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diff --git a/src/_posts/2021-07-14-how-to-secure-a-webapp.md b/src/_posts/2021-07-14-how-to-secure-a-webapp.md new file mode 100644 index 0000000..2ddbb51 --- /dev/null +++ b/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. |