Ssrf Server Side in Buffalo (Go)

Server-side request forgery (SSRF) in a Buffalo application written in Go occurs when the app accepts a URL from an attacker and uses it to make an outbound HTTP request without adequate validation or network controls. Buffalo’s request handling flow—parsing parameters, query strings, or JSON payloads, then passing them to an HTTP client—can inadvertently allow an attacker to direct the server to internal services, metadata endpoints, or external systems. Because Buffalo is a convention-driven framework, developers may assume routing and parameter handling are automatically safe, but the underlying Go HTTP client and custom route logic still require explicit safeguards.

In practice, this often manifests when an endpoint accepts a url or redirect_to parameter and uses http.Get or http.NewRequest without restricting hostnames, ports, or schemes. For example, a handler that fetches a remote resource for “preview” or “health check” purposes can be tricked into reaching the Go metadata service at 169.254.169.254 (AWS instance metadata), internal Kubernetes services like kubernetes.default.svc, or other sensitive internal endpoints. Buffalo’s flexibility in composing handlers and middleware means such a vulnerable handler might be nested inside authentication or logging layers that do not inspect the target URL, expanding the attack surface. SSRF can lead to internal network scanning, service enumeration, or exfiltration of credentials from internal metadata endpoints.

middleBrick’s unauthenticated scanning flow is relevant here: by submitting a Buffalo app’s public endpoint, the scanner can probe input vectors that reach server-side HTTP calls. The scanner’s checks for SSRF map findings to frameworks like OWASP API Top 10 and can surface indicators such as unvalidated redirects or missing network segmentation. Because SSRF is about the server acting as an unintended proxy, remediation must focus on constraining what the server is allowed to contact, not just validating client-side expectations.

To remediate SSRF in Buffalo with Go, constrain outbound requests using a strict allowlist of hosts and schemes, avoid forwarding raw user input to the HTTP client, and enforce timeouts and network-level restrictions. Below are concrete, idiomatic examples for Buffalo handlers.

1. Validate against an allowlist and reject dangerous schemes

Parse the user-supplied URL and ensure it matches only expected hosts and safe schemes. Do not rely on manual string checks; use Go’s url.URL methods to inspect components.

import (
	"net/url"
	"strings"

	"github.com/gobuffalo/buffalo"
)

func safeURL(raw string) (*url.URL, error) {
	hostport := strings.Split(raw, ":")
	host := hostport[0]
	allowedHosts := map[string]bool{
		"api.example.com": true,
		"data.example.com": true,
	}
	if !allowedHosts[host] {
		return nil, buffalo.NewError(400, "disallowed host")
	}
	u, err := url.Parse(raw)
	if err != nil {
		return nil, err
	}
	if u.Scheme != "https" {
		return nil, buffalo.NewError(400, "only https allowed")
	}
	return u, nil
}

func FetchResource(c buffalo.Context) error {
	raw := c.Param("url")
	u, err := safeURL(raw)
	if err != nil {
		return c.Render(400, r.JSON(err.Error()))
	}
	// proceed with httpClient that has timeouts
	return c.Render(200, r.JSON(map[string]string{"target": u.String()}))
}

2. Use a custom HTTP client with timeouts and restricted transport

Prevent long-running or uncontrolled requests by configuring an http.Client with timeouts and, if needed, a custom Transport that limits exposure to internal networks.

import (
	"net/http"
	"time"
)

var httpClient = &http.Client{
	Timeout: 10 * time.Second,
}

func MakeRequest(c buffalo.Context) error {
	u, err := safeURL(c.Param("url"))
	if err != nil {
		return c.Render(400, r.JSON(err.Error()))
	}
	req, err := http.NewRequest("GET", u.String(), nil)
	if err != nil {
		return c.Render(500, r.JSON("request error"))
	}
	resp, err := httpClient.Do(req)
	if err != nil {
		return c.Render(502, r.JSON("upstream error"))
	}
	defer resp.Body.Close()
	// process resp.Body safely
	return c.Render(200, r.JSON(map[string]int{"status": resp.StatusCode}))
}

3. Avoid forwarding Host headers and use context cancellation

Ensure that forwarded headers do not leak sensitive information or enable hop-by-hop attacks. Use request context to enforce deadlines and prevent resource exhaustion.

func MakeRequestWithContext(c buffalo.Context) error {
	u, err := safeURL(c.Param("url"))
	if err != nil {
		return c.Render(400, r.JSON(err.Error()))
	}
	req, err := http.NewRequestWithContext(c.Request().Context(), "GET", u.String(), nil)
	if err != nil {
		return c.Render(500, r.JSON("request error"))
	}
	// explicitly drop potentially dangerous headers
	req.Header.Del("Authorization")
	req.Header.Del("Cookie")
	resp, err := httpClient.Do(req)
	if err != nil {
		return c.Render(502, r.JSON("upstream error"))
	}
	defer resp.Body.Close()
	return c.Render(200, r.JSON(map[string]int{"status": resp.StatusCode}))
}

4. Restrict internal access via network controls

Even with code-level validation, deploy the Buffalo app in an environment that blocks outbound traffic to sensitive internal ranges (e.g., 127.0.0.0/8, 10.0.0.0/8, 172.16.0.0/12, 169.254.0.0/16). Combine this with service mesh or firewall rules to prevent the server from reaching metadata or internal Kubernetes endpoints.