Out Of Bounds Write in Fiber with Api Keys

An Out Of Bounds Write in a Fiber application becomes significant when API keys are used for endpoint authorization. API keys are commonly passed via headers (for example, Authorization or custom headers such as x-api-key), and if the application uses these keys to index into buffers, arrays, or string builders without proper bounds checking, the unchecked key length or malformed key values can lead to memory corruption. This typically occurs when the key is copied into a fixed-size buffer using functions that do not enforce length limits, allowing an attacker-supplied key to write past the allocated memory region.

Consider a route that retrieves an API key from headers and uses its value to determine a lookup index or to copy into a fixed-size structure:

package main

import (
	"github.com/gofiber/fiber/v2"
)

func main() {
	app := fiber.New()
	app.Get('/secure', func(c *fiber.Ctx) error {
		key := c.Get('X-API-Key', '')
		// Unsafe: using key length to index a fixed-size buffer
		buf := make([]byte, 256)
		if len(key) > 0 {
			// Potential out-of-bounds write if key is used as an index or copied without validation
			idx := int(key[0]) % 256
			buf[idx] = 0x01 // unsafe if idx is derived from key without proper validation
		}
		return c.SendString('ok')
	})
	app.Listen(':3000')
}

In this example, the first byte of the API key is used to compute an index into buf. If the key is controlled by an attacker, they can craft a key that leads to an index that, while masked by modulo, may still interact with other logic or be used in a way that triggers an out-of-bounds condition in downstream processing (e.g., when the index is used in a copy operation with an incorrect length). More critically, if the key is directly used in a memory copy such as copy(buf, []byte(key)) without verifying that len(key) <= len(buf), an out-of-bounds write can occur.

Additionally, when API keys are stored in slices or buffers that are later passed to Cgo or other low-level operations, missing length validation can corrupt memory outside the Go-managed heap. This is especially dangerous when the key is expected to be short (e.g., 32 characters) but an attacker provides a much longer string, exploiting missing validation to overwrite adjacent memory. The risk is compounded if the application processes keys in bulk or uses them to influence buffer allocations, where an oversized key can lead to excessive allocations or slice growth that corrupts memory layout.

An attacker can exploit this by sending a carefully crafted API key in requests to endpoints that perform unsafe operations based on the key. The impact may include crashes, information disclosure, or potentially arbitrary code execution depending on how the corrupted memory is used later. Because API keys are often treated as trusted identifiers, developers may skip rigorous validation, inadvertently creating an out-of-bounds write vector.

To prevent Out Of Bounds Write when using API keys in Fiber, enforce strict length checks and avoid using raw key bytes as indices or direct memory copy sources. Always validate the length of the key before using it in any buffer operation, and prefer constant-time comparisons for authentication.

Here is a secure pattern for handling API keys in Fiber:

package main

import (
	"errors"
	"github.com/gofiber/fiber/v2"
)

const maxAPIKeyLength = 64

func isValidAPIKey(key string) bool {
	if len(key) == 0 || len(key) > maxAPIKeyLength {
		return false
	}
	// Optionally, validate allowed character set (e.g., alphanumeric)
	for i := 0; i < len(key); i++ {
		c := key[i]
		if !((c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z') || (c >= '0' && c <= '9') || c == '-' || c == '_') {
			return false
		}
	}
	return true
}

func main() {
	app := fiber.New()
	app.Get('/secure', func(c *fiber.Ctx) error {
		key := c.Get('X-API-Key', '')
		if !isValidAPIKey(key) {
			return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{'error': 'invalid api key'})
		}
		// Safe: key length is validated before any buffer use
		buf := make([]byte, 256)
		// Use a deterministic, bounded operation, e.g., hash the key instead of using raw bytes as index
		// Example: use key to derive an index via a hash to avoid direct byte usage
		// This is illustrative; in practice, use a proper authentication mechanism
		return c.SendString('ok')
	})
	app.Listen(':3000')
}

Always treat API keys as opaque strings. Do not use individual bytes or substrings as memory indices. If you must map keys to buffer positions, use a cryptographic hash function to derive a bounded index:

import (
	"crypto/sha256"
	"encoding/binary"
)

func keyToIndex(key string) uint32 {
	h := sha256.Sum256([]byte(key))
	return binary.BigEndian.Uint32(h[:4]) % 256
}

For production, combine this with a constant-time comparison for authentication and store keys securely (e.g., hashed in a database). The middleware approach in Fiber allows centralized validation:

func APIKeyAuth(next fiber.Handler) fiber.Handler {
	return func(c *fiber.Ctx) error {
		key := c.Get('X-API-Key', '')
		if !isValidAPIKey(key) || !checkKeyAgainstStore(key) { // implement checkKeyAgainstStore
			return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{'error': 'forbidden'})
		}
		return next(c)
	}
}

app.Get('/secure', APIKeyAuth(func(c *fiber.Ctx) error {
	return c.SendString('authenticated')
}))

These practices eliminate unsafe memory operations tied to API key values and align with secure handling of secrets in HTTP services.