Open Redirect Chain in Buffalo (Go)

An Open Redirect Chain occurs when an application redirects a user to an arbitrary, attacker-controlled URL, often by concatenating user-supplied input with a redirect operation. In Buffalo, a Go web framework, this commonly arises when a handler reads a URL parameter such as next and passes it directly to a redirect helper without validation. Because Buffalo is built on Go’s net/http, unsafe use of http.Redirect or http.RedirectHandler enables an attacker to craft a chain of redirects that traverse multiple domains, potentially bypassing browser-based referrer leakage controls and obscuring the final destination.

The vulnerability is amplified when the application constructs a redirect URL by joining a base path with an untrusted input, and when middleware or helper utilities do not enforce an allowlist of trusted hosts. For example, an attacker might supply next=https://evil.com/steal?token=SESSION, and if the application naively uses http.Redirect(w, r, url, http.StatusFound), the victim’s browser is sent to the malicious site. Because the redirect appears to originate from a trusted domain, users may not notice the transition, enabling phishing or session token exfiltration via the referrer header or embedded resources.

Buffalo applications that also integrate external authentication providers or OAuth flows are especially susceptible to open redirect chains if the return URL is taken directly from query parameters without canonicalization or host validation. Go’s standard library does not provide high-level redirect safety utilities, so developers must explicitly validate the target host against a known set of origins. Without such checks, the framework’s productivity features can inadvertently expose a critical client-side attack vector that aligns with the OWASP API Top 10 and can be detected by security scans such as those performed by middleBrick, which tests for BOLA/IDOR, Property Authorization, and Unsafe Consumption in unauthenticated mode.

To remediate open redirect chains in Buffalo, validate and sanitize all redirect targets using an allowlist of hostnames and by resolving relative paths. Avoid directly using user input in http.Redirect. Below are concrete, working examples that demonstrate safe patterns.

Unsafe Example (Vulnerable)

package actions

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

func RedirectHandler(c buffalo.Context) error {
    next := c.Param("next")
    http.Redirect(c.Response(), c.Request(), next, http.StatusFound)
    return nil
}

Safe Example with Host Allowlist

package actions

import (
    "errors"
    "net/url"
    "strings"

    "github.com/gobuffalo/buffalo"
    "github.com/gobuffalo/buffalo/middleware"
    "net/http"
)

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

func safeRedirect(target string) (string, error) {
    parsed, err := url.Parse(target)
    if err != nil {
        return "", errors.New("invalid url")
    }
    if parsed.Scheme != "https" {
        return "", errors.New("only https is allowed")
    }
    if !allowedRedirectHosts[parsed.Host] {
        return "", errors.New("host not allowed")
    }
    // Ensure no path traversal or unexpected fragments
    clean := url.URL{
        Scheme: parsed.Scheme,
        Host: parsed.Host,
        Path: parsed.Path,
    }
    return clean.String(), nil
}

func RedirectHandler(c buffalo.Context) error {
    next := c.Param("next")
    safe, err := safeRedirect(next)
    if err != nil {
        // Fallback to a default location
        http.Redirect(c.Response(), c.Request(), "/", http.StatusSeeOther)
        return nil
    }
    http.Redirect(c.Response(), c.Request(), safe, http.StatusSeeOther)
    return nil
}

Additional best practices include using relative internal paths when possible, leveraging Buffalo’s middleware.Redirect helper with strict rules, and incorporating automated scans via the middleBrick CLI (middlebrick scan <url>) to detect insecure redirects in CI/CD. For teams using the Pro plan, continuous monitoring can alert when a new endpoint introduces an unsafe redirect pattern, and the GitHub Action can fail builds if a risk score exceeds your defined threshold.