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diff --git a/static/src/_posts/2021-07-14-how-to-secure-a-webapp.md b/static/src/_posts/2021-07-14-how-to-secure-a-webapp.md deleted file mode 100644 index 155068d..0000000 --- a/static/src/_posts/2021-07-14-how-to-secure-a-webapp.md +++ /dev/null @@ -1,315 +0,0 @@ ---- -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. |