Open Redirect in Gorilla Mux with Jwt Tokens

An Open Redirect occurs when an application redirects a user to an arbitrary URL without proper validation. In Gorilla Mux, this typically arises when a route parameter or query parameter is used to determine the redirect target. When JWT tokens are involved—such as passing a token in a query parameter (e.g., ?token=...) or embedding a redirect URL inside a token payload—an insecure implementation can allow an attacker to control the final redirect destination after token validation.

Consider a scenario where a handler decodes a JWT token to extract a next claim used for post-login redirection. If the token is not validated strictly (e.g., issuer, audience, expiration), or if the application trusts the next value without canonicalization, an attacker can supply a malicious token with a crafted redirect URL. Because Gorilla Mux matches routes using path patterns and host headers, an attacker might also manipulate host or path variables to point to a malicious site. This combination increases risk: the JWT may appear to carry trusted context, while the routing logic fails to enforce a strict allowlist of safe redirect targets.

Moreover, if the application exposes an endpoint that accepts a redirect URL as a query parameter and also requires a valid JWT for authorization (e.g., /login?redirect=https://example.com), misconfiguration can lead to token leakage or token replay across domains. Even when tokens are validated, if the redirect logic does not enforce same-origin policies or strict URL allowlists, the endpoint becomes an open redirect. Attackers can craft phishing links that include a valid-looking JWT but redirect users to malicious sites after successful authentication checks, bypassing browser referrer protections due to the server-side redirect.

In practice, this vulnerability stems from two failures: (1) insufficient validation of redirect targets, and (2) improper handling of JWT claims that influence routing. Gorilla Mux does not inherently protect against these logic flaws; it is the developer’s responsibility to ensure that any user-influenced redirect target is normalized, restricted to trusted domains, and decoupled from authorization tokens.

To remediate open redirect risks when using JWT tokens in Gorilla Mux, enforce strict allowlists for redirect targets and avoid using untrusted input to construct redirect URLs. Below are concrete code examples demonstrating secure patterns.

1. Validate redirect targets against an allowlist

Never use raw user input for redirect URLs. Instead, map allowed destinations by name or path, and validate against a predefined set.

import (
    "net/http"
    "strings"

    "github.com/gorilla/mux"
)

var allowedRedirects = map[string]string{
    "dashboard": "/app/dashboard",
    "profile":  "/app/profile",
    "settings": "/app/settings",
}

func redirectHandler(w http.ResponseWriter, r *http.Request) {
    vars := mux.Vars(r)
    targetName := vars["page"]

    // Normalize input
    targetName = strings.ToLower(targetName)

    // Allowlist check
    if path, ok := allowedRedirects[targetName]; ok {
        http.Redirect(w, r, path, http.StatusFound)
        return
    }

    http.Error(w, "invalid redirect target", http.StatusBadRequest)
}

2. Validate URLs when using query parameters with JWT

If you must accept a redirect URL (e.g., for deep linking), parse and validate the URL strictly, ensuring the host is in a trusted set and the scheme is HTTPS.

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

    "github.com/gorilla/mux"
)

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

func safeRedirectHandler(w http.ResponseWriter, r *http.Request) {
    raw := r.URL.Query().Get("url")
    if raw == "" {
        http.Error(w, "missing redirect url", http.StatusBadRequest)
        return
    }

    parsed, err := url.Parse(raw)
    if err != nil {
        http.Error(w, "invalid url", http.StatusBadRequest)
        return
    }

    // Enforce HTTPS
    if parsed.Scheme != "https" {
        http.Error(w, "insecure scheme", http.StatusBadRequest)
        return
    }

    // Enforce trusted host
    if !trustedHosts[parsed.Host] {
        http.Error(w, "untrusted host", http.StatusBadRequest)
        return
    }

    http.Redirect(w, r, parsed.String(), http.StatusFound)
}

3. Avoid embedding redirect URLs in JWT claims

Do not store redirect URLs inside JWT payloads. If you must pass post-login context, use opaque server-side session state keyed by a random token, and pass only the token in the JWT. This prevents token tampering and keeps routing logic independent of the JWT.

import (
    "net/http"

    "github.com/gorilla/mux"
    "github.com/golang-jwt/jwt/v5"
)

func postLoginHandler(w http.ResponseWriter, r *http.Request) {
    tokenString := r.Header.Get("Authorization")
    if tokenString == "" {
        http.Error(w, "missing token", http.StatusUnauthorized)
        return
    }

    token, err := jwt.Parse(tokenString, func(token *jwt.Token) (interface{}, error) {
        // validate signing method and key
        return []byte("your-secret"), nil
    })
    if err != nil || !token.Valid {
        http.Error(w, "invalid token", http.StatusUnauthorized)
        return
    }

    // Use server-side session to determine redirect
    sessionID := generateSecureSessionID()
    storeSession(sessionID, "/app/dashboard")

    // Return only the session ID; client redirects to /session/redirect?sid=...
    http.Redirect(w, r, "/session/redirect?sid="+sessionID, http.StatusFound)
}

By combining strict allowlists, URL parsing with host/scheme validation, and avoiding redirect instructions inside JWTs, you mitigate open redirect risks while retaining the usability of JWT-based flows in Gorilla Mux.