Uninitialized Memory in Fiber with Hmac Signatures

Uninitialized memory in a Fiber-based application occurs when a buffer or variable is declared but not explicitly set before being used in cryptographic operations such as HMAC signing. In Go, memory is not automatically zeroed at declaration; a slice or byte array may contain residual data from prior allocations. When such uninitialized memory is fed into an HMAC signing routine, the computed signature becomes nondeterministic, because the input to the hash function includes unpredictable bytes. This nondeterminism directly undermines signature verification, since the same payload with different uninitialized bytes will produce different HMACs. An attacker does not need to exploit a weakness in the HMAC algorithm itself; they can leverage the inconsistency caused by uninitialized memory to bypass integrity checks or cause application logic to behave incorrectly.

In the context of middleBrick’s security checks, uninitialized memory used with HMAC signatures may surface under the Input Validation and Authentication categories. The scanner tests whether API endpoints that rely on HMAC-based integrity mechanisms can produce stable, verifiable signatures across repeated requests with equivalent payloads. If signatures vary without changes to the message or key, the scan can flag instability that may indicate nondeterministic inputs such as uninitialized memory. Because HMAC relies on exact byte-level input, any unknown content introduces risk: an attacker might induce edge cases where verification passes or fails unpredictably, enabling signature confusion or information leakage through error handling.

Consider a Fiber route that signs a JSON body using HMAC without clearing or initializing a temporary buffer:

var temp [32]byte // uninitialized or partially reused
// ... some logic that may leave temp with stale data
data := append(temp[:0], requestBody...)
signature := computeHMAC(data, key)

Here, temp may contain arbitrary leftover bytes, causing data to diverge between calls even when requestBody is identical. If this data is included in the HMAC computation, the resulting signature will vary. A verification routine that compares signatures using a non-constant-time check may inadvertently leak information about which bytes differ, and an attacker could use this behavior to infer memory contents or influence authorization decisions. The combination of Fiber’s rapid request handling and uninitialized buffers increases the likelihood that stale data affects security-sensitive operations.

To detect this pattern, middleBrick’s unauthenticated scanning sends requests designed to observe signature consistency for identical payloads. While the tool does not inspect memory directly, it can identify instability in authentication outcomes that may point to nondeterministic inputs. Remediation focuses on ensuring that all inputs to HMAC are explicitly defined and zeroed when no longer needed, preventing uninitialized memory from affecting cryptographic results.

Remediation centers on guaranteeing that every byte contributing to an HMAC computation is explicitly initialized and that keys are handled securely. In Fiber, this means initializing buffers, avoiding reuse of slices that may retain stale data, and using constant-time comparison functions for signature verification. Below are concrete, working examples that demonstrate secure HMAC handling.

Example 1: Deterministic HMAC with explicit buffer initialization

Instead of reusing an uninitialized array, create a fresh slice for each request and ensure no residual data influences the input.

import (
	"crypto/hmac"
	"crypto/sha256"
)

func computeHMAC(data, key []byte) []byte {
	h := hmac.New(sha256.New, key)
	h.Write(data)
	return h.Sum(nil)
}

func signHandler(c *fiber.Ctx) error {
	key := []byte("my-secure-key-32-bytes-long-12345678")
	var body []byte = c.Body() // explicit, initialized source
	sig := computeHMAC(body, key)
	return c.SendString(fmt.Sprintf("%x", sig))
}

This approach ensures that the data passed to HMAC is exactly the request body, with no extraneous bytes. The slice returned by c.Body() is a clean, initialized input.

Example 2: Avoiding slice reuse and ensuring zeroing of sensitive buffers

If you must reuse a buffer, clear it before each use and do not rely on uninitialized memory.

var buf [1024]byte // reused buffer

func processRequest(payload []byte, key []byte) []byte {
	// Explicitly zero the buffer before use
	for i := range buf {
		buf[i] = 0
	}
	copy(buf[:], payload)
	return computeHMAC(buf[:len(payload)], key)
}

Zeroing the buffer prevents stale data from mixing with the intended input. Note that for high-performance scenarios, you may allocate fresh slices per request instead of reusing buffers to keep the code simpler and safer.

Example 3: Constant-time signature verification

Prevent timing attacks by using hmac.Equal when comparing signatures, especially if you compare stored and computed HMACs.

func verifyHandler(c *fiber.Ctx) error {
	key := []byte("my-secure-key-32-bytes-long-12345678")
	expectedSig, err := hex.DecodeString(c.Params("sig"))
	if err != nil {
		return c.SendStatus(fiber.StatusBadRequest)
	}
	actualSig := computeHMAC(c.Body(), key)
	if !hmac.Equal(expectedSig, actualSig) {
		return c.SendStatus(fiber.StatusUnauthorized)
	}
	return c.SendStatus(fiber.StatusOK)
}

Using hmac.Equal ensures that comparison time does not depend on the number of matching bytes, mitigating timing-based side channels. Combined with initialized inputs, this provides a robust approach to HMAC security in Fiber.

For teams using middleBrick’s Pro plan, continuous monitoring can help detect instability in authentication outcomes across scans, reinforcing confidence that HMAC-based integrity checks remain deterministic and resistant to side channels introduced by uninitialized memory.