Open Redirect in Buffalo with Basic Auth

An Open Redirect in a Buffalo application becomes more risky when Basic Authentication is used because the authentication flow and redirect logic can interact in ways that bypass intended access controls. In Buffalo, a developer might protect a route with HTTP Basic Auth and then redirect users based on a URL parameter or a stored destination (often called a "return-to" or "next" parameter). If the redirect target is not strictly validated, an attacker who knows or guesses the protected endpoint can craft a URL like /login?next=https://evil.com. After the user supplies valid Basic Auth credentials, Buffalo processes the login and follows the redirect to the attacker-controlled host.

This combination exposes a few specific issues. First, Basic Auth credentials are sent on each request; if a redirect leads to an external site, the browser may include the credentials in a Referer header or preauthenticate the external origin depending on browser behavior, partially leaking secrets. Second, the route handler might not enforce authentication on the redirect target, so the redirect occurs after successful auth but before any additional authorization checks. Third, because Buffalo apps often render links or set Location headers dynamically, an open redirect can appear in authenticated UI elements (e.g., a post-login dashboard that redirects based on role or stored intent), making the vector less obvious during manual testing.

An attacker does not need to exploit server-side code execution; they only need a valid set of Basic Auth credentials (which can be obtained through phishing, accidental exposure, or weak password policies) and a predictable redirect path. Tools like middleBrick can detect this pattern by scanning the unauthenticated attack surface and, when configured with Basic Auth scenarios, identifying endpoints that perform redirects without strict allow-listing of destinations. While middleBrick does not fix the issue, its findings include prioritized severity, a description of the redirect chain, and remediation guidance mapped to the OWASP API Top 10 and relevant compliance frameworks.

To fix an open redirect in Buffalo when using Basic Auth, validate and restrict the redirect target before issuing the Location header. Do not trust user-supplied URLs. Instead of passing raw parameters to redirect.Login or redirect.To, compare the target against a strict allow-list or a known set of routes, and default to a safe internal path when validation fails.

Example of a vulnerable handler:

func Login(c buffalo.Context) error {
    user, pass, ok := c.Request().BasicAuth()
    if !ok || !isValidUser(user, pass) {
        c.Response().Header().Set("WWW-Authenticate", `Basic realm="restricted"`)
        return c.Render(401, rtext.String("Unauthorized"))
    }
    next := c.Param("next")
    if next != "" {
        http.Redirect(c.Response(), c.Request(), next, http.StatusFound)
        return nil
    }
    return c.Render(200, rhtml.String("login_form"))
}

Example of a remediated handler with allow-list validation:

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

func safeLogin(c buffalo.Context) error {
    user, pass, ok := c.Request().BasicAuth()
    if !ok || !isValidUser(user, pass) {
        c.Response().Header().Set("WWW-Authenticate", `Basic realm="restricted"`)
        return c.Render(401, rtext.String("Unauthorized"))
    }
    next := c.Param("next")
    if next != "" && allowedRedirects[next] {
        http.Redirect(c.Response(), c.Request(), next, http.StatusFound)
        return nil
    }
    // Default to a known safe internal route
    http.Redirect(c.Response(), c.Request(), "/dashboard", http.StatusFound)
    return nil
}

Additional measures include removing the next parameter from authentication routes when possible, using session-based post-login navigation instead of URL parameters, and ensuring that any Location header does not contain a different host. middleBrick can be used to verify that these changes remove the open redirect by rescanning the endpoint and confirming that no external redirects are detected in the findings.