Double CSRF

A utility package to help implement stateless CSRF protection using the Double Submit Cookie Pattern in express.

Dos and Don'ts • Getting Started • Configuration • Utilities • Support

Background

This module provides the necessary pieces required to implement CSRF protection using the Double Submit Cookie Pattern. This is a stateless CSRF protection pattern, if you are using sessions and would prefer a stateful CSRF strategy, please see csrf-sync for the Synchroniser Token Pattern.

Since csurf has been deprecated I struggled to find alternative solutions that were accurately implemented and configurable, so I decided to write my own! Thanks to NextAuth as I referenced their implementation. From experience CSRF protection libraries tend to complicate their configuration, and if misconfigured, can render the protection completely useless.

This is why csrf-csrf aims to provide a simple and targeted implementation to simplify it's use.

Dos and Don'ts

Do read the OWASP - Cross-Site Request Forgery Prevention Cheat Sheet
Do read the OWASP - Secrets Management Cheat Sheet
Do follow the recommendations when configuring csrf-csrf.
Do join the Discord server and ask questions in the psifi-support channel if you need help.
Do follow fastify/csrf-protection recommendations for secret security.
Do keep secure as true in production.
Do make sure you do not compromise your security by not following best practices.
Do not use the same secret for csrf-csrf and cookie-parser.
Do not use the cookie value within getTokenFromRequest>.
Do not expose your CSRF tokens or hash in any log output or transactions other than the CSRF exchange.
Do not transmit the token hash by any other means.

Getting Started

This section will guide you through using the default setup, which does sufficiently implement the Double Submit Cookie Pattern. If you'd like to customise the configuration, see the configuration section.

You will need to be using cookie-parser and the middleware should be registered before Double CSRF. In case you want to use signed CSRF cookies, you will need to provide cookie-parser with a unique secret for cookie signing. This utility will set a cookie containing both the csrf token and a hash of the csrf token and provide the non-hashed csrf token so you can include it within your response.

If you're using TypeScript, requires TypeScript >= 3.8

``` npm install cookie-parser csrf-csrf ``` ```js // ESM import { doubleCsrf } from "csrf-csrf"; // CommonJS const { doubleCsrf } = require("csrf-csrf"); ``` ```js const { invalidCsrfTokenError, // This is just for convenience if you plan on making your own middleware. generateToken, // Use this in your routes to provide a CSRF hash + token cookie and token. validateRequest, // Also a convenience if you plan on making your own middleware. doubleCsrfProtection, // This is the default CSRF protection middleware. } = doubleCsrf(doubleCsrfOptions); ```

This will extract the default utilities, you can configure these and re-export them from your own module. You should only transmit your token to the frontend as part of a response payload, do not include the token in response headers or in a cookie, and do not transmit the token hash by any other means.

To create a route which generates a CSRF token and a cookie containing ´${token|tokenHash}´:

```js const myRoute = (req, res) => { const csrfToken = generateToken(req, res); // You could also pass the token into the context of a HTML response. res.json({ csrfToken }); }; const myProtectedRoute = (req, res) => res.json({ unpopularOpinion: "Game of Thrones was amazing" }); ```

Instead of importing and using generateToken, you can also use req.csrfToken any time after the doubleCsrfProtection middleware has executed on your incoming request.

```js request.csrfToken(); // same as generateToken(req, res); ```

You can also put the token into the context of a templated HTML response. Just make sure you register this route before registering the middleware so you don't block yourself from getting a token.

```js // Make sure your session middleware is registered before these express.use(session); express.get("/csrf-token", myRoute); express.use(doubleCsrfProtection); // Any non GET routes registered after this will be considered "protected" ```

By default, any request that are not GET, HEAD, or OPTIONS methods will be protected. You can configure this with the ignoredMethods option.

You can also protect routes on a case-to-case basis:

```js app.get("/secret-stuff", doubleCsrfProtection, myProtectedRoute); ```

Once a route is protected, you will need to ensure the hash cookie is sent along with the request and by default you will need to include the generated token in the x-csrf-token header, otherwise you'll receive a `403 - ForbiddenError: invalid csrf token`. If your cookie is not being included in your requests be sure to check your withCredentials and CORS configuration.

Sessions

If you plan on using express-session then please ensure your cookie-parser middleware is registered after express-session, as express session parses it's own cookies and may conflict.

Using asynchronously

csrf-csrf itself will not support promises or async, however there is a way around this. If your csrf token is stored externally and needs to be retrieved asynchronously, you can register an asynchronous middleware first, which exposes the token.

