HIGH use after freefiberhmac signatures

Use After Free in Fiber with Hmac Signatures

Use After Free in Fiber with Hmac Signatures — how this specific combination creates or exposes the vulnerability

Use After Free (UAF) in the context of an API built with Fiber and Hmac Signatures occurs when memory that was previously associated with a parsed request or signature state is reused after being freed, leading to unpredictable behavior or potential data leakage. Although the scanning engine is black-box and does not inspect memory internals, the observable effect is that runtime checks based on Hmac validation may reference stale or corrupted data, bypassing intended integrity checks.

Consider a typical Fiber route that relies on Hmac Signatures to verify request integrity. The application reads a signature from a header, computes the expected Hmac using a shared secret, and compares the two values. If the logic holds references to buffers or objects that are released or overwritten before the comparison completes—due to asynchronous processing, object pooling, or improper scoping—a Use After Free condition can manifest. For example, if the computed Hmac or intermediate byte slices are stored in variables that get reused across requests in a high-concurrency environment, a later request might inadvertently use memory that was freed and reallocated for another purpose.

In practice, this can lead to signature validation anomalies where a valid signature is rejected or, worse, an invalid signature is accepted because the comparison logic references corrupted memory. The scanner detects such issues under the BFLA/Privilege Escalation and Input Validation checks, highlighting mismatches between expected and observed behavior in Hmac verification flows. Real-world attack patterns like CVE-2023-28878 (which involved memory handling flaws in web frameworks) illustrate how signature verification can be undermined when memory is mishandled, even if the cryptographic primitive itself is sound.

Because Fiber is a high-performance framework that favors low-latency request handling, developers may inadvertently introduce subtle scoping issues when using buffers or context objects across middleware and handlers. The scanner flags these as potential UAF risks under the Unsafe Consumption and Property Authorization checks, especially when Hmac operations depend on request-scoped data that may be prematurely released.

Hmac Signatures-Specific Remediation in Fiber — concrete code fixes

To mitigate Use After Free risks when using Hmac Signatures in Fiber, ensure that all cryptographic buffers, computed digests, and comparison values are scoped to the request lifecycle and not reused across concurrent operations. Below are concrete code examples demonstrating safe handling.

Vulnerable pattern (illustrative):

package main

import (
	"crypto/hmac"
	"crypto/sha256"
	"github.com/gofiber/fiber/v2"
)

var sharedSecret = []byte("super-secret-key")
var computedDigest []byte // Dangerous: reused across requests

func VerifyHmac(c *fiber.Ctx) error {
	payload := c.Body()
	signature := c.Get("X-Signature")
	h := hmac.New(sha256.New, sharedSecret)
	h.Write(payload)
	computedDigest = h.Sum(nil) // Reassigning a shared variable
	if !hmac.Equal(computedDigest, []byte(signature)) {
		return c.Status(fiber.StatusUnauthorized).SendString("Invalid signature")
	}
	return c.Next()
}

In this example, computedDigest is a package-level variable that can be overwritten by concurrent requests, leading to Use After Free if the memory is accessed after being reallocated. The scanner would flag this under BFLA/Privilege Escalation due to unsafe state sharing.

Secure pattern:

package main

import (
	"crypto/hmac"
	"crypto/sha256"
	"github.com/gofiber/fiber/v2"
)

func VerifyHmac(c *fiber.Ctx) error {
	payload := c.Body()
	signature := c.Get("X-Signature")
	h := hmac.New(sha256.New, []byte("super-secret-key"))
	h.Write(payload)
	digest := h.Sum(nil) // Scoped to the function, no reuse
	if !hmac.Equal(digest, []byte(signature)) {
		return c.Status(fiber.StatusUnauthorized).SendString("Invalid signature")
	}
	return c.Next()
}

By declaring digest inside the handler, each request operates on its own memory, eliminating cross-request contamination. The scanner validates this approach under the Authentication and Input Validation checks, confirming that Hmac verification is isolated and reliable.

Additionally, when integrating with the middleBrick CLI (middlebrick scan <url>) or GitHub Action, ensure that your codebase avoids global state for cryptographic operations. The continuous monitoring capabilities of the Pro plan can alert you if changes reintroduce unsafe patterns, while the dashboard helps track improvements over time.

Frequently Asked Questions

Can the scanner detect Use After Free risks in Hmac validation logic?

The scanner identifies observable symptoms such as inconsistent signature validation under the BFLA/Privilege Escalation and Input Validation checks. It does not inspect memory directly but flags patterns that may indicate unsafe handling of cryptographic state.

Does using middleware alter Hmac verification safety in Fiber?

Yes. Middleware that shares buffers or cryptographic state across routes can introduce Use After Free risks. Always scope Hmac computations to the request handler and avoid reusing variables across middleware and endpoints.

Use After Free in Fiber with Hmac Signatures