Open Redirect in Buffalo with Hmac Signatures

An Open Redirect in a Buffalo application that uses Hmac Signatures can occur when a redirect target is derived from user-controlled input and the application relies solely on Hmac Signatures to validate that input without additional context checks. In Buffalo, a typical pattern is to generate a signed token containing a destination URL (e.g., next) and then redirect the user after verifying the signature. If the application does not validate the URL’s host or path against an allowlist before using it in a redirect, an attacker can supply a malicious external URL and have the signature validated successfully, leading to an open redirect.

Hmac Signatures ensure integrity and authenticity of data, but they do not guarantee safety. For example, a developer might sign a URL parameter like url with a secret key and then, upon verification, directly pass that URL to Buffalo’s redirect helper. Because the signature matches, the framework assumes the URL is safe, even if it points to an attacker-controlled domain. This becomes a stored or reflected open redirect when the signed value is reused across requests or embedded in emails. The risk is compounded if the signature verification does not enforce a strict Host check or if the redirect logic does not reject non-absolute paths that resolve to external hosts via relative resolution.

In the context of security scanning, this pattern is flagged because the unauthenticated attack surface includes endpoints that perform redirects with signed parameters. The scanner tests whether the redirect can be manipulated to point to arbitrary external domains while still producing a valid Hmac Signature. If successful, it indicates that the signature mechanism is being used for integrity alone, without enforcing destination safety, which maps to the OWASP API Top 10 #5 (Broken Function Level Authorization) and #1 (Broken Object Level Authorization) when the redirect influences access control flows.

To remediate open redirect risks when using Hmac Signatures in Buffalo, you must combine signature verification with strict destination validation. The signed payload should only contain a relative path or a pre-approved host, and the application must enforce allowlisting before performing any redirect. Below are concrete code examples for a safe implementation in a Buffalo application using Go’s hmac package.

1. Sign and verify a relative redirect path

Instead of signing a full URL, sign a relative path and store the allowed host in server-side configuration. This ensures the redirect never leaves your domain.

import (
    "crypto/hmac"
    "crypto/sha256"
    "encoding/base64"
    "fmt"
    "net/http"

    "github.com/gobuffalo/buffalo"
)

func generateSignedPath(path string, secret []byte) string {
    mac := hmac.New(sha256.New, secret)
    mac.Write([]byte(path))
    signature := base64.URLEncoding.EncodeToString(mac.Sum(nil))
    return fmt.Sprintf("%s?signature=%s", path, signature)
}

func verifySignedPath(path, signature string, secret []byte) bool {
    mac := hmac.New(sha256.New, secret)
    mac.Write([]byte(path))
    expected := mac.Sum(nil)
    decoded, _ := base64.URLEncoding.DecodeString(signature)
    return hmac.Equal(expected, decoded)
}

func SafeRedirectHandler(c buffalo.Context) error {
    secret := []byte("your-32-byte-secret-key-here-secure-and-unique")
    requestedPath := c.Param("path")
    sig := c.Param("signature")

    if !verifySignedPath(requestedPath, sig, secret) {
        return c.Error(http.StatusBadRequest, fmt.Errorf("invalid signature"))
    }

    // Only allow known safe relative paths
    allowedPrefixes := []string{"/dashboard", "/settings", "/profile"}
    allowed := false
    for _, p := range allowedPrefixes {
        if requestedPath == p {
            allowed = true
            break
        }
    }
    if !allowed {
        return c.Error(http.StatusForbidden, fmt.Errorf("path not allowed"))
    }

    http.Redirect(c.Response(), c.Request(), requestedPath, http.StatusFound)
    return nil
}

2. Validate host when full URLs are necessary

If you must redirect to an external URL, extract the host from the signed payload and compare it against an allowlist. Never trust the URL’s host directly from user input, even if signed.

import (
    "crypto/hmac"
    "crypto/sha256"
    "encoding/base64"
    "encoding/json"
    "fmt"
    "net/http"
    "net/url"

    "github.com/gobuffalo/buffalo"
)

type RedirectPayload struct {
    URL string `json:"url"`
}

func generateSignedRedirect(payload RedirectPayload, secret []byte) (string, error) {
    data, err := json.Marshal(payload)
    if err != nil {
        return "", err
    }
    mac := hmac.New(sha256.New, secret)
    mac.Write(data)
    signature := base64.URLEncoding.EncodeToString(mac.Sum(nil))
    encodedPayload := base64.URLEncoding.EncodeToString(data)
    return fmt.Sprintf("%s?payload=%s&signature=%s", "/safe-redirect", encodedPayload, signature), nil
}

func isHostAllowed(u *url.URL, allowedHosts map[string]bool) bool {
    return allowedHosts[u.Host]
}

func SafeExternalRedirectHandler(c buffalo.Context) error {
    secret := []byte("your-32-byte-secret-key-here-secure-and-unique")
    payloadB64 := c.Param("payload")
    sig := c.Param("signature")

    decoded, err := base64.URLEncoding.DecodeString(payloadB64)
    if err != nil {
        return c.Error(http.StatusBadRequest, fmt.Errorf("invalid encoding"))
    }

    var payload RedirectPayload
    if err := json.Unmarshal(decoded, &payload); err != nil {
        return c.Error(http.StatusBadRequest, fmt.Errorf("invalid payload"))
    }

    // Verify signature
    mac := hmac.New(sha256.New, secret)":
    mac.Write(decoded)
    expected := mac.Sum(nil)
    decodedSig, _ := base64.URLEncoding.DecodeString(sig)
    if !hmac.Equal(expected, decodedSig) {
        return c.Error(http.StatusBadRequest, fmt.Errorf("invalid signature"))
    }

    // Parse and validate host
    targetURL, err := url.Parse(payload.URL)
    if err != nil || !isHostAllowed(targetURL, map[string]bool{"app.example.com": true, "cdn.example.com": true}) {
        return c.Error(http.StatusForbidden, fmt.Errorf("host not allowed"))
    }

    http.Redirect(c.Response(), c.Request(), targetURL.String(), http.StatusFound)
    return nil
}

These examples ensure that Hmac Signatures are used to protect integrity while explicit host and path allowlists enforce safe redirect behavior, mitigating open redirect risks in Buffalo applications.