Andrew Ayer

If your application makes requests to URLs provided by untrusted sources (such as users), you must take care to avoid server side request forgery (SSRF) attacks. Otherwise, an attacker might be able to induce your application to make a request to a service on your server's localhost or internal network. Since the service thinks the request is coming from a trusted source, it might perform a privileged action or return sensitive data that gets relayed by your application back to the attacker. This is particularly a problem when running in EC2, which exposes sensitive credentials over its metadata service, which is accessible over HTTP at a private IP address. SSRF attacks can be serious; one was exploited earlier this year to steal more than 100 million credit applications from Capital One.

One way to prevent SSRF attacks is to validate all addresses before connecting to them. However, you must do the validation at a very low layer to be effective. It's not sufficient to simply block URLs that contain "localhost" or an internal IP address, since an attacker could publish a DNS record under a public domain that resolves to an internal IP address. It's also insufficient to do the DNS lookup yourself and block a URL if the hostname resolves to an unsafe address; an attacker could set up a special DNS server that returns a safe address the first time it's queried, and the target address the second time when your application actually connects to the URL.

Instead, you need to hook deep into your HTTP client's networking stack and check for a safe address right before the HTTP client tries to access it.

Fortunately, Go makes it easy to hook in at just the right place, thanks to the Control field of net.Dialer, introduced in Go 1.11:

// If Control is not nil, it is called after creating the network
// connection but before actually dialing.
//
// Network and address parameters passed to Control method are not
// necessarily the ones passed to Dial. For example, passing "tcp" to Dial
// will cause the Control function to be called with "tcp4" or "tcp6".
Control func(network, address string, c syscall.RawConn) error // Go 1.11

This function is called by Go's standard library after the address has been resolved, but before connecting. The network argument is tcp4, udp4, tcp6, or udp6, and the address argument is an IP address and port number separated by a colon (e.g. 192.0.2.0:80 or [2001:db8:f942::3ab2]:443; split it with net.SplitHostPort, not strings.Split, to avoid IPv6 breakage). If the control function returns an error, the dial is aborted.

Here's an example control function that returns an error if the address is not safe. It's quite conservative, permitting only TCP connections to port 80 and 443 on public IP addresses (see here for the implementation of isPublicIPAddress). You may want to customize the control function to suit your application's needs.

func safeSocketControl(network string, address string, conn syscall.RawConn) error {
	if !(network == "tcp4" || network == "tcp6") {
		return fmt.Errorf("%s is not a safe network type", network)
	}

	host, port, err := net.SplitHostPort(address)
	if err != nil {
		return fmt.Errorf("%s is not a valid host/port pair: %s", address, err)
	}

	ipaddress := net.ParseIP(host)
	if ipaddress == nil {
		return fmt.Errorf("%s is not a valid IP address", host)
	}

	if !isPublicIPAddress(ipaddress) {
		return fmt.Errorf("%s is not a public IP address", ipaddress)
	}

	if !(port == "80" || port == "443") {
		return fmt.Errorf("%s is not a safe port number", port)
	}

	return nil
}

Once you have a control function, you can use it to make HTTP requests as follows (the various numbers below match those used by http.DefaultClient):

safeDialer := &net.Dialer{
	Timeout:   30 * time.Second,
	KeepAlive: 30 * time.Second,
	DualStack: true,
	Control:   safeSocketControl,
}

safeTransport := &http.Transport{
	Proxy:                 http.ProxyFromEnvironment,
	DialContext:           safeDialer.DialContext,
	ForceAttemptHTTP2:     true,
	MaxIdleConns:          100,
	IdleConnTimeout:       90 * time.Second,
	TLSHandshakeTimeout:   10 * time.Second,
	ExpectContinueTimeout: 1 * time.Second,
}

safeClient := &http.Client{
	Transport: safeTransport,
}

resp, err := safeClient.Get(untrustedURL)

The above code examples are in the public domain.