Null Pointer Dereference in Fiber with Api Keys

A null pointer dereference in a Fiber-based API occurs when code attempts to access members or invoke methods on an object reference that is null. When API keys are handled without proper validation, this pattern becomes more likely because key extraction and assignment may produce null values.

In Fiber, middleware commonly reads API keys from headers and stores them in request locals. If a key is missing, the assignment may result in null, and subsequent access without a guard triggers a runtime exception. For example, using a struct to hold key metadata and failing to initialize fields can lead to dereferencing null fields.

Consider a handler that expects a validated key object:

const ( 
    keyHeader = "X-API-Key"
)

type KeyInfo struct { 
    Value string 
    Scopes []string // may be nil if not parsed
}

app.Get("/resource", func(c *fiber.Ctx) error { 
    keyRaw := c.Get(keyHeader)
    // Simulating a lookup that may return nil KeyInfo
    keyInfo := getKeyInfo(keyRaw) // may return nil
    // Risk: keyInfo or keyInfo.Scopes may be nil
    if contains(keyInfo.Scopes, "read:data") { 
        c.Send("OK")
    } else { 
        return c.Status(fiber.StatusForbidden).SendString("insufficient scope")
    }
})

If getKeyInfo returns nil (e.g., when the key is unknown), accessing keyInfo.Scopes causes a null pointer dereference. Similarly, if scopes are not initialized, iterating over keyInfo.Scopes without checking for nil can crash the handler. This pattern is common in APIs where key validation is split across middleware and handlers, and the contract between components assumes non-null objects.

An unauthenticated endpoint lacking proper key checks may still pass nil values through the context chain, increasing the chance of dereference in downstream handlers. The vulnerability is not in the key format itself but in the assumptions about presence and structure after validation steps.

Impacts include runtime panics, service instability, and potential exposure of stack traces if error handling is inadequate. While this is a logic error rather than a direct bypass, it can degrade availability and complicate secure key usage patterns.