```js (req, res, next) => { getCsrfTokenAsync(req) .then((token) => { req.asyncCsrfToken = token; next(); }) .catch((error) => next(error)); }; ```

And in this example, your getTokenFromRequest would look like this:

```js (req) => req.asyncCsrfToken; ```

Configuration

When creating your doubleCsrf, you have a few options available for configuration, the only required option is getSecret, the rest have sensible defaults (shown below). ```js const doubleCsrfUtilities = doubleCsrf({ getSecret: () => "Secret", // A function that optionally takes the request and returns a secret getSessionIdentifier: (req) => "", // A function that should return the session identifier for a given request cookieName: "__Host-psifi.x-csrf-token", // The name of the cookie to be used, recommend using Host prefix. cookieOptions: { sameSite = "lax", // Recommend you make this strict if posible path = "/", secure = true, ...remainingCookieOptions // See cookieOptions below }, size: 64, // The size of the generated tokens in bits ignoredMethods: ["GET", "HEAD", "OPTIONS"], // A list of request methods that will not be protected. getTokenFromRequest: (req) => req.headers["x-csrf-token"], // A function that returns the token from the request }); ```

getSecret

```ts (request?: Request) => string | string[] ```

Required

This should return a secret key or an array of secret keys to be used for hashing the CSRF tokens.

In case multiple are provided, the first one will be used for hashing. For validation, all secrets will be tried, preferring the first one in the array. Having multiple valid secrets can be useful when you need to rotate secrets, but you don't want to invalidate the previous secret (which might still be used by some users) right away.

getSessionIdentifier

```ts (req: Request) => string; ```

Optional
Default: () => ""

A function that takes in the request and returns the unique session identifier for that request. For example:

```ts (req: Request) => req.session.id; ```

This will ensure that CSRF tokens are signed with the unique identifier included, this means tokens will only be valid for the session that they were requested by and generated for.

cookieName

```ts string; ```

Optional
Default: "__Host-psifi.x-csrf-token"

Optional: The name of the httpOnly cookie that will be used to track CSRF protection. If you change this it is recommend that you continue to use the __Host- or __Secure- security prefix.

Change for development

f

The security prefix requires the secure flag to be true and requires requests to be received via HTTPS, unless you have your local instance running via HTTPS, you will need to change this value in your development environment.

cookieOptions

```ts { sameSite?: string; path?: string; secure?: boolean ...remainingCookieOptions // See below. } ```

Optional
Default:

```ts { sameSite: "lax", path: "/", secure: true } ```
The options provided to the cookie, see cookie attributes. The remaining options are all undefined by default and consist of:
```ts maxAge?: number | undefined; signed?: boolean | undefined; expires?: Date | undefined; domain?: string | undefined; encode?: (val: string) => string ```
Change for development

For development you will need to set secure to false unless you're running HTTPS locally. Ensure secure is true in your live environment by using environment variables.

delimiter

```ts string; ```

Optional
Default: "|"

The delimiter is used when concatenating the plain CSRF token with the hash, constructing the value for the cookie. It is also used when splitting the cookie value. This is how a token can be reused when there is no state. Note that the plain token value within the cookie is only intended to be used for token re-use, it is not used as the source for token validation.

getTokenFromRequest

```ts (req: Request) => string | null | undefined; ```

Optional
Default:

```ts (req: Request) => req.headers["x-csrf-token"]; ```

This function should return the token sent by the frontend, either in the request body/payload, or from the `x-csrf-token` header. Do NOT return the value from the cookie in this function, this would be the same as having no csrf protection at all. The doubleCsrfProtection middleware will validate the value returned by this function against the value in the cookie.

ignoredMethods

```ts Array; ```

Optional
Default: ["GET", "HEAD", "OPTIONS"]

An array of request types that the doubleCsrfProtection middleware will ignore, requests made matching these request methods will not be protected. It is recommended you leave this as the default.

