Open Redirect in Buffalo with Firestore

An open redirect in a Buffalo application that uses Firestore typically occurs when user-controlled data (e.g., a Firestore document field or query parameter) is used to construct a redirect URL without strict validation. Because Firestore stores document data that may include URLs, slugs, or redirect targets, a developer might read a document and redirect the user to a location stored in a field such as redirect_url. If that data is not canonicalized and strictly validated, an attacker can supply a malicious URL (e.g., https://evil.com) which gets stored in Firestore or provided via a parameter, leading to a reflected or stored open redirect.

In the context of an unauthenticated scan, middleBrick tests this attack surface by probing endpoints that accept a redirect target and then follow the response to determine whether the final URL differs from an expected origin. When a Firestore-backed Buffalo handler does not enforce an allowlist of trusted domains or does not ensure the target path is relative, the scan can identify the open redirect as a high-severity finding. This is especially relevant when combined with other findings such as Missing Authentication or Improper Authorization, because an open redirect can be used to escalate impact by luring authenticated users to malicious sites.

An example vulnerable handler might read a document ID from the request, fetch a Firestore document, and redirect to a value stored in the document without verifying the URL scheme and host. middleBrick’s checks for BOLA/IDOR and Property Authorization highlight cases where the document is user-controlled and the redirect target is not constrained, producing a finding mapped to the OWASP API Top 10 and common compliance frameworks.

To remediate open redirect in a Buffalo app using Firestore, validate and restrict redirect targets to a safe set of paths or domains. Prefer relative URLs for internal redirects, and if an external URL is required, enforce an allowlist of acceptable hosts and use strict parsing to avoid scheme or hostname confusion.

Below are Firestore-integrated code examples demonstrating a safe approach in Buffalo.

• Safe redirect with an allowlist and relative path:

// handlers/app.go
package handlers

import (
    "net/http"
    "net/url"

    "github.com/gobuffalo/buffalo"
    "cloud.google.com/go/firestore"
    "context"
)

var allowedRedirectHosts = map[string]bool{
    "app.example.com": true,
    "dashboard.example.com": true,
}

func SafeRedirect(c buffalo.Context) error {
    client, err := firestore.NewClient(c.Request().Context(), <your-project>)
    if err != nil {
        return c.Render(500, r.String("internal error"))
    }
    defer client.Close()

    docID := c.Param("doc_id")
    docRef := client.Collection("redirects").Doc(docID)
    doc, err := docRef.Get(c.Request().Context())
    if err != nil || !doc.Exists() {
        return c.Render(404, r.String("not found"))
    }

    var data struct {
        Path string `firestore:"path"`
    }
    if err := doc.DataTo(&data); err != nil {
        return c.Render(500, r.String("internal error"))
    }

    // Ensure path is relative and does not contain disallowed schemes
    if data.Path == "" || data.Path[0] != '/' {
        return c.Render(400, r.String("invalid path"))
    }

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

• Redirect with external URL allowlist:

func ExternalRedirect(c buffalo.Context) error {
    client, err := firestore.NewClient(c.Request().Context(), <your-project>)
    if err != nil {
        return c.Render(500, r.String("internal error"))
    }
    defer client.Close()

    target := c.Param("target")
    parsed, err := url.Parse(target)
    if err != nil || parsed.Scheme == "" || parsed.Host == "" {
        return c.Render(400, r.String("invalid url"))
    }

    if !allowedRedirectHosts[parsed.Host] {
        return c.Render(403, r.String("host not allowed"))
    }

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

These examples emphasize input validation, canonicalization, and allowlisting, which reduce the risk of open redirect when combined with Buffalo’s routing and Firestore document access. middleBrick can verify the effectiveness of these controls by scanning the endpoints and confirming that redirects are constrained and that findings such as BOLA/IDOR or Property Authorization do not expose unconstrained redirect targets.