Open Redirect in Buffalo with Api Keys

An Open Redirect occurs when an application redirects a user to an arbitrary URL without proper validation. In the Buffalo framework, this often arises when a handler uses user-controlled input to construct a redirect location. Combining this with Api Keys introduces a scenario where an authenticated client may be redirected to a malicious site if the redirect destination is derived from or influenced by the request context, such as an intended post-login flow.

Consider a typical Buffalo application that uses an Api Key for authentication. The Api Key might identify a client or tenant, and after validation the application may redirect to a client-specific dashboard, for example /dashboard?client_id=123. If the redirect target is built from concatenated strings or taken directly from request parameters, an attacker who knows or guesses a valid Api Key could manipulate the redirect location through parameters like next or return_to. Because the Api Key proves identity, the application may trust the redirect destination implicitly, leading to an authenticated open redirect.

The risk is compounded when the Api Key is passed in headers or query parameters and the redirect logic does not strictly enforce an allowlist of safe paths. An attacker could craft a link such as https://app.example.com/login?api_key=VALID_KEY&next=https://evil.com. If the handler validates the Api Key and then redirects to the value of next without canonicalization and strict allowlisting, the user is sent to the attacker’s site while the application appears to behave normally for a legitimate authenticated session.

In the context of middleBrick’s security checks, this pattern would be flagged under Input Validation and Property Authorization. The scanner examines whether redirects incorporate untrusted data and whether authorization checks are applied consistently for authenticated contexts, including those where Api Keys are used for identification. Even though Api Keys are not secrets in the URL in this example, their presence can increase the impact of an open redirect by enabling authenticated phishing or session fixation scenarios.

Real-world mappings include OWASP API Top 10 A05:2023 Security Misconfiguration and aspects of A01:2027 Broken Access Control, because the redirect may bypass expected access boundaries. Unlike server-side request forgery, the payload is a client-side navigation to a malicious domain, but the misuse of identity (Api Key) makes the abuse more convincing.

Remediation focuses on strict validation of redirect targets and ensuring that Api Key authentication does not inadvertently authorize redirection to untrusted locations. Always use an allowlist of safe paths for post-authentication redirects and avoid reflecting user input into the redirect location.

Example of a vulnerable handler that uses an Api Key header and a user-supplied next parameter:

package actions

import (
    "net/http"
    "github.com/gobuffalo/buffalo"
)

func LoginHandler(c buffalo.Context) error {
    apiKey := c.Param("api_key")
    next := c.Param("next")
    if !validateApiKey(apiKey) {
        return c.Error(401, errors.New("unauthorized"))
    }
    // Vulnerable: redirect to user-controlled next
    http.Redirect(c.Response(), c.Request(), next, http.StatusFound)
    return nil
}

Fixed version with strict allowlisting and no reflection of untrusted input:

package actions

import (
    "net/http"
    "github.com/gobuffalo/buffalo"
)

var allowedRedirectPaths = map[string]bool{
    "/dashboard": true,
    "/welcome":   true,
}

func LoginHandler(c buffalo.Context) error {
    apiKey := c.Param("api_key")
    if !validateApiKey(apiKey) {
        return c.Error(401, errors.New("unauthorized"))
    }
    // Safe default
    target := "/dashboard"
    // If a safe next is explicitly allowed, use it
    if c.Param("next") != "" && allowedRedirectPaths[c.Param("next")] {
        target = c.Param("next")
    }
    http.Redirect(c.Response(), c.Request(), target, http.StatusFound)
    return nil
}

When using query parameters for routing, prefer path-based parameters and validate against a strict set. For multi-tenant scenarios where Api Keys map to organizations, resolve the redirect target server-side rather than relying on client-supplied values:

package actions

import (
    "net/http"
    "github.com/gobuffalo/buffalo"
)

func PostAuthHandler(c buffalo.Context) error {
    apiKey := c.Request().Header.Get("X-API-Key")
    org, err := lookupOrgByApiKey(apiKey)
    if err != nil {
        return c.Render(401, r.JSON(map[string]string{"error": "unauthorized"}))
    }
    // Server-side resolved target, no user input in URL
    target := org.DashboardURL
    http.Redirect(c.Response(), c.Request(), target, http.StatusFound)
    return nil
}

Additional measures include setting the Referrer-Policy header and avoiding open redirects for any authenticated flow, regardless of the identification mechanism. middleBrick’s scans can validate that no open redirect patterns remain by checking input flows and response location headers across the unauthenticated attack surface.