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----
-title: >-
-    How to Secure a Webapp
-description: >-
-    Get ready to jump through some hoops.
-tags: tech
----
-
-In this post I will be documenting all security hoops that one must jump through
-in order to consider their webapp secure. This list should not be considered
-comprehensive, as I might have forgotten something or some new hoop might have
-appeared since writing.
-
-For the context of this post a "webapp" will be considered to be an HTML/CSS/JS
-website, loaded in a browser, with which users create and access accounts using
-some set of credentials (probably username and password). In other words, most
-popular websites today. This post will only cover those concerns which apply to
-_all_ webapps of this nature, and so won't dive into any which might be incurred
-by using one particular technology or another.
-
-Some of these hoops might seem redundant or optional. That may be the case. But
-if you are building a website and are beholden to passing some third-party
-security audit for any reason you'll likely find yourself being forced to
-implement most, if not all, of these measures anyway.
-
-So without further ado, let's get started!
-
-## HTTPS
-
-At this point you have to use HTTPS, there's not excuse for not doing so. All
-attempts to hit an HTTP endpoint should redirect to the equivalent HTTPS
-endpoint, and you should be using [HSTS][hsts] to ensure that a browser is never
-tricked into falling back to HTTP via some compromised DNS server.
-
-[hsts]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Strict-Transport-Security
-
-## Cookies
-
-Cookies are an old web technology, and have always been essentially broken. Each
-cookie can have certain flags set on it which change their behavior, and some of
-these flags are required at this point.
-
-### Secure
-
-If you're storing anything sensitive in a cookie (spoiler alert: you will be)
-then you need to have the Secure flag set on it. This prevents the cookie from
-being sent in a non-HTTPS request.
-
-### HTTPOnly
-
-The HTTPOnly flag protects a cookie from XSS attacks by preventing it from being
-accessible from javascript. Any cookie which is storing sensitive information
-_must_ have this flag set. In the **Authentication** section we will cover the
-storage of session tokens, but the TLDR is that they have to be stored in an
-HTTPOnly cookie.
-
-Practically, this means that your sessions architecture _must_ account for the
-fact that the webapp itself will not have direct access to its persistent
-session token(s), and therefore must have some other way of knowing that it's
-logged in (e.g. a secondary, non-HTTPOnly cookie which contains no secrets but
-only signals that the browser is logged in).
-
-### SameSite
-
-The SameSite attribute can be set to `Strict`, `Lax`, or `None`. `Lax` is the
-default in modern browsers and is sufficient for most security concerns, but if
-you can go with `Strict` that would be better. The downside of `Strict` is that
-cookies won't be sent on initial page-load of a site.
-
-In any case, even though `Lax` is the default you should still set this
-attribute manually (or your auditor might get to add another bullet point to
-their report).
-
-## Authentication
-
-Authentication is obviously one of the juiciest targets for an attacker. It's
-one thing to be able to trick a user into performing this or that action, but if
-one can just log in _as_ the user then they essentially have free-reign over all
-their information.
-
-### Password History
-
-Most websites use a username/password system as the first step of login. This
-is.... fine. We've accepted it, at any rate. But there's a couple of hoops which
-must be jumped through as a result of it, and the first is password history.
-
-I hope it goes without saying that one should be using a hashing algorithm like
-bcrypt to store user passwords. But what is often not said is that, for each
-user, you need to store the hashes of their last N passwords (where N is
-something like 8). This way if they attempt to re-use an old password they are
-not able to do so. The users must be protected from themselves, afterall.
-
-### Credential Stuffing/Account Enumeration
-
-A credential stuffing attack is one where credentials are stolen from one
-website and then attempted to be used on another, in the hope that users have
-re-used their username/password across multiple sites. When they occur it'll
-often look like a botnet spamming the authentication endpoint with tons of
-different credentials.
-
-Account enumeration is a similar attack: it's where an attacker finds a way to
-get the webapp to tell them whether or not an account email/username exists in
-the system, without needing to have the right password. This is often done by
-analyzing the error messages returned from login or a similar endpoint (e.g.
-"Sorry this username is taken"). They then run through all possible values for
-that endpoint to try and enumerate which users actually exist in the system.
-
-Account enumeration is tricky because often those errors are extremely helpful,
-and we'd _like_ to keep them if we can.
