Bola Idor in Buffalo with Hmac Signatures

BOLA (Broken Level Authorization) / IDOR occurs when an API exposes object-level access controls that a user can manipulate simply by changing an identifier. When endpoints in Buffalo are protected only with HMAC signatures, the vulnerability arises if the signature does not bind the request to the specific resource and its owner. A server may validate that the HMAC is correct, but if the signed payload does not include a user identifier or a tenant/context identifier, an attacker can take a valid signature from one resource and reuse it against another resource they do not own.

For example, imagine an endpoint /api/v1/buffalo/{buffaloId} that uses an HMAC signature to authenticate requests. The signature might be computed over parameters like buffaloId, method, and timestamp, but omit the user ID or organization context. An authenticated user can copy the signature from their own buffalo record and change the buffaloId in the URL to access another user’s buffalo. Because the server verifies the cryptographic integrity of the parameters but does not verify ownership, the request is processed in the context of the attacker’s permissions, leading to unauthorized data access or modification. This is BOLA/IDOR rooted in a weak HMAC binding strategy.

In Buffalo, a typical handler might parse query or path parameters and verify an x-signature header without ensuring that the signed scope matches the actor’s identity. If the signature generation uses only the resource ID and does not incorporate the principal ID or a shared context such as an organization slug, the authorization boundary collapses. An attacker does not need to break the HMAC algorithm; they simply replay a valid signature across different resource IDs. Because the scan methodology of middleBrick includes unauthenticated endpoint analysis, such authorization flaws can be detected by correlating endpoint behavior with missing ownership checks, even when HMACs are present.

Real-world attack patterns mirror this: an attacker enumerates buffalo identifiers, captures a signed request, and replays it with altered IDs. If the server does not tie the HMAC to the actor and to the specific resource instance, the authorization decision incorrectly assumes the requester is allowed. This maps to OWASP API Top 10 API1:2023 Broken Object Level Authorization and can expose sensitive buffalo records or allow modification where it should be prohibited. The presence of HMAC signatures can give a false sense of integrity if developers assume cryptographic integrity alone prevents IDOR.

To fix BOLA/IDOR when using HMAC signatures in Buffalo, the signature must bind the request to both the actor and the resource. This means including the user ID (or a stable principal identifier) and the resource owner context (such as organization or tenant) in the signed payload. The server must then recompute the HMAC using the same scope and reject the request if the actor in the signature does not match the authenticated actor or if the resource does not belong to that actor.

Below are concrete code examples using Go with the Buffalo framework and a standard HMAC-SHA256 scheme. The examples show how to generate and verify a signature that includes user ID, buffalo ID, HTTP method, and a timestamp to prevent replay attacks.

Signing on the client

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

func buildSignedParams(baseURL string, buffaloID string, userID string, apiKey string, secret string) (url.Values, error) {
    params := url.Values{}
    timestamp := fmt.Sprintf("%d", time.Now().Unix())
    params.Set("user_id", userID)
    params.Set("buffalo_id", buffaloID)
    params.Set("timestamp", timestamp)
    params.Set("method", "GET")

    // Deterministic key order for signing
    keys := []string{"buffalo_id", "method", "timestamp", "user_id"}
    var parts []string
    for _, k := range keys {
        parts = append(parts, k+"="+params.Get(k))
    }
    message := strings.Join(parts, "&")

    mac := hmac.New(sha256.New, []byte(secret))
    mac.Write([]byte(message))
    signature := hex.EncodeToString(mac.Sum(nil))
    params.Set("signature", signature)
    params.Set("api_key", apiKey)
    return params, nil
}

Verifying on the server in Buffalo

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

func verifyHmac(next http.Handler) http.Handler {
    return http.HandlerFunc(func(rw http.ResponseWriter, r *http.Request) {
        // Extract query parameters
        query := r.URL.Query()
        receivedSig := query.Get("signature")
        if receivedSig == "" {
            http.Error(rw, "missing signature", http.StatusUnauthorized)
            return
        }

        userID := query.Get("user_id")
        buffaloID := query.Get("buffalo_id")
        timestamp := query.Get("timestamp")
        method := query.Get("method")
        apiKey := query.Get("api_key")

        // Optional: enforce timestamp window to mitigate replay
        // if time.Since(parsed) > 5*time.Minute { reject }

        // Recompute using the same key order as client
        keys := []string{"buffalo_id", "method", "timestamp", "user_id"}
        var parts []string
        for _, k := range keys {
            val := query.Get(k)
            if val == "" {
                http.Error(rw, "missing parameter in signature scope", http.StatusBadRequest)
                return
            }
            parts = append(parts, k+"="+val)
        }
        message := strings.Join(parts, "")

        // Fetch the secret per apiKey/user (e.g., from a secure store)
        secret, err := getSecretForAPIKey(apiKey, userID)
        if err != nil {
            http.Error(rw, "invalid credentials", http.StatusUnauthorized)
            return
        }

        mac := hmac.New(sha256.New, []byte(secret))
        mac.Write([]byte(message))
        expected := hex.EncodeToString(mac.Sum(nil))

        if !hmac.Equal([]byte(expected), []byte(receivedSig)) {
            http.Error(rw, "invalid signature", http.StatusUnauthorized)
            return
        }

        // Additional ownership check: ensure buffaloID belongs to userID
        if !doesBuffaloBelongToUser(buffaloID, userID) {
            http.Error(rw, "forbidden: buffalo not owned by user", http.StatusForbidden)
            return
        }

        next.ServeHTTP(rw, r)
    })

Key remediation points: include the user principal and the resource owner context in the signed payload; enforce ownership checks after signature validation; use a monotonic or timestamp check to reduce replay risk; and avoid signing only the resource ID. Using middleBrick’s CLI (middlebrick scan <url>) or GitHub Action can help detect whether your endpoints validate ownership in addition to signature integrity.