secure webapp

src/_posts/2021-07-14-how-to-secure-a-webapp.md

@@ -0,0 +1,314 @@
+---
+title: >-
+    How to Secure a Webapp
+description: >-
+    Get ready to jump through some hoops.
+---
+
+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.