Excessive Data Exposure in Buffalo with Hmac Signatures

Excessive Data Exposure occurs when an API returns more information than necessary for a given operation. In Buffalo, a Go web framework, this commonly intersects with Hmac Signatures when responses include sensitive fields alongside a signature that was intended only for authentication or integrity. For example, an endpoint might return user profile data, internal IDs, or debug details together with a computed Hmac signature in a header or body field. Because the signature does not protect the content of the response, exposing sensitive data in the same payload defeats the purpose of protecting integrity and can lead to information disclosure.

Consider a scenario where an API returns a JSON object containing email, role, and internal identifiers along with an X-Response-Signature header computed over a canonical subset of the response. If the response includes fields that should be restricted based on authorization context, an attacker who gains read access to network traffic or logs can harvest personal data while the signature merely confirms the payload was not tampered with. MiddleBrick identifies this pattern as a finding under Data Exposure, noting that Hmac Signatures verify integrity but do not limit what is exposed. The scanner checks whether sensitive data appears in responses that carry integrity markers and highlights the mismatch between integrity protection and data minimization.

In Buffalo, developers sometimes compute an Hmac over a canonical string that includes selected headers and a timestamp, then include the signature in a header while returning the full request and response payloads. If the payload contains sensitive data and the server does not strip or mask those fields before serialization, the signature does nothing to prevent exposure. Real-world attack patterns such as log injection or insecure storage of logs can amplify the risk, as exposed fields remain in clear text. The scanner evaluates whether endpoints that use Hmac Signatures still leak information like PII, secrets, or internal tokens, and maps findings to OWASP API Top 10 and relevant compliance frameworks.

Remediation focuses on ensuring that sensitive data is not present in API responses when integrity mechanisms such as Hmac Signatures are used, and on correctly constructing and verifying signatures in Buffalo handlers. You should limit response payloads to the minimal required fields and avoid including sensitive or internal data in structures that are signed or returned to the client. Below are concrete code examples demonstrating how to implement Hmac Signatures safely in Buffalo.

Example 1: Signing a canonical payload and returning only necessary fields

import (
    "crypto/hmac"
    "crypto/sha256"
    "encoding/hex"
    "encoding/json"
    "net/http"
    "time"

    "github.com/gobuffalo/buffalo"
)

type minimalResponse struct {
    UserID   string `json:"user_id"`
    Username string `json:"username"`
    Scope    string `json:"scope"`
}

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

func userProfileHandler(c buffalo.Context) error {
    // Build a minimal response structure
    resp := minimalResponse{
        UserID:   c.Param("user_id"),
        Username: "alice",
        Scope:    "read",
    }
    body, _ := json.Marshal(resp)

    // Compute Hmac over the canonical JSON body
    signature := signPayload(body, "your-256-bit-secret")

    // Return only the necessary data and the signature in a header
    c.Response().Header().Set("X-Response-Signature", signature)
    return c.Render(200, r.JSON(body))
}

Example 2: Verifying signature and avoiding data leakage in middleware

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

    "github.com/gobuffalo/buffalo"
)

func VerifyHmac(secret string) buffalo.MiddlewareFunc {
    return func(next buffalo.Handler) buffalo.Handler {
        return func(c buffalo.Context) error {
            // Read the request body if present and verify integrity
            var payload map[string]interface{}
            if err := json.NewDecoder(c.Request().Body).Decode(&payload); err != nil {
                return c.Render(400, r.JSON(map[string]string{"error": "invalid body"}))
            }
            // Recompute signature over the received body
            body, _ := json.Marshal(payload)
            expected := signPayload(body, secret)
            provided := c.Request().Header.Get("X-Request-Signature")
            if !hmac.Equal([]byte(expected), []byte(provided)) {
                return c.Render(401, r.JSON(map[string]string{"error": "invalid signature"}))
            }
            // Restore body for downstream handlers
            c.Request().Body = ioutil.NopCloser(bytes.NewBuffer(body))
            return next(c)
        }
    }
}

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

Example 3: Avoiding inclusion of sensitive fields in signed responses

import (
    "crypto/hmac"
    "crypto/sha256"
    "encoding/hex"
    "encoding/json"
    "net/http"

    "github.com/gobuffalo/buffalo"
)

type safeProfile struct {
    UserID   string `json:"user_id"`
    Username string `json:"username"`
}

func buildSafeProfile(u User) safeProfile {
    // Explicitly exclude sensitive fields such as email, password hash, tokens
    return safeProfile{
        UserID:   u.ID,
        Username: u.Username,
    }
}

func profileHandler(c buffalo.Context) error {
    user := getCurrentUser(c) // hypothetical function
    safe := buildSafeProfile(user)
    body, _ := json.Marshal(safe)

    signature := signPayload(body, "your-256-bit-secret")
    c.Response().Header().Set("X-Response-Signature", signature)
    return c.Render(200, r.JSON(body))
}

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

Operational guidance

• Define a strict response schema that excludes sensitive or unnecessary fields before computing Hmac Signatures.
• Compute the signature over a canonical representation (e.g., sorted keys or a defined subset) to avoid discrepancies between client and server verification.
• Use constant-time comparison when verifying signatures to mitigate timing attacks.
• Audit logs and transport layers should treat exposed fields as sensitive even when signatures are present, because signatures do not encrypt data.