csp-middleware

HTTP content security policy(CSP) middleware.

Compliant with Content Security Policy Level 3.

Middleware

For a definition of Universal HTTP middleware, see the http-middleware project.

Usage

Middleware adds the Content-Security-Policy header to the response.

import {
  csp,
  type Handler,
} from "https://deno.land/x/csp_middleware@$VERSION/mod.ts";
import { assert } from "https://deno.land/std/testing/asserts.ts";

declare const request: Request;
declare const handler: Handler;

const middleware = csp();
const response = await middleware(request, handler);

assert(response.headers.has("content-security-policy"));

yield:

Content-Security-Policy: default-src 'none'; script-src 'self'; connect-src 'self'; img-src 'self'; style-src 'self'; base-uri 'self'; form-action 'self'

The default header field value is compliant with Content Security Policy (CSP) Quick Reference Guide, Stater policy.

Options

Middleware factory takes following fields.

Name	Type	Description
directives	`CSPDirectives`	CSP directives.
reportOnly	`boolean`	Whether the policy report only or not.

Directives

directives can be one of the following.

CSPDirective
Camel casing CSPDirective

CSP directives

CSPDirectives are structured Content-Security-Policy header field objects.

Base types are as follows:

interface Directives {
  [k: string]: string | string[];
}

Each key represents a directive name and each value represents a directive value.

The Directive supports all directives in Content Security Policy Level 3.

Each directive may be restricted to a more strict type.

For example, a webrtc directive is restricted to 'allow' or 'block'.

import { csp } from "https://deno.land/x/csp_middleware@$VERSION/middleware.ts";

const middleware = csp({
  directives: {
    "default-src": "'none'",
    webrtc: "'allow'",
  },
});

Check deno doc for about CSPDirectives.

Camel casing

The directive name can also be defined in camel case. Overloading makes it exclusive.

import { csp } from "https://deno.land/x/csp_middleware@$VERSION/middleware.ts";

const middleware = csp({
  directives: {
    defaultSrc: "'none'",
    scriptSrc: ["'self'", "*.example.test"],
  },
});

Report Only

The header field changes depending on the value of reportOnly.

Value	Header field
`true`	Content-Security-Policy-Report-Only
`false`	Content-Security-Policy

The default reportOnly is false.

import {
  csp,
  type Handler,
} from "https://deno.land/x/csp_middleware@$VERSION/mod.ts";
import { assert } from "https://deno.land/std/testing/asserts.ts";

declare const request: Request;
declare const handler: Handler;

const middleware = csp({ reportOnly: true });
const response = await middleware(request, handler);

assert(response.headers.has("content-security-policy-report-only"));

Serialization error

CSP directives will serialize into string.

In Serialization, the directive name and directive value are validated based on ABNF. If they are invalid, an error may be thrown.

Errors are thrown in the following cases:

None of the directive is present
Directive key does not compliant with <directive-name>
Directive value does not compliant with <VCHAR> without ";" and ","
Directive values has a duplicate value

import { csp } from "https://deno.land/x/csp_middleware@$VERSION/middleware.ts";
import { assertThrows } from "https://deno.land/std/testing/asserts.ts";

assertThrows(() => csp({ directives: {} }));
assertThrows(() => csp({ directives: { defaultSrc: "<invalid>" } }));
assertThrows(() =>
  csp({ directives: { defaultSrc: ["<duplicate>", "<duplicate>"] } })
);

Effects

Middleware may make changes to the following elements of the HTTP message.

HTTP Headers
- Content-Security-Policy
- Content-Security-Policy-Report-Only

Conditions

Middleware will execute if all of the following conditions are met:

Depends on reportOnly:

Content-Security-Policy header does not exists in response
Content-Security-Policy-Report-Only header does not exists in response

API

All APIs can be found in the deno doc.

License

Released under the MIT license

httpland/csp-middleware