Insecure Deserialization in Fiber with Hmac Signatures

Insecure deserialization in a Fiber-based API occurs when an endpoint accepts a serialized payload (e.g., JSON, gob, or other formats), deserializes it without strict validation, and uses an HMAC signature only to assert integrity rather than authenticity or authorization. Even when an HMAC is used to verify that the payload has not been tampered with, deserialization of untrusted data can lead to arbitrary code execution, logic bypass, or sensitive data exposure. For example, an attacker might supply a maliciously crafted object that, when deserialized, triggers dangerous methods or reads internal resources. The presence of an HMAC does not prevent unsafe deserialization; it only ensures the bytes have not been altered after signing. If the server trusts the HMAC and proceeds to deserialize the content without type checks, length limits, or schema validation, the signature remains valid while the deserialization behavior is unsafe.

Consider a scenario where a Fiber endpoint receives a JSON body with a special field that is later unmarshaled into an interface{} and then type-asserted. If the client computes an HMAC over the raw body using a shared secret and includes it in a header, the server verifies the HMAC before deserialization. This flow might look correct at first glance, but if the server uses a generic deserializer (e.g., json.Unmarshal into an empty interface or a struct with an untyped field), an attacker can embed nested structures, timestamps, or specially crafted types that lead to unexpected behavior upon deserialization. The HMAC verification passes because the attacker can obtain a valid signature only if they know the secret; however, if the secret leaks or is weak, they can forge payloads. More importantly, even with a valid signature, unsafe deserialization can exploit gadget chains or invoke methods during unmarshaling, leading to Remote Code Execution (RCE). This pattern is commonly seen in APIs that prioritize integrity checks but overlook input validation and the principle of least privilege during deserialization.

Real-world impacts mirror known attack patterns such as CVE-2020-26160 in related ecosystems, where insecure deserialization allowed attackers to execute code via crafted objects. In a Fiber service, this could manifest as unauthorized privilege escalation, data exfiltration, or service disruption. The root cause is not the HMAC itself but the combination of weak deserialization practices with over-trust in signed payloads. Attackers who do not know the HMAC secret cannot forge valid signatures, but they might exploit other vectors such as signature replay or insufficient scope checks. Therefore, defenses must include strict type definitions, allowlisting of known fields, and avoiding interface{}-based unmarshaling for untrusted input, even when an HMAC is present.

To remediate insecure deserialization in Fiber when using HMAC signatures, apply strict input validation and avoid generic deserialization of untrusted data. Use strongly typed structs with explicit field types and omit unknown fields. Do not rely on interface{} or map[string]interface{} for untrusted payloads unless you enforce rigorous allowlisting. Verify the HMAC before processing, but treat deserialization as a separate security boundary. Below are concrete code examples showing a safe approach in Fiber.

First, define a typed request structure and compute/verify HMAC using HMAC-SHA256. The client computes the signature over the raw JSON body and sends it in a header; the server recomputes and compares in constant time.

// client side: compute HMAC-SHA256 over raw JSON body
package main

import (
    "crypto/hmac"
    "crypto/sha256"
    "encoding/hex"
    "net/http"
    "strings"
)

const secret = "my-strong-shared-secret"

func signBody(body string) string {
    mac := hmac.New(sha256.New, []byte(secret))
    mac.Write([]byte(body))
    return hex.EncodeToString(mac.Sum(nil))
}

func main() {
    rawBody := `{"userId":"123","action":"transfer"}`
    signature := signBody(rawBody)
    req, _ := http.NewRequest("POST", "https://api.example.com/transfer", strings.NewReader(rawBody))
    req.Header.Set("X-Request-Signature", signature)
    // send req
}

On the server side in Fiber, verify the signature before deserialization and use a strongly typed struct. Do not deserialize into interface{}.

// server side: Fiber handler with HMAC verification and strict unmarshaling
package main

import (
    "crypto/hmac"
    "crypto/sha256"
    "encoding/hex"
    "io/ioutil"
    "net/http"
    "strings"

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

const secret = "my-strong-shared-secret"

func verifyHMAC(body, receivedSig string) bool {
    mac := hmac.New(sha256.New, []byte(secret))
    mac.Write([]byte(body))
    expected := hex.EncodeToString(mac.Sum(nil))
    return hmac.Equal([]byte(expected), []byte(receivedSig))
}

type TransferRequest struct {
    UserID string `json:"userId"`
    Action string `json:"action"`
}

func main() {
    app := fiber.New()
    app.Post("/transfer", func(c *fiber.Ctx) error {
        rawBody := c.Request().Body()
        bodyStr := string(rawBody)
        sig := c.Get("X-Request-Signature", "")
        if !verifyHMAC(bodyStr, sig) {
            return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "invalid signature"})
        }
        var req TransferRequest
        if err := c.BodyParser(&req); err != nil {
            return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{"error": "invalid payload"})
        }
        // proceed with validated req.UserID and req.Action
        return c.JSON(fiber.Map{"status": "ok", "userId": req.UserID, "action": req.Action})
    })
    app.Listen(":3000")
}

Additional hardening steps include: setting explicit JSON decoder options to disallow unknown fields, applying schema validation (e.g., using a library that supports go-playground/validator), and rate limiting to reduce abuse surface. These measures ensure that HMAC integrity checks complement, rather than replace, secure deserialization practices.