Open Redirect in Gorilla Mux with Mongodb

An open redirect in a Gorilla Mux service that uses MongoDB for user or content storage arises when an endpoint accepts a user-supplied URL or host value and then redirects the client without strict validation. If the redirect destination is built from data stored in MongoDB (for example, a configured callback URL, a stored referrer, or a user profile field such as redirect_url), an attacker who can write to the database can influence where authenticated or unauthenticated users are sent.

Consider a pattern where a handler reads a redirect target from a MongoDB document and uses http.Redirect or a similar mechanism. If the stored value is an untrusted string and the handler does not enforce an allowlist or validate the scheme and host, an attacker who can inject a malicious URL (e.g., http://evil.example.com) into MongoDB can cause the application to redirect any visiting user to a phishing or malware site.

Gorilla Mux routes often use path or query parameters to select resources. If a route like /users/{uid}/profile fetches profile data from MongoDB and then redirects to a stored external URL (perhaps to an identity provider or an analytics endpoint), and that URL is not validated, the application becomes an open redirect. The risk is compounded when the MongoDB-stored value is derived from user input earlier in the application lifecycle, making the redirect chain indirect and harder to audit.

Even if MongoDB itself does not perform the redirect, the combination creates a dependency chain: user input enters the system, is stored in the database, and later is read and used for navigation. If any stage lacks validation, the chain inherits the weakness. Common real-world patterns include OAuth callback URLs stored per client, dynamic destination links for marketing campaigns, or return-to URLs saved after login. In each case, the application must treat MongoDB values as untrusted input.

middleBrick scans such API surfaces and flags open redirect risks under BFLA/Privilege Escalation and Property Authorization checks, providing prioritized findings with severity and remediation guidance. This is important because open redirects can erode user trust and are often leveraged in phishing campaigns that abuse the legitimate domain.

To remediate open redirect when using Gorilla Mux and MongoDB, validate and constrain any redirect target read from the database. Prefer allowlists over denylists, enforce strict URL parsing, and avoid using raw user-controlled strings in redirect responses.

Example 1: Validate against an allowlist of permitted hosts. This ensures that only known, safe destinations are used regardless of what is stored in MongoDB.

import (
    "net/url"
    "net/http"
    "strings"
)

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

func redirectHandler(w http.ResponseWriter, r *http.Request) {
    vars := mux.Vars(r)
    userID := vars["uid"]
    var profile struct {
        RedirectURL string `bson:"redirect_url"`
    }
    // Fetch from MongoDB; error handling omitted for brevity
    // collection.FindOne(ctx, filter).Decode(&profile)

    // Validate the stored URL
    parsed, err := url.Parse(profile.RedirectURL)
    if err != nil || !allowedHosts[parsed.Host] {
        http.Error(w, "invalid redirect target", http.StatusBadRequest)
        return
    }
    http.Redirect(w, r, profile.RedirectURL, http.StatusFound)
}

Example 2: Normalize and restrict schemes to HTTPS only, preventing insecure redirects and mixed-content issues. This is particularly important when MongoDB stores URLs that might have been set before stricter policies were enforced.

func secureRedirect(raw string) (string, error) {
    parsed, err := url.Parse(raw)
    if err != nil {
        return "", err
    }
    if parsed.Scheme != "https" {
        return "", fmt.Errorf("insecure scheme: %s", parsed.Scheme)
    }
    if !strings.HasPrefix(parsed.Host, "trusted.") {
        return "", fmt.Errorf("host not trusted: %s", parsed.Host)
    }
    return parsed.String(), nil
}

func handler(w http.ResponseWriter, r *http.Request) {
    // Assume doc.RedirectURL is fetched from MongoDB
    safeURL, err := secureRedirect(doc.RedirectURL)
    if err != nil {
        http.Error(w, "invalid redirect configuration", http.StatusInternalServerError)
        return
    }
    http.Redirect(w, r, safeURL, http.StatusFound)
}

Example 3: Use structured configuration instead of free-form URLs when possible. Store only path segments or predefined keys in MongoDB and map them to full URLs server-side. This reduces the attack surface presented by database content.

type RedirectKey string
const (
    KeyDashboard RedirectKey = "dashboard"
    KeySupport   RedirectKey = "support"
)

var redirectMap = map[RedirectKey]string{
    KeyDashboard: "https://dashboard.example.com",
    KeySupport:   "https://support.example.com",
}

func handleByKey(w http.ResponseWriter, r *http.Request, key RedirectKey) {
    target, exists := redirectMap[key]
    if !exists {
        http.Error(w, "unknown redirect key", http.StatusBadRequest)
        return
    }
    http.Redirect(w, r, target, http.StatusFound)
}

These patterns ensure that even if MongoDB contains untrusted or mutable data, the application does not blindly follow user-influenced redirects. Combine these practices with runtime security scanning that checks for open redirect indicators across your API surface; middleBrick can surface relevant findings and map them to frameworks such as OWASP API Top 10 to guide remediation.