size

```ts number; ```

Optional
Default: 64

The size in bytes of the tokens that will be generated, if you plan on re-generating tokens consider dropping this to 32.

errorConfig

```ts statusCode?: number; message?: string; code?: string | undefined; ```

Optional
Default:

```ts { statusCode: 403, message: "invalid csrf token", code: "EBADCSRFTOKEN" } ``` Used to customise the error response statusCode, the contained error message, and it's code, the error is constructed via createHttpError. The default values match that of csurf for convenience.

Utilities

Below is the documentation for what doubleCsrf returns.

doubleCsrfProtection

```ts (request: Request, response: Response, next: NextFunction) => void ```

The middleware used to actually protect your routes, see the getting started examples above, or the examples included in the repository.

generateToken

```ts ( request: Request, response: Response, overwrite?: boolean, // Set to true to force a new token to be generated validateOnReuse?: boolean, // Set to false to generate a new token if token re-use is invalid ) => string; ```

By default if a csrf-csrf cookie already exists on an incoming request, generateToken will not overwrite it, it will simply return the existing token so long as the token is valid. If you wish to force a token generation, you can use the third parameter:

```ts generateToken(req, res, true); // This will force a new token to be generated, and a new cookie to be set, even if one already exists ```

If the 'overwrite' parameter is set to false (default), the existing token will be re-used and returned. However, the cookie value will also be validated. If the validation fails an error will be thrown. If you don't want an error to be thrown, you can set the 'validateOnReuse' (by default, true) to false. In this case instead of throwing an error, a new token will be generated and returned.

```ts generateToken(req, res, true); // As overwrite is true, an error will never be thrown. generateToken(req, res, false); // As validateOnReuse is true (default), an error will be thrown if the cookie is invalid. generateToken(req, res, false, false); // As validateOnReuse is false, an error will never be thrown, even if the cookie is invalid. Instead, a new cookie will be generated if it is found to be invalid. ```

Instead of importing and using generateToken, you can also use req.csrfToken any time after the doubleCsrfProtection middleware has executed on your incoming request.

```ts req.csrfToken(); // same as generateToken(req, res) and generateToken(req, res, false); req.csrfToken(true); // same as generateToken(req, res, true); req.csrfToken(false, false); // same as generateToken(req, res, false, false); ```

The generateToken function serves the purpose of establishing a CSRF (Cross-Site Request Forgery) protection mechanism by generating a token and an associated cookie. This function also provides the option to utilize a third parameter called overwrite, and a fourth parameter called validateOnReuse. By default, overwrite is set to false, and validateOnReuse is set to true.

It returns a CSRF token and attaches a cookie to the response object. The cookie content is `${token}|${tokenHash}`.

You should only transmit your token to the frontend as part of a response payload, do not include the token in response headers or in a cookie, and do not transmit the token hash by any other means.

When overwrite is set to false, the function behaves in a way that preserves the existing CSRF cookie and its corresponding token and hash. In other words, if a valid CSRF cookie is already present in the incoming request, the function will reuse this cookie along with its associated token.

On the other hand, if overwrite is set to true, the function will generate a new token and cookie each time it is invoked. This behavior can potentially lead to certain complications, particularly when multiple tabs are being used to interact with your web application. In such scenarios, the creation of new cookies with every call to the function can disrupt the proper functioning of your web app across different tabs, as the changes might not be synchronized effectively (you would need to write your own synchronization logic).

If overwrite is set to false, the function will also validate the existing cookie information. If the information is found to be invalid (for instance, if the secret has been changed from the time the cookie was generated), an error will be thrown. If you don't want an error to be thrown, you can set the validateOnReuse (by default, true) to false. If it is false, instead of throwing an error, a new cookie will be generated.

invalidCsrfTokenError

This is the error instance that gets passed as an error to the next call, by default this will be handled by the default error handler. This error is customizable via the errorConfig.

validateToken

```ts (req: Request) => boolean; ```

This function is used by the doubleCsrfProtection middleware to determine whether an incoming request has a valid CSRF token. You can use this to make your own custom middleware (not recommended).

Support

Join the Discord and ask for help in the psifi-support channel.
Pledge your support through the Patreon

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