Language: Go
Label: safe
CWE: CWE-918 \u2014 Server-Side Request Forgery (SSRF)
The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination.

Code:
func safeAddr(ctx context.Context, resolver *net.Resolver, hostport string, opts ...Option) (string, error) {
	c := basicConfig()
	for _, opt := range opts {
		opt(c)
	}
	host, port, err := net.SplitHostPort(hostport)
	if err != nil {
		return "", err
	}

	ip := net.ParseIP(host)
	if ip != nil {
		if ip.IsUnspecified() || (ip.To4() != nil && c.isIPForbidden(ip)) {
			return "", fmt.Errorf("bad ip is detected: %v", ip)
		}
		return net.JoinHostPort(ip.String(), port), nil
	}

	if c.isHostForbidden(host) {
		return "", fmt.Errorf("bad host is detected: %v", host)
	}

	r := resolver
	if r == nil {
		r = net.DefaultResolver
	}
	addrs, err := r.LookupIPAddr(ctx, host)
	if err != nil || len(addrs) <= 0 {
		return "", err
	}
	safeAddrs := make([]net.IPAddr, 0, len(addrs))
	for _, addr := range addrs {
		// only support IPv4 address
		if addr.IP.To4() == nil {
			continue
		}
		if c.isIPForbidden(addr.IP) {
			return "", fmt.Errorf("bad ip is detected: %v", addr.IP)
		}
		safeAddrs = append(safeAddrs, addr)
	}
	if len(safeAddrs) == 0 {
		return "", fmt.Errorf("fail to lookup ip addr: %v", host)
	}
	return net.JoinHostPort(safeAddrs[0].IP.String(), port), nil
}