the input. If I send a header saying "I have 2 billion items," the Go runtime dutifully attempts to find contiguous memory for 2 billion redirectionMsg structs. It doesn't care that I only sent 5 bytes of actual data.

You don't need a botnet. You don't need a supercomputer. You just need curl and ten hex characters. The attack vector is shockingly simple and affects every endpoint on the server.

The Payload: dd7fffffff

Let's break that down:

• dd: The MessagePack marker for Array32 (an array with a 32-bit length).
• 7fffffff: The hexadecimal representation of 2,147,483,647 (MaxInt32).

The Attack:

curl -H "Cookie: fiber_flash=dd7fffffff" http://target:3000/

When the server receives this, it decodes the hex and hits the allocator. It tries to allocate a slice for 2.14 billion structs. Even if the struct is small (say, 40 bytes), that requires requesting ~85 Gigabytes of RAM instantly.

Unless your server is running on a high-end mainframe, the Go runtime immediately panics with runtime: out of memory. The process dies. If you're running this in Kubernetes without strict restart policies, you just entered a crash loop. If you're on a standard VPS, your service is dead until you wake up and restart it.

The fix in version 3.1.0 is simple sanity checking. You can't fit a gallon of water in a shot glass, and you can't fit 2 billion array elements in a 5-byte payload.

The patched code introduces a check against the remaining bytes in the buffer. Before allocating, it calculates: "Does the remaining input stream have enough bytes to theoretically hold X elements?"

// The Fix Logic (Conceptual)
if availableBytes < (arraySize * minElementSize) {
    return error("array size exceeds available data")
}
make(slice, arraySize)

By validating that the declared size is physically possible given the input, the allocation is blocked before it hits the OS. This is a classic defense-in-depth strategy: never trust the client's metadata without verifying it against the actual data.