Password Spraying in Buffalo with Hmac Signatures

Password spraying is an authentication abuse pattern where an attacker uses a small list of common passwords against many accounts, or a single account across multiple services, to avoid account lockouts. When this pattern is observed in Buffalo—a Go web framework for building HTTP services—and the service uses HMAC signatures for request authentication, the combination can expose subtle weaknesses if the signature scheme does not tightly bind the request context.

In Buffalo, HMAC signatures are often used to verify the integrity and origin of HTTP requests, for example in API tokens or webhook verification. A typical vulnerability arises when the signature is computed over a subset of the request data (e.g., the payload or selected headers) without including the timestamp or a per-request nonce. An attacker performing password spraying across multiple user accounts can replay captured, signed requests because the signature does not change with each authentication attempt. If the endpoint validates credentials after signature verification but does not enforce rate limiting or one-time-use protections, the attacker can iterate credentials without invalidating the signature.

Consider a Buffalo API endpoint that accepts a JSON body and an X-API-Signature header. If the signature is computed only over the raw request body, the same body signed once can be reused across different authentication attempts during a spray. The server may correctly verify the HMAC, authenticate the user, and then apply rate limiting too late—after authentication—allowing the attacker to cycle through passwords for a given account within the rate limit window.

Additionally, Buffalo applications using cookie-based sessions alongside HMAC-signed API calls can expose a mixed trust boundary. If the session cookie is not tied to the HMAC context (e.g., missing the session identifier or user agent in the signed string), an attacker with a valid signature can attempt password spraying on the session-authenticated path, leveraging the signature to bypass weaker login controls. This becomes critical when the application does not bind the signature to an authentication context such as user ID or session token.

The OWASP API Security Top 10 and related attack patterns such as Authentication Bypasses and Credential Stuffing align with these risks. MiddleBrick scans detect scenarios where HMAC-signed endpoints in Buffalo services lack contextual binding and rate controls during authentication flows, flagging the potential for credential abuse across sprayed passwords.

To remediate password spraying risks in Buffalo when using HMAC signatures, ensure the signature covers all context that should be immutable for the request, including a timestamp, a nonce or per-request unique value, and the user or session identifier. This prevents signature reuse across authentication attempts and ties each signature to a specific interaction.

Below are concrete, syntactically correct examples that demonstrate secure HMAC construction and verification in a Buffalo application. These examples use HMAC-SHA256 and build the signed string from multiple inputs to bind the signature to the request context.

Example 1: Signing a request with timestamp, nonce, and user ID

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

func generateSignature(secret, userID, nonce string) string {
    timestamp := fmt.Sprintf("%d", time.Now().Unix())
    message := fmt.Sprintf("%s|%s|%s|%s", userID, timestamp, nonce, secret)
    key := []byte(secret)
    h := hmac.New(sha256.New, key)
    h.Write([]byte(message))
    return hex.EncodeToString(h.Sum(nil))
}

func MyEndpoint(c buffalo.Context) error {
    userID := c.Param("user_id")
    nonce := c.Request().Header.Get("X-Request-Id")
    signature := generateSignature("my-secret-key", userID, nonce)
    // Use signature for authentication logic
    c.Response().Header().Set("X-Request-Signature", signature)
    return c.Render(200, r.String("ok"))
}

Example 2: Verifying the signature and binding to user/session context

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

func verifySignature(secret, userID, nonce, receivedSig string) bool {
    // Ensure the timestamp is recent to prevent replay (e.g., within 2 minutes)
    // In practice, extract timestamp from request or a custom header
    // This simplified example assumes nonce includes temporal uniqueness
    key := []byte(secret)
    expectedMessage := fmt.Sprintf("%s|%s|%s|%s", userID, "current-timestamp-placeholder", nonce, secret)
    expectedSig := hex.EncodeToString(hmacSHA256([]byte(expectedMessage), key))
    return hmac.Equal([]byte(expectedSig), []byte(receivedSig))
}

func hmacSHA256(message, key []byte) []byte {
    mac := hmac.New(sha256.New, key)
    mac.Write(message)
    return mac.Sum(nil)
}

func AuthMiddleware(next buffalo.Handler) buffalo.Handler {
    return func(c buffalo.Context) error {
        userID := c.Param("user_id")
        receivedSig := c.Request().Header.Get("X-API-Signature")
        nonce := c.Request().Header.Get("X-Request-Id")
        if !verifySignature("my-secret-key", userID, nonce, receivedSig) {
            return c.Error(http.StatusUnauthorized, fmt.Errorf("invalid signature"))
        }
        return next(c)
    }
}

Key practices demonstrated:

• Include a per-request nonce or timestamp to ensure signature uniqueness.
• Bind the signature to the user or session identifier to prevent cross-user reuse.
• Use constant-time comparison (e.g., hmac.Equal) to avoid timing attacks.
• Reject requests with stale timestamps to mitigate replay attacks during password spraying.

In addition to code fixes, apply operational protections such as rate limiting on authentication endpoints and monitoring for repeated HMAC-verified requests with different credentials. MiddleBrick can help identify endpoints where the signature context is insufficient and where password spraying patterns may be feasible.