Insecure Direct Object Reference in Echo Go with Hmac Signatures

Insecure Direct Object Reference (IDOR) occurs when an API exposes a reference—such as a numeric ID or a UUID—to a resource without verifying that the requesting user is authorized to access that specific object. In Echo Go, this often manifests when a route like /users/:id/profile uses an identifier directly from the URL, looks up the corresponding record, and returns it without confirming the caller owns or is permitted to view that record. When Hmac Signatures are introduced to add request integrity and replay protection, the interaction with ID handling can inadvertently preserve or expose IDOR if the signature does not bind the resource identifier to the authorization context.

Consider an endpoint that accepts an Hmac-signed query or header to prove the request has not been tampered with. A typical pattern includes a signature generated over selected parameters, for example the resource ID, timestamp, and a shared secret. If the server validates the Hmac signature correctly but then directly uses the provided ID to fetch data without checking whether the authenticated subject (e.g., via session or token claims) has access to that ID, the endpoint remains vulnerable to IDOR. An attacker who can guess or enumerate numeric IDs could forge valid Hmac signatures if the signing key is leaked or the signing scope is too broad, or simply omit the signature if unsigned paths exist, and then iterate over IDs to harvest other users’ data. Even when signatures are enforced, if the signature scope does not explicitly include the subject’s identity or tenant context, the resource reference becomes a direct object reference that bypasses authorization checks.

In Echo Go, this can occur when middleware validates the Hmac signature and then passes the raw :id path parameter to a service that performs a direct lookup. For example, a signature might cover id=123 and a timestamp, but if the handler does not cross-reference the authenticated user’s ID with the requested id, the lookup proceeds purely on the supplied reference. This is especially risky when the ID is predictable, such as an auto-incrementing integer, or when UUIDs are not truly random and can be enumerated. The vulnerability is not in the Hmac algorithm itself, but in how the signed parameters are mapped to authorization decisions. If the signature silently implies trust in the ID, the application effectively treats the object reference as an authorization token, which it is not.

Real-world analogies include scenarios where an attacker iterates over invoice IDs signed by a legitimate user who inadvertently exposes a signature scheme that does not include user context, as seen in cases tied to OWASP API Top 10 A1: Broken Object Level Authorization. In Echo Go applications that use JWTs for identity alongside Hmac for request integrity, developers must ensure that the subject claim within the JWT is explicitly checked against the resource identifier before any data access. Without this check, the combination of a verifiable signature and a direct object reference creates a pathway for unauthorized enumeration and data exposure, even when the transport integrity is cryptographically assured.

Remediation centers on binding the resource identifier to the authenticated subject and enforcing strict authorization before data access. In Echo Go, this means extending the Hmac validation logic to include the user’s unique ID or tenant context in the signed payload, and ensuring every data lookup is gated by a permissions check that uses the authenticated principal rather than the raw URL parameter alone.

First, when constructing the Hmac signature, incorporate the subject identifier. For example, sign a concatenation of the user ID, the resource ID, a timestamp, and a nonce. This ensures that even if an attacker discovers or guesses a valid signature for one user, they cannot reuse it for another user because the subject is part of the signed scope.

// Example: building an Hmac signature that includes user ID and resource ID
package main

import (
    "crypto/hmac"
    "crypto/sha256"
    "encoding/hex"
    "fmt"
    "net/url"
    "strconv"
    "strings"
    "time"
)

func buildHmac(userID, resourceID string, secret []byte) string {
    timestamp := strconv.FormatInt(time.Now().Unix(), 10)
    message := strings.Join([]string{userID, resourceID, timestamp, "nonce123"}, "|")
    mac := hmac.New(sha256.New, secret)
    mac.Write([]byte(message))
    sig := hex.EncodeToString(mac.Sum(nil))
    // Return signature and timestamp for validation
    return url.QueryEscape(sig + ":" + timestamp)
}

Second, in the handler, validate the Hmac and then explicitly verify that the resource being accessed belongs to the authenticated user. Do not rely on the ID from the URL as the sole authorization determinant. Fetch the object using a compound lookup that includes the user identifier, or perform an explicit ownership check before returning data.

// Example: Echo handler with Hmac validation and IDOR protection
package main

import (
    "net/http"
    "strings"
    "github.com/labstack/echo/v4"
)

func profileHandler(secret []byte) echo.HandlerFunc {
    return func(c echo.Context) error {
        userID := c.Get("user_id").(string) // from JWT or session
        requestedID := c.Param("id")
        sigQuery := c.QueryParam("sig")
        sigParts := strings.Split(sigQuery, ":")
        if len(sigParts) != 2 {
            return echo.NewHTTPError(http.StatusBadRequest, "invalid signature")
        }
        receivedSig := sigParts[0]
        timestamp := sigParts[1]
        // Recompute expected signature
        message := strings.Join([]string{userID, requestedID, timestamp, "nonce123"}, "|")
        mac := hmac.New(sha256.New, secret)
        mac.Write([]byte(message))
        expectedSig := hex.EncodeToString(mac.Sum(nil))
        if !hmac.Equal([]byte(expectedSig), []byte(receivedSig)) {
            return echo.NewHTTPError(http.StatusForbidden, "invalid signature")
        }
        // Critical: verify ownership/authorization before fetching
        var profile Profile
        if err := db.Where("id = ? AND user_id = ?", requestedID, userID).First(&profile).Error; err != nil {
            return echo.NewHTTPError(http.StatusForbidden, "access denied")
        }
        return c.JSON(http.StatusOK, profile)
    }
}

Third, prefer using UUIDs over predictable integers for resource identifiers, and ensure that enumeration is mitigated through consistent permission checks rather than relying on obscurity. Combine this with logging and monitoring for repeated signature validation failures, which may indicate probing attempts. These steps ensure that Hmac Signatures improve integrity without inadvertently weakening object-level authorization.