Open Redirect in Gorilla Mux with Api Keys

An open redirect occurs when an application redirects a user to an arbitrary URL supplied in the request without proper validation. In Gorilla Mux, a common pattern is to use query parameters to determine the target location, for example a next or redirect_url parameter. When api keys are used for authentication but the redirect logic does not validate the destination, the api key can be exposed in the redirect chain.

Consider a handler that accepts an api key in a header or query parameter and then redirects based on an unchecked URL parameter:

func redirectHandler(w http.ResponseWriter, r *http.Request) {
    vars := mux.Vars(r)
    apiKey := vars["apikey"]
    target := r.URL.Query().Get("url")
    if target != "" {
        http.Redirect(w, r, target, http.StatusFound)
        return
    }
    // default behavior
    http.Redirect(w, r, "/dashboard", http.StatusFound)
}

In this setup, the api key is used to identify the client but is never validated against the redirect target. An attacker can craft a link such as https://api.example.com/redirect?apikey=ABC123&url=https://evil.com. Because the handler does not verify that the redirect target is safe, the user is sent to the malicious site while the api key remains in the request and may be leaked in Referer headers, logs, or browser history. This becomes particularly dangerous if the api key grants elevated permissions or if the redirect is part of an OAuth flow where the token is exposed.

Additionally, if the api key is passed in the URL rather than a header, it can end up in server logs or be leaked to third-party sites via the Referer header when the browser follows the redirect. The combination of Gorilla Mux's flexible routing and loosely validated redirect parameters creates a vector where authentication credentials are inadvertently exposed through an open redirect.

Another scenario involves host-based redirection where the hostname is used to determine the target. If the application constructs a redirect URL using user-controlled input without validating the host, an attacker can supply a malicious host that leads to phishing:

func hostRedirect(w http.ResponseWriter, r *http.Request) {
    host := r.URL.Query().Get("host")
    if host != "" {
        redirectURL := "https://" + host + "/callback"
        http.Redirect(w, r, redirectURL, http.StatusFound)
        return
    }
    http.Redirect(w, r, "/", http.StatusFound)
}

Here, an api key may be embedded in session or query parameters, and the unchecked host value allows redirection to any external domain. This pattern is common in multi-tenant setups where routing is determined by the request host, but without strict allow-listing, it can lead to open redirects and credential leakage.

To mitigate open redirect risks when using api keys with Gorilla Mux, you must validate and restrict redirect destinations. The safest approach is to avoid using user-supplied URLs for redirection entirely. If redirection is necessary, use a strict allow-list of trusted paths or hosts and ensure the api key is never exposed in the redirect target.

Below is a secure example that validates the redirect path against a predefined set of allowed paths and uses a session or context to carry the api key instead of including it in the URL:

var allowedPaths = map[string]bool{
    "/dashboard": true,
    "/profile":   true,
    "/settings":  true,
}

func secureRedirect(w http.ResponseWriter, r *http.Request) {
    apiKey := r.Header.Get("X-API-Key")
    if apiKey == "" {
        http.Error(w, "Unauthorized", http.StatusUnauthorized)
        return
    }
    // Validate api key via your auth service here
    path := r.URL.Query().Get("path")
    if !allowedPaths[path] {
        http.Error(w, "Invalid redirect target", http.StatusBadRequest)
        return
    }
    http.Redirect(w, r, path, http.StatusFound)
}

When you must accept a full URL, perform strict parsing and host allow-listing:

func safeExternalRedirect(w http.ResponseWriter, r *http.Request) {
    apiKey := r.Header.Get("X-API-Key")
    raw := r.URL.Query().Get("url")
    if raw == "" {
        http.Redirect(w, r, "/", http.StatusFound)
        return
    }
    parsed, err := url.Parse(raw)
    if err != nil {
        http.Error(w, "Invalid URL", http.StatusBadRequest)
        return
    }
    // Only allow same-host redirects or explicitly trusted hosts
    allowedHosts := map[string]bool{
        "app.example.com": true,
        "trusted.com":     true,
    }
    if !allowedHosts[parsed.Host] {
        http.Error(w, "Redirect target not allowed", http.StatusBadRequest)
        return
    }
    // Ensure no fragment or unexpected parameters leak the api key
    parsed.RawQuery = ""
    http.Redirect(w, r, parsed.String(), http.StatusFound)
}

For Gorilla Mux routes, ensure that api keys are handled as headers or in the request body rather than as route variables that might be inadvertently exposed. Combine these validations with server-side logging of suspicious redirect attempts to detect abuse. Using the middleBrick CLI (middlebrick scan <url>) can help verify that your redirect endpoints do not reflect uncontrolled user input.