Content Security Policy (CSP) Middleware

A simple middleware for configuring CSP headers with support for websockets.

This middleware was inspired by github.com/unrolled/secure which provides a good way to apply security policy to your HTTP server response headers to help mitigate some common web based XSS attacks.

Unfortunately the unrolled/secure CSP configuration is a static string. Combining this with web sockets in dynamic environments where the server may be running behind a reverse proxy or in a Container, means that it is not possible to use the connect-src policy if you also wish to use websockets.

This middleware provides just the CSP header, but also supports reading the host value directly out of the HTTP request and configuring the appropriate WS (HTTP) or WSS (TLS) protocol based on the request configuration.

If you don't specify a specific CSP policy field, the middleware will not configure a policy for it.

Policy strings

csp has some commonly used constants defined

const (
	Self      = "'self'"
	None      = "'none'"
	Any       = "*"
	CSPHeader = "Content-Security-Policy"
)

Starter Config

csp has a starter config policy, with reasonable defaults, which you can use as a basis for customizing your own.

csp := csp.New(csp.StarterConfig())

... use of csp middleware ...

CSP violation reports

Some browsers support sending CSP violation reports to a uri. You can specify this in the config with ReportURI.

csp := csp.New(csp.Config{
	Default:   csp.None,
	Script:    csp.Self,
	Connect:   csp.Self,
	Img:       csp.Self,
	ReportURI: "http://example.com/csp-violations",
})

Dynamic WebSocket Support

If you specify WebSocket in the config, the middleware will dynamically permit the Host specified in the Request.Host field to use Web Sockets.

TODO: consider adding a white-list feature to prevent weird attacks against things like vhosts.

csp := csp.New(csp.Config{
	Default:   csp.None,
	Script:    csp.Self,
	Connect:   csp.Self,
	Img:       csp.Self,
	Style:     csp.Self,
	WebSocket: true,
})

Integration

Negroni

package main

import (
  "github.com/yanfali/csp"
  "github.com/urfave/negroni"
)

func main() {

    ... set up code for router ...

	n := negroni.Classic()

	csp := csp.New(csp.Config{
		Default: csp.None,
		Script:  csp.Self,
		Connect: csp.Self,
		Img:     csp.Self,
		Style:   csp.Self,
	})
	n.UseFunc(csp.NegroniHandlerFunc())

	... startup code for http server ...
}

Alice

csp := csp.New(csp.Config{
	Default: csp.None,
	Script:  csp.Self,
	Connect: csp.Self,
	Img:     csp.Self,
	Style:   csp.Self,
})
stdChain := alice.New(csp.Middleware)
mux := http.NewServeMux()
mux.Handle("/", stdChain.ThenFunc(func(w http.ResponseWriter, req *http.Request) {
	fmt.Fprintf(w, "Hello World")
}

csp also supports anything that accepts a standard http.HandlerFunc (csp.HandlerFunc()) or http.Handler (csp.Middleware).