Open Redirect Chain in Buffalo

In the Go-based Buffalo web framework, open redirect chains often arise from improper handling of redirect URLs in middleware or handler functions. A common pattern occurs when developers use c.Redirect() with user-controlled input without proper validation, especially in authentication flows. For example, after a login attempt, Buffalo applications frequently redirect users to a return_to parameter. If this parameter is not validated against an allowlist of trusted domains, attackers can chain redirects through multiple malicious domains before reaching a final phishing site.

This vulnerability is exacerbated in Buffalo when using the github.com/gobuffalo/buffalo/render package with dynamic template data that influences redirect logic. Consider a scenario where a handler extracts a next query parameter and passes it directly to c.Redirect():

func LoginHandler(c buffalo.Context) error {
    next := c.Param("next")
    if next == "" {
        next = "/"
    }
    // Vulnerable: no validation of next parameter
    return c.Redirect(302, next)
}

An attacker could craft a URL like https://example.com/login?next=https://evil.com/phish?continue=https://trusted.com. If the application does not validate the next parameter, the user is first redirected to evil.com, which then immediately redirects to trusted.com — creating a redirect chain that bypasses security checks relying on single-hop validation. This chain can be used to obscure the final malicious destination from users and security tools that only inspect the first redirect.

Buffalo's convention-over-configuration approach sometimes leads to implicit redirect logic in generated code (e.g., from buffalo g action), where developers may overlook the need to validate redirect URLs, especially in custom authentication implementations.

Detecting open redirect chains in Buffalo applications requires analyzing both static code patterns and dynamic behavior during scanning. middleBrick identifies this issue by simulating unauthenticated requests to endpoints that perform redirects, particularly those handling login, OAuth callbacks, or form submissions. It checks for redirect responses (3xx status codes) where the Location header contains user-controlled input, then follows the redirect chain to see if it leads to an external, untrusted domain.

For Buffalo-specific detection, middleBrick looks for patterns such as:

• Handlers using c.Param(), c.Request().URL.Query().Get(), or c.Context().Value() to obtain redirect targets without validation.
• Use of c.Redirect() with variables derived from request parameters, headers, or cookies.
• Missing calls to validation functions like net/url.Parse() combined with host allowlist checks before redirection.

During a scan, middleBrick sends probes like ?next=https://evil.com to suspected endpoints and monitors the redirect chain. If the chain exceeds one hop and the final destination differs from the initial domain, it flags a potential open redirect chain. For example, scanning a Buffalo login endpoint might reveal:

Request: GET /login?next=https://attacker.com/step1
Response 1: 302 Location: https://attacker.com/step1
Response 2: 302 Location: https://trusted.com/login
Response 3: 200 OK (phishing page)

This three-hop chain indicates a vulnerability where the initial redirect to attacker-controlled territory is not blocked. middleBrick reports this under the "Property Authorization" check (as improper validation of redirect URLs constitutes a failure to enforce business logic on resource access) and provides the full redirect chain in the findings for forensic analysis.

Developers can also use the middleBrick CLI to scan a Buffalo API locally: middlebrick scan https://localhost:3000/login — which will test for such redirect chains without requiring agents or configuration.

Fixing open redirect chains in Buffalo applications involves validating and sanitizing redirect URLs before using them in c.Redirect(). The most effective approach is to maintain an allowlist of trusted relative paths or domains and reject any input that does not match. Buffalo’s standard library provides the net/url package for safe URL parsing.

Here is a secure implementation of the login handler using Buffalo’s context and proper validation:

import (
    "net/url"
    "strings"
)

func LoginHandler(c buffalo.Context) error {
    next := c.Param("next")
    if next == "" {
        next = "/"
    }

    // Parse and validate the redirect URL
    u, err := url.Parse(next)
    if err != nil {
        return c.Error(400, next)
    }

    // Allow only relative paths or trusted domains
    if u.IsAbs() {
        trusted := []string{"https://example.com", "https://trusted.example.com"}
        allowed := false
        for _, t := range trusted {
            if strings.HasPrefix(u.String(), t) {
                allowed = true
                break
            }
        }
        if !allowed {
            next = "/" // Fallback to safe default
        }
    } else if !strings.HasPrefix(next, "/") {
        // Prevent path traversal or malformed relative URLs
        next = "/"
    }

    return c.Redirect(302, next)
}

This code ensures that:

• Absolute URLs are only allowed if they match a predefined list of trusted domains.
• Relative URLs must start with / to prevent paths like ../../external.
• Any invalid input defaults to a safe landing page (/).

For applications with many redirect points, consider creating a reusable Buffalo middleware function:

func RedirectValidator(next http.Handler) http.Handler {
    return http.HandlerFunc(func(c buffalo.Context, next http.Handler) {
        // Logic to validate redirect URLs in context values
        // Call next only if validation passes
        next.ServeHTTP(c.Response(), c.Request())
    })
}

Register this middleware globally or on specific routes to enforce consistent validation. After fixing, rescan with middleBrick (e.g., middlebrick scan https://your-buffalo-app.com) to confirm the redirect chain vulnerability is resolved and the security score improves.