Insecure Direct Object Reference in Fiber with Hmac Signatures

Insecure Direct Object Reference (IDOR) occurs when an API exposes internal object references (such as numeric IDs or UUIDs) without proper authorization checks, allowing one user to access or modify data belonging to another user. In a Fiber application, this risk can be amplified when developers rely on Hmac Signatures for request integrity but overlook authorization at the resource level. Hmac Signatures help ensure that a request has not been tampered with in transit, yet they do not enforce that the authenticated subject has permission to access the targeted resource. For example, an endpoint like GET /users/:id may validate a valid Hmac header, but if the handler directly uses params.id to query a database without verifying that the authenticated user owns that ID, an attacker can simply change the ID to access other users’ data.

Consider a scenario where a client computes an Hmac over selected headers and a timestamp, sends it in x-api-signature, and the server verifies the signature before processing the request. The signature verification may pass, but if the server uses the raw resource identifier from the URL to fetch a record, the trust boundary is at the signature, not at the resource ownership. An authenticated user can iterate through sequential numeric IDs, and each valid Hmac (perhaps tied to a session token) will be accepted, leading to a BOLA/IDOR finding. This is especially relevant when IDs are predictable (e.g., incremental integers) and no contextual authorization is enforced. Even with a strong Hmac implementation, missing ownership checks mean the API remains vulnerable to horizontal privilege escalation across user boundaries.

In a black-box scan, middleBrick tests endpoints that use identifiers in paths or query parameters while validating Hmac Signatures. It confirms whether a valid signature on one user’s context can be reused to access another user’s resources. The scanner does not assume that signature validation equates to authorization; it checks whether each operation enforces proper access controls relative to the requesting identity. Real-world findings often trace back to missing or incomplete authorization logic around endpoints that already use cryptographic integrity mechanisms. Attack patterns such as IDOR via predictable IDs or missing tenant checks are common in APIs that conflate integrity with authorization.

To remediate IDOR in Fiber when Hmac Signatures are in use, you must couple signature validation with explicit resource ownership checks. After verifying the Hmac, resolve the current user (from session, JWT, or another mechanism) and ensure that any data access is scoped to that user. Below is a concrete example in Go using Fiber, where a middleware validates an Hmac signature and attaches the user identity to the context, and a downstream handler enforces ownership before querying a mock data store.

// main.go
package main

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

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

// Shared secret for Hmac (in practice, store securely and rotate)
const secret = "super-secret-key"

// Helper to compute Hmac for testing; not used in production request validation
func computeHmac(message string) string {
    key := []byte(secret)
    h := hmac.New(sha256.New, key)
    h.Write([]byte(message))
    return hex.EncodeToString(h.Sum(nil))
}

// Middleware to validate Hmac signature and extract user
func hmacAuth(c *fiber.Ctx) error {
    // Incoming signature header
    sigHeader := c.Get("X-Api-Signature")
    // Recompute over a canonical string: method + path + timestamp
    timestamp := c.Get("X-Request-Timestamp")
    method := c.Method()
    path := c.Path()
    payload := fmt.Sprintf("%s|%s|%s|%s", method, path, timestamp, "static-body-or-params")
    expected := computeHmac(payload)

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

    // Simulate user resolution — in real apps, decode JWT or look up session
    userID, err := strconv.Atoi(c.Get("X-User-Id", "0"))
    if err != nil || userID == 0 {
        return c.Status(http.StatusUnauthorized).JSON(fiber.Map{"error": "invalid user"})
    }

    c.Locals("userID", userID)
    return c.Next()
}

// Handler with ownership check
func GetUserProfile(c *fiber.Ctx) error {
    requestedID, err := strconv.Atoi(c.Params("id"))
    if err != nil {
        return c.Status(http.StatusBadRequest).JSON(fiber.Map{"error": "invalid user id"})
    }

    currentUser := c.Locals("userID").(int)
    if requestedID != currentUser {
        return c.Status(http.StatusForbidden).JSON(fiber.Map{"error": "access denied"})
    }

    // Mock database fetch — scope query to currentUser
    profile := map[string]interface{}{
        "id":   currentUser,
        "name": "Alice",
    }
    return c.JSON(profile)
}

func main() {
    app := fiber.New()
    app.Use(hmacAuth)
    app.Get("/users/:id", GetUserProfile)
    app.Listen(":3000")
}

The key remediation steps are:

• After Hmac verification, resolve the authenticated user identity (e.g., from a JWT claim or session store).
• Ensure that any data access uses this identity to scope queries (e.g., WHERE user_id = $1).
• Explicitly compare the requested resource ID with the authenticated user ID and reject if they do not match for user-specific endpoints.
• Avoid using raw IDs from the client for indirect lookups without a tenant or ownership filter.

By combining Hmac integrity checks with strict ownership validation, you prevent IDOR while preserving the integrity guarantees of your signature scheme. middleBrick’s scans validate that both signature verification and resource-level authorization are present, helping you close the gap between cryptographic integrity and access control.