Use After Free in Fiber with Api Keys

Use After Free (UAF) occurs when memory is deallocated but remains referenced, leading to undefined behavior if later accessed. In a Fiber-based API service that relies on API keys for routing or authorization, UAF can emerge when key material is freed while in-flight requests or asynchronous handlers still hold pointers or references to it.

Consider a Fiber application that stores API keys in request-scoped contexts for validation. If keys are stored as C-style character arrays or managed via custom allocators, freeing the underlying buffer before all dependent goroutines complete can expose stale memory. An attacker may exploit timing differences—such as rapid key rotation or concurrent request cancellation—to cause the application to read or write through freed memory. This can leak private key material or enable manipulation of authorization decisions, effectively bypassing intended access controls.

In practice, this risk is heightened when API keys are cached for performance but invalidation logic does not guarantee safe memory lifecycle across concurrent requests. For example, a key might be removed from an in-memory map and freed while a background validation routine still holds a reference. The routine may then dereference the pointer, reading uninitialized data or previously freed key bytes. If those bytes correspond to other sensitive objects, an attacker can infer internal state or influence control flow.

Because Fiber is a high-performance framework, developers often manage resources manually to reduce allocations. Without careful synchronization and guaranteed lifetime management, the interplay of fast request handling and key material can turn seemingly safe patterns into UAF vectors. The vulnerability does not require malformed requests; it emerges from legitimate concurrency patterns interacting with unsafe memory handling.

Middleware that inspects or modifies API key headers is particularly susceptible if it retains references beyond the request context. Even when the framework abstracts memory, logical references—such as closures capturing key identifiers—can keep objects alive longer than intended or reuse memory in unexpected ways. This can lead to data exposure or privilege escalation when freed memory is repurposed.

To mitigate Use After Free in Fiber when working with API keys, ensure key material remains valid for the entire duration of request processing and any associated asynchronous operations. The safest approach is to rely on managed memory and immutable copies rather than raw pointers or manual deallocation.

Below are concrete, safe patterns for handling API keys in Fiber using Go. These examples avoid direct pointer manipulation and ensure keys are copied into request-local structures with clear ownership.

Example 1: Safe key extraction and validation (no raw pointer retention)

// Safe key handling: copy the key into a request-local value.
func ValidateAPIKey(c *fiber.Ctx) error {
    apiKey := c.Get("X-API-Key")
    if apiKey == "" {
        return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "missing api key"})
    }
    // Copy the key into a new string; the underlying bytes are managed by Go.
    keyCopy := apiKey
    if !isValidKey(keyCopy) {
        return c.Status(fiber.StatusForbidden).JSON(fiber.Map{"error": "invalid api key"})
    }
    // Store a copy in context for downstream handlers; context values should be immutable.
    c.Locals("apiKey", keyCopy)
    return c.Next()
}

func isValidKey(key string) bool {
    // Compare against a secure store; avoid holding raw pointers to key bytes.
    allowed := map[string]bool{
        "abc123...": true,
        "def456...": true,
    }
    return allowed[key]
}

Example 2: Avoiding closure captures that extend lifetime unexpectedly

// Ensure handlers do not accidentally capture and retain key pointers.
func SetupRoutes(app *fiber.App) {
    app.Get("/resource", func(c *fiber.Ctx) error {
        // Extract and copy the key; do not pass a pointer to a long-lived structure.
        key := c.Locals("apiKey").(string)
        // Use the copied value, not the address of a context internal.
        if !hasPermission(key, "read") {
            return c.Status(fiber.StatusForbidden).JSON(fiber.Map{"error": "insufficient scope"})
        }
        return c.JSON(fiber.Map{"data": "secure"})
    })
}

func hasPermission(key string, scope string) bool {
    // Perform permission checks using the copied key value.
    // This function should not store references to key beyond its call.
    return key != "" && scope == "read"
}

Example 3: Synchronization when rotating keys (safe invalidation)

// Use atomic swaps or versioned keys to ensure no reader sees freed memory.
type keyRegistry struct {
    mu   sync.RWMutex
    keys map[string]struct{}
}

func (r *keyRegistry) Validate(c *fiber.Ctx) error {
    r.mu.RLock()
    defer r.mu.RUnlock()
    apiKey := c.Get("X-API-Key")
    if _, ok := r.keys[apiKey]; !ok {
        return c.Status(fiber.StatusForbidden).JSON(fiber.Map{"error": "key revoked"})
    }
    return c.Next()
}

func (r *keyRegistry) Rotate(newKeys map[string]struct{}) {
    r.mu.Lock()
    defer r.mu.Unlock()
    // Replace the map atomically; old maps can be garbage collected safely.
    r.keys = newKeys
}

// Usage in main:
// registry := &keyRegistry{keys: initialKeys}
// app.Use(registry.Validate)

These patterns emphasize immutable copies, clear ownership, and synchronization rather than manual memory management. They prevent Use After Free by ensuring API key material is not freed while in use and that no handler retains references to deallocated memory.