-
-I've bucketed both of these attacks in the same section because they have a
-similar solution: proof-of-work. The idea is that, for each request to some
-sensitive endpoint, the client must send some proof that they've done an
-intensive CPU computation.
-
-Compared to IP-based rate-limiting, PoW is much more effective against botnets
-(which have a limitless set of IPs from which to spam you), while also being
-much less intrusive on your real users than a captcha.
-
-PoW stymies botnets because they are generally being hosted by low-power,
-compromised machines. In addition the systems that run these botnets are pretty
-shallow in capability, because it's more lucrative to rent the botnet out then
-to actually use it yourself, so it's rare for a botnet operator to go to the
-trouble of implementing your PoW algorithm in the first place.
-
-So stick a PoW requirement on any login or account creation endpoint, or any
-other endpoint which might be used to enumerate accounts in the system. You can
-even make the PoW difficulty rise in relation to number of recent attempts on
-these endpoints, if you're feeling spry.
-
-### MFA
-
-All the PoW checks in the world won't help your poor user who isn't using a
-different username/password for each website, and who got unlucky enough to have
-those credentials leaked in a hack of a completely separate site than your own.
-They also won't help your user if they _are_ using different username/passwords
-for everything, but their machine gets straight up stolen IRL and the attacker
-gets access to their credential storage.
-
-What _will_ help them in these cases, however, is if your site supports
-multi-factor authentication, such as [TOTP][totp]. If it does then your user
-will have a further line of defense in the form of another password which
-changes every 30 seconds, and which can only be accessed from a secondary device
-(like their phone). If your site claims to care about the security of your
-user's account then MFA is an absolute requirement.
-
-It should be noted, however, that not all MFA is created equal. A TOTP system
-is great, but a one-time code being sent over SMS or email is totally different
-and not nearly as great. SMS is vulnerable to [SIM jacking][sim], which can be
-easily used in a targeted attack against one of your users. One-time codes over
-email are pointless for MFA, as most people have their email logged in on their
-machine all the time, so if someone steals your user's machine they're still
-screwed.
-
-In summary: MFA is essentially required, _especially_ if the user's account is
-linked to anything valuable, and must be done with real MFA systems like TOTP,
-not SMS or email.
-
-[totp]: https://www.twilio.com/docs/glossary/totp
-[sim]: https://www.vice.com/en/article/3kx4ej/sim-jacking-mobile-phone-fraud
-
-### Login Notifications
-
-Whenever a user successfully logs into their account you should send them email
-(or some other notification) letting them know it happened. This way if it
-wasn't actually them who did so, but an attacker, they can perhaps act quickly
-to lock down their account and prevent any further harm. The login notification
-email should have some kind of link in it which can be used to immediately lock
-the account.
-
-### Token Storage
-
-Once your user has logged into your webapp, it's up to you, the developer, to
-store their session token(s) somewhere. The question is... where? Well this
-one's easy, because there's only one right answer: HTTPOnly cookies (as alluded
-to earlier).
-
-When storing session tokens you want to guard against XSS attacks which might
-grab the tokens and send them to an attacker, allowing that attacker to hijack
-the session and pose as the user. This means the following are not suitable
-places to store the tokens:
-
-* Local storage.
-* `window`, or anything which can be accessed via `window`.
-* Non-HTTPOnly cookies.
-
-Any of these are trivial to find by a script running in the browser. If a
-session token is ephemeral then it may be stored in a "normal" javascript
-variable somewhere _as long as_ that variable isn't accessible from a global
-context. But for any tokens which need to be persisted across browser restarts
-an HTTPOnly cookie is your only option.
-
-## Cross-Site
-
-Speaking of XSS attacks, we have some more mitigation coming up...
-
-### CSP
-
-Setting a [CSP][csp] for your website is key to preventing XSS. A CSP allows you
-to more tightly control the allowed origins of the various entities on your site
-(be they scripts, styles, images, etc...). If an entity of unexpected origin
-shows up it is disallowed.
-
-Be sure to avoid any usages of the policies labeled "unsafe" (go figure),
-otherwise the CSP is rendered somewhat pointless. Also, when using hostname
-based allowlisting try to be as narrow as you can in your allowlist, and
-especially only include https hosts. If you can you should opt for the `nonce`
-or `sha` policies.
