Jezen Thomas

Jezen Thomas

I'm a programmer and entrepreneur. Occasionally I write about technology, and speak at conferences. Find me on Instagram, or on GitHub, or write me an email.

Implementing a Content Security Policy in Yesod

If you use Mozilla’s Observatory website security scanner, you’ll quickly find you won’t get an A+ grade without correctly implementing a Content Security Policy.

A CSP is just a HTTP response header, so you’d think it would be trivial to add. Unfortunately, that isn’t the case. There are currently three different CSP versions, and as usual the different browser vendors can’t agree on how this feature should be implemented or even what name to use for the header.

It also doesn’t help that documentation you might assume to be authoritative is sometimes misleading. In fact, following various tutorials on the Internet — many of which are contradictory — initially lead me to implement a CSP using host-based whitelisting. The benefit of this approach is that you can just hardcode your whitelist, and implement your CSP in nginx without touching your application. There are a number of drawbacks though; not only are host-based whitelists cumbersome to maintain, they’re also generally insecure as evidenced in a Google research paper. The short story is that it’s better to use a nonce-based approach.

This presents its own set of problems though. For a nonce to be effective, it must be:

  • Encoded in Base64
  • Randomly generated on every request
  • Present in both the CSP header and the HTML tag of the script to which you’re granting execution permission

The first two constraints are trivial to solve — we can use the uuid and base64-bytestring libraries to produce our nonces.

The nextRandom function generates our random value. The quality of the randomness of this value doesn’t really matter, so we could have used randomIO from the System.Random module here. The reason I decided against using that function is that it’s quite slow. There are other libraries available for generating random values with good performance, but I’m already using UUIDV4 values in the rest of my application.

The toASCIIBytes function turns our random UUID value into a ByteString, which is the type that our Base64 encoding function expects. Finally, we use decodeUtf8 to turn our bytestring into a Text value, as this is the specific string-like type that Yesod’s HTML tag and response header functions expect.

Now that we have our nonce, how do we shoehorn it into the script tags we wish to whitelist? Typically when writing an application in Yesod, you’d use the addScript function to generate the script tags and insert them in the document. This won’t work though, because the nonce should be inserted into its own HTML attribute in that script tag. Luckily, Yesod provides us a way to generate a script tag with arbitrary additional attributes.

This works, but the ergonomics aren’t great and it adds quite a lot of noise. Fortunately Haskell makes it easy to abstract away this kind of thing, so we can write our own small helper functions to clean this up. The same principle applies whether we’re adding scripts we host ourselves, or scripts hosted elsewhere.

This takes care of all the JavaScript libraries we wish to import, but there is one more crucial script tag left to cover. When we compose widgets together which contain some Julius (Shakespeare-flavoured JavaScript) code, Yesod joins those disparate bits of widget-specific code together into a file typically called autogen-xxxxxxxx.js, where those xs are [I think] the hash of the file.

This is done in the core of the Yesod framework, so you can’t directly override this behaviour. As of yesod-core-1.6.16 however, there’s a new typeclass method that allows us to add arbitrary attributes to that generated tag too. We can use this method together with the nonce generator function we wrote earlier.

Now that we have our nonce in all our HTML script tags, we also need to stuff it into the script-src directive of our CSP response header. We could use the addHeader function in every handler in our application, but that would be cumbersome to maintain and adds unnecessary noise. Instead, we can add this response header to every request from just one place using a middleware.

The middleware function that we want to write will need to know the value of the nonce we generated earlier in the request lifecycle. Usually when you want to store some state in the application memory, you’d add a new field to the App record, initialise it when the application starts up, and manipulate the value it holds from your handler functions. In our case however, we don’t actually want the value to exist outside the lifecycle of a single request so this is a great opportunity to take advantage of Yesod’s per-request caching.

How this works is rather clever. You might be curious about the cacheGet function. How does it know exactly what to fetch from the cache? We didn’t even tell it which key to use for the lookup! Well, the per-request cache stores data by type, and Haskell can infer the type of cached value we’re interested in because we’ve passed our cached nonce value to the csp function which expects a Maybe Text argument.

Since values are cached by type, it logically follows that we can only cache one value for any given type. If we wanted to cache some other Text value, we’d be stuck. The way around this is to wrap our Text value with a newtype.

Making this change necessitates we also change the functions we wrote earlier. Our generator will not only have to create a value of the correct type, but it will also need to add that value to the per-request cache. Since this function will potentially be called multiple times during a single request, we should make it idempotent by having it first check the cache, and using the CSPNonce value if it already exists. Again, type inference is working here (notice we specified the type in the function’s type signature) to magically select the correct cache key.

I won’t show the final implementations of the other functions, because it’s mostly a case changing Text to CSPNonce and unwrapping that newtype in a couple of places. If you’re following along at home, you can just let the compiler guide you to the end. There are a couple of final steps for a more complete implementation, but these do nothing to further explain the mechanics of the above approach and are left as an exercise for the reader. They pertain to CSP not working correctly in [Mobile] Safari. More specifically:

  • Safari uses a non-standard header name. Use the ua-parse library to determine the user’s browser and set the header name appropriately.
  • Safari does not support the 'strict-dynamic' directive, so you need host-based whitelisting anyway. Use another newtype and cache a set of CSPHost values.

When you do implement your own CSP, be sure to check your syntax with Google’s CSP evaluator, and also make good use of the report-uri directive so you can see if some of your content is unintentionally blocked for some users in production.