Ssrf Cloud Metadata in Buffalo (Go)

Server-Side Request Forgery (SSRF) against cloud metadata endpoints is a high-impact pattern when a Go service built with the Buffalo framework makes outbound HTTP requests to instance metadata services (for example, 169.254.169.254). Buffalo’s convention-driven request handling and automatic binding can inadvertently forward attacker-controlled input into these calls, enabling SSRF that reaches cloud metadata APIs.

In Buffalo, a typical handler might bind parameters from the request and use them to construct a client request. If a parameter such as url or host is taken directly from user input and passed to an HTTP client without strict validation, an attacker can supply a malicious URL like http://169.254.169.254/latest/meta-data/iam/security-credentials/. Because Buffalo does not inherently block internal IPs, the request proceeds from the server, revealing sensitive cloud metadata that should never be exposed to unauthenticated endpoints.

The risk is compounded when the application runs inside a cloud environment (e.g., EC2, ECS, or similar) where the metadata service is reachable from within the instance network. An SSRF against this endpoint can lead to overprivileged IAM role discovery, exposure of instance identity tokens, or enumeration of internal services that rely on metadata for configuration. middleBrick detects this pattern during its unauthenticated scan by observing whether responses from the metadata IP are returned and whether sensitive fields such as iam/security-credentials/ are included in responses, flagging the endpoint with a high severity finding and guidance to restrict destination IPs and validate URLs.

Because Buffalo encourages rapid prototyping, developers may skip host validation or use generic HTTP clients without tightening the default transport. The framework’s flexibility in defining routes and parameters means that without explicit safeguards, endpoints accepting free-form URLs become prime targets. The scanner’s checks for SSRF and cloud metadata probing are designed to surface these weaknesses before an attacker can weaponize them, emphasizing the need to treat metadata endpoints as strictly internal and never reachable from external inputs.

Remediation in Go with Buffalo centers on input validation, strict URL parsing, and blocking access to reserved IP ranges, including the metadata address 169.254.169.254. Below are concrete, idiomatic examples that you can apply in your Buffalo handlers.

1. Validate and restrict destination hosts

Use a whitelist approach or a strict allowlist of domains, and reject requests targeting internal IPs or metadata addresses. The following helper checks the target host and returns a 400 error if it is disallowed:

import (
    "net"
    "net/http"
    "strings"
)

func isAllowedHost(host string) bool {
    // Reject loopback and link-local ranges
    if ip := net.ParseIP(host); ip != nil {
        return ip.IsGlobalUnicast() && !ip.IsLoopback() && !ip.IsLinkLocalMulticast() && !ip.IsPrivate()
    }
    // Optionally allow only specific external domains
    allowed := map[string]bool{
        "api.example.com": true,
        "cdn.example.com": true,
    }
    return allowed[host]
}

func validateReq(c buffalo.Context) error {
    target := c.Param("url")
    u, err := url.Parse(target)
    if err != nil || !isAllowedHost(u.Host) {
        return c.Error(http.StatusBadRequest, errors.New("invalid or disallowed host"))
    }
    return nil
}

2. Use a custom HTTP transport to block metadata IPs

Create a custom transport that rejects connections to known cloud metadata addresses. This prevents any request from reaching 169.254.169.254 regardless of how the URL is constructed:

import (
    "context"
    "errors"
    "net"
    "net/http"
)

var (
    errBlockedMetadata = errors.New("request to metadata endpoint is not allowed")
    metadataIP        = "169.254.169.254"
)

type secureTransport struct{ http.RoundTripper }

func (t secureTransport) RoundTrip(req *http.Request) (*http.Response, error) {
    host := req.URL.Host
    if ip := net.ParseIP(host); ip != nil && ip.String() == metadataIP {
        return nil, errBlockedMetadata
    }
    if net.ParseIP(host).IsPrivate() && host == metadataIP {
        return nil, errBlockedMetadata
    }
    return t.RoundTripper.RoundTrip(req)
}

func newSecureClient() *http.Client {
    return &http.Client{Transport: secureTransport{&http.Transport{}}}
}

3. Apply middleware to reject risky parameters in Buffalo routes

Buffalo middleware can intercept requests before they reach your action. Use it to sanitize or reject incoming URL parameters that could lead to SSRF against metadata endpoints:

func SSRFProtection() func(*buffalo.Request) *buffalo.Response {
    return func(r *buffalo.Request) *buffalo.Response {
        raw := r.Param("url")
        if raw == "" {
            return nil
        }
        u, err := url.Parse(raw)
        if err != nil {
            return r.Render(http.StatusBadRequest, r.JSON(map[string]string{"error": "invalid url"}))
        }
        if ip := net.ParseIP(u.Host); ip != nil && ip.String() == "169.254.169.254" {
            return r.Render(http.StatusForbidden, r.JSON(map[string]string{"error": "forbidden target"}))
        }
        return nil
    }
}

// In actions file: app.GET("/fetch", SSRFProtection(), fetchHandler)

4. Use net/http client with context timeouts

Always configure timeouts and prefer context control to avoid hanging requests. Combine this with validated hosts to keep calls safe:

import (
    "context"
    "net/http"
    "time"
)

func fetchExternal(ctx context.Context, target string) ([]byte, error) {
    u, err := url.Parse(target)
    if err != nil {
        return nil, err
    }
    if !isAllowedHost(u.Host) {
        return nil, errors.New("host not allowed")
    }
    req, _ := http.NewRequestWithContext(ctx, "GET", target, nil)
    client := newSecureClient()
    resp, err := client.Do(req)
    if err != nil {
        return nil, err
    }
    defer resp.Body.Close()
    return io.ReadAll(resp.Body)
}