-
-[csp]: https://developer.mozilla.org/en-US/docs/Web/HTTP/CSP
-
-### SVG
-
-A small but important note: if you're website allows users to upload images,
-then be _very_ careful about allowing users to upload SVGs. SVGs are actually
-XML documents, and even worse than that they allow `<script>` tags within them!
-So you need to be very careful about allowing SVGs to be uploaded. If you can
-get away with it, it's better to disallow their use at all.
-
-## CSRF
-
-The web was designed in a time when cross-site requests were a considered
-feature. This has proven to be a massive mistake. We have two cross-site request
-prevention techniques in this list. The first is CSRF.
-
-CSRF protection will cover you from a variety of attacks, mostly of the kind
-where an attacker embeds a `<form>` on their own webpage, with the form set up
-to POST to _your_ website in some way. When a user of your website lands on the
-attacker's page and triggers the POST, the POST will be sent along with whatever
-cookies the user has stored in their browser for _your_ site!
-
-The attacker could, potentially, trick a user into submitting a password-reset
-request using a known value, or withdrawing all their money into the attacker's
-bank account, or anything else the user might be able to do on their own.
-
-The idea with CSRF is that any HTTP request made against an API should have an
-unguessable token as a required parameter, called the CSRF token. The CSRF token
-should be given to your webapp in a way where only your webapp could know it.
-There are many ways to accomplish this, including a cookie, server-side embedded
-value, etc... OWASP has put together an [entire cheatsheet full of CSRF
-methods][csrf] which is well worth checking out.
-
-[csrf]: https://cheatsheetseries.owasp.org/cheatsheets/Cross-Site_Request_Forgery_Prevention_Cheat_Sheet.html
-
-## CORS
-
-CORS is the other half of cross-site protection. With CSRF in place it's
-somewhat redundant, but it's good to have multiple layers of protection in place
-(in case you fuck up one of them by accident).
-
-The key thing one must do for CORS protection is to set the
-`Access-Control-Allow-Origin` to the origin a request is being sent from _only
-if you trust that origin_. If you stick a wildcard in that header then you're
-not doing anything.
-
-## Random Headers
-
-The rest of this is random HTTP headers which must be set in various contexts to
-protect your users.
-
-### Permissions Policy
-
-The [Permissions-Policy][pp] header is fairly new and not fully standardized
-yet, but there is support for it so it's worth using. It allows you to specify
-exactly which browser features you expect your webapp to need, and therefore
-prevent an attacker from taking advantage of some other feature that you were
-never going to use anyway.
-
-[pp]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Feature-Policy
-
-### X-Content-Type-Options
-
-It's important to set `X-Content-Type-Options: nosniff` on virtually all
-HTTP responses, in order to (theoretically) prevent a browser from inferring the
-MIME of the returned content.
-
-### X-Frame-Options
-
-Set `X-Frame-Options: deny` to prevent your webapp from being rendered in a
-frame or iframe on someone else's site, which might then be used to trick one of
-your users into doing something stupid.
-
-### X-XSS-Protection
-
-Set `X-XSS-Protection: 1; mode=block` to give older browsers which lack CSP
-support some extra defense against XSS attacks. It's not super clear to me what
-exactly this actually does, but it's easy enough to set.
-
-### Referrer-Policy
-
-Set the `Referrer-Policy` to inform your users' browsers to not send the
-`Referer` header to third-party sites when your users navigate away from your
-site. You don't want other websites to be able to see _yours_ in their logs, as
-they could then correlate which users of theirs have accounts with you (and so
-potentially have some easy targets).
-
-### Cache-Control/Pragma
-
-For all requests which return sensitive information (i.e. any authenticated
-requests) it's important to set `Cache-Control: no-store` and `Pragma: no-cache`
-on the response. This prevents some middle server or the browser from caching
-the response, and potentially returning it later to someone else using your site
-from the same location.
-
-## That's It
-
-It's probably not it, actually, these are just what I could think of off the top
-of my head. Please email me if I missed any.
-
-If you, like me, find yourself asking "how is anyone supposed to have figured
-this out?" then you should A) thank me for writing it all down for you and B)
-realize that at least 50% of this list has nothing to do with the web, really,
-and everything to do with covering up holes that backwards compatibility has
-left open. We can cover these holes, we just need everyone to agree on the path
-to doing so, and to allow ourselves to leave some ancient users behind.