Open Redirect in Buffalo

Open Redirect vulnerabilities occur when an application accepts a user-supplied URL and redirects the browser to that location without proper validation. In Buffalo, this commonly appears in authentication flows or state transitions where a redirect_to, next, or return_to parameter is used to preserve the original destination after a login or action.

Buffalo handlers often use c.Redirect() to send responses. A vulnerable pattern directly uses query or form parameters to determine the redirect target. For example, a login handler might read a redirect_to parameter and pass it straight to c.Redirect(). Attackers exploit this by crafting URLs like https://api.example.com/login?redirect_to=https://evil.com. When a user authenticates, they are sent to the attacker's site, enabling phishing or credential harvesting.

Another manifestation involves Buffalo's c.Param() method when route paths include catch-all parameters (e.g., /redirect/*path) that are later used in redirects. Buffalo's routing does not inherently validate these values as safe relative paths, allowing absolute URLs to be injected. This aligns with OWASP API Top 10:API8:2023 — Security Misconfiguration, where improper handling of redirects creates an attack vector.

Real-world CVEs like CVE-2021-44228 (Log4j) involved redirect-like behaviors in LDAP lookups, but in Buffalo, the issue is purely in application logic around HTTP redirects. The vulnerability is critical because it facilitates social engineering attacks with high impact on user trust.

Manually testing for Open Redirect in Buffalo involves identifying endpoints that accept redirect-related parameters and probing them with external URLs. Look for handlers that call c.Redirect() with values derived from c.Param(), c.Query(), or form data. Test payloads include:

• https://attacker.com
• //attacker.com
• https:attacker.com
• /\attacker.com

If the Location header in the response points to an external domain, the vulnerability exists.

Automated scanning with middleBrick's Input Validation check detects this by submitting these payloads to the API endpoint and analyzing redirect responses. The scanner tests both query parameters and JSON body fields commonly used in Buffalo APIs. For example, a Buffalo route like POST /api/v1/auth/login that accepts JSON with a redirect_url field will be probed. middleBrick reports the finding under the Input Validation category with a high severity, showing the exact parameter and payload that triggered the external redirect.

To use middleBrick for Buffalo APIs, simply scan the endpoint URL. No credentials or setup are needed. The scanner will also correlate the finding with the OpenAPI/Swagger spec if available, highlighting where the redirect parameter is defined. For instance:

middlebrick scan https://api.buffalo-app.com

The report will include a prioritized finding with remediation guidance specific to Buffalo's context.

Fix Open Redirect in Buffalo by validating that redirect URLs are either relative paths (starting with /) or match an allowed list of trusted domains. Never accept absolute URLs from user input. Implement a validation helper in your Buffalo app.

Vulnerable Buffalo Handler Example:

func LoginHandler(c buffalo.Context) error {
    type LoginRequest struct {
        Username     string `json:"username"`
        Password     string `json:"password"`
        RedirectTo   string `json:"redirect_to"`
    }
    var req LoginRequest
    if err := c.Bind(&req); err != nil {
        return c.Error(400, err)
    }
    // Authenticate user...
    if req.RedirectTo != "" {
        return c.Redirect(req.RedirectTo) // Vulnerable!
    }
    return c.Redirect("/dashboard")
}

Remediated Version Using a Validation Helper:

func isSafeRedirect(redirect string) bool {
    if redirect == "" {
        return false
    }
    // Allow only relative paths (no scheme, no host)
    if !strings.HasPrefix(redirect, "/") {
        return false
    }
    // Optional: allow specific absolute domains if needed
    // allowedHosts := []string{"app.example.com"}
    // u, err := url.Parse(redirect)
    // if err == nil && u.Host != "" && !slices.Contains(allowedHosts, u.Host) {
    //     return false
    // }
    return true
}

func LoginHandler(c buffalo.Context) error {
    type LoginRequest struct {
        Username   string `json:"username"`
        Password   string `json:"password"`
        RedirectTo string `json:"redirect_to"`
    }
    var req LoginRequest
    if err := c.Bind(&req); err != nil {
        return c.Error(400, err)
    }
    // Authenticate user...
    target := "/dashboard"
    if req.RedirectTo != "" && isSafeRedirect(req.RedirectTo) {
        target = req.RedirectTo
    }
    return c.Redirect(target)
}

This fix ensures only relative paths are used. If your Buffalo app must redirect to external domains (e.g., SSO callbacks), maintain an explicit allowlist and validate the host against it. Buffalo's c.Redirect() will automatically set the Location header, so the validation must happen before that call.

Additionally, review your Buffalo routes for catch-all parameters (e.g., c.Param("path:path")) and apply similar validation. Integrate this check into any middleware that handles redirects to ensure consistency across the application.