Type Confusion in Fiber with Hmac Signatures

Type confusion in the Fiber web framework combined with improper handling of Hmac Signatures can lead to security weaknesses where an attacker manipulates the expected type of a value used in signature verification. In Go, Fiber often uses middleware to validate Hmac signatures to ensure request integrity. If the code that extracts or casts values for signing or verification does not enforce strict types, an attacker can supply a value of a different type that still passes type checks at runtime due to interface{} usage or reflection, causing the signature verification logic to misinterpret the data.

For example, consider a handler that reads a JSON payload containing an identifier and a signature. If the identifier is expected to be a string but is instead received as a number or a nested object, and the verification logic uses a loosely typed assertion (e.g., v, ok := parsed["id"].(string)), the assertion may succeed incorrectly or produce a zero value when the type does not match. This can allow an attacker to forge a valid Hmac signature by providing a numeric or object value that, when hashed with the shared secret, produces a different but accepted signature due to mismatched canonicalization rules.

When combined with Hmac Signatures, type confusion can expose the application to signature bypass or data tampering. The signature is typically computed over a canonical representation of the payload or selected fields. If the fields used in the canonicalization process are not strictly typed or are serialized differently than expected (e.g., integers formatted without quotes vs strings with quotes), an attacker may craft inputs that result in the same signature despite altered content. This is especially dangerous when the application uses interface{} to handle dynamic payloads and fails to validate types before including them in the Hmac computation or verification.

Moreover, reflection-based unmarshaling in Fiber can silently assign default types when the expected type is ambiguous, leading to subtle mismatches between the runtime types and the types assumed during signature generation. An attacker can exploit this by sending values that appear correct in JSON but resolve to different Go types, causing the Hmac verification to incorrectly validate the request as authentic. This undermines the integrity guarantees provided by Hmac Signatures and can lead to unauthorized actions or data exposure.

To remediate type confusion with Hmac Signatures in Fiber, enforce strict type assertions and canonicalization before computing or verifying signatures. Use explicit type checks and avoid interface{} for fields involved in signature generation. Below are concrete code examples demonstrating secure handling in Fiber.

Example 1: Strict type assertion and canonical JSON serialization

package main

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"fmt"
	"github.com/gofiber/fiber/v2"
)

type SignedRequest struct {
	Action string `json:"action"`
	UserID int64  `json:"user_id"`
}

func verifyHmacSignature(c *fiber.Ctx) error {
	var req SignedRequest
	if err := c.BodyParser(&req); err != nil {
		return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{"error": "invalid payload"})
	}

	// Canonical JSON serialization with strict field ordering
	canonical, err := json.Marshal(req)
	if err != nil {
		return c.Status(fiber.StatusInternalServerError).JSON(fiber.Map{"error": "serialization failed"})
	}

	secret := []byte("super-secret-key")
	mac := hmac.New(sha256.New, secret)
	mac.Write(canonical)
	expectedSignature := hex.EncodeToString(mac.Sum(nil))

	providedSignature := c.Get("X-Signature")
	if !hmac.Equal([]byte(expectedSignature), []byte(providedSignature)) {
		return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "invalid signature"})
	}

	return c.JSON(fiber.Map{"status": "ok"})
}

func main() {
	app := fiber.New()
	app.Post("/secure", verifyHmacSignature)
	app.Listen(":3000")
}

Example 2: Explicit field validation and avoiding interface{} type assertions

package main

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/hex"
	"fmt"
	"net/http"
	"strconv"

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

func secureHandler(c *fiber.Ctx) error {
	userIDStr := c.FormValue("user_id")
	amountStr := c.FormValue("amount")

	// Strict parsing and type validation
	userID, err := strconv.ParseInt(userIDStr, 10, 64)
	if err != nil {
		return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{"error": "invalid user_id"})
	}

	amount, err := strconv.ParseFloat(amountStr, 64)
	if err != nil {
		return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{"error": "invalid amount"})
	}

	// Build canonical string without interface{} ambiguity
	canonical := userIDStr + amountStr
	mac := hmac.New(sha256.New, []byte("secret"))
	mac.Write([]byte(canonical))
	expected := hex.EncodeToString(mac.Sum(nil))

	if !hmac.Equal([]byte(expected), []byte(c.Get("X-Signature"))) {
		return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "invalid signature"})
	}

	return c.JSON(fiber.Map{"user_id": userID, "amount": amount})
}

func main() {
	fiberApp := fiber.New()
	fiberApp.Post("/payment", secureHandler)
	fiberApp.Listen(":3001")
}

In both examples, the remediation focuses on avoiding ambiguous type assertions, using explicit struct unmarshaling or strict parsing, and ensuring the canonical input to Hmac is deterministic and type-consistent. This prevents attackers from exploiting type confusion to forge valid Hmac Signatures.