cookie 是否保护令牌免受 XSS 攻击?

2022-01-10 00:00:00 jwt security cookies javascript access-token

我正在为基于浏览器的 Javascript Web 应用程序构建基于 JWT(JSON Web 令牌)的身份验证机制，使用无状态服务器(无用户会话！)，我想一劳永逸地知道，如果使用将我的 JWT 令牌存储在 cookie 中将保护我的令牌免受 XSS 攻击，或者如果没有保护，那么在我的 Javascript 应用程序中使用浏览器本地存储并没有真正的优势.

我在 SO 和许多博客中看到过这个问题的提问和回答，但我从未见过真正让我满意的答案.

这个问题最初是基于它征求意见的基础上提出的 - 并且按照我原来的措辞，正确地如此.因此，让我在此明确说明，我不希望基于开发人员懒惰等模糊概念发表意见 - 这就是基本规则旨在消除的内容.我想要的是有证据支持的是/否的答案.要么:

  • 是的，可以保护 cookie 免受 XSS 和 CSRF 的影响，方法如下" 或
  • 不，通过保护您的 cookie 免受 CSRF 的影响，您总是可以将它们打开到与最初使 cookie 成为一个好主意的 XSS 攻击相同的类型"

所以我要重述这个问题，用一些简化的基本规则，并提前指出漏洞，以便专家们能够纠正我.

基本规则

  • 您的应用程序是一个 javascript 浏览器应用程序 - 它可能在 AngularJS 中，但它可能是定制的.它通过 REST 调用与服务器通信.假设，jQuery $ajax 调用.

  • 服务器是无状态的:没有会话管理.

  • 应用使用 JWT 作为主要身份验证令牌(OAuth2 用语中的访问令牌")，并使用秘密签名密钥在服务器上验证它们

  • 忽略 cookie 的其他重要优势:浏览器管理、减少编码不佳的机会等.对于这场战斗，我想考虑绝对安全性，并假设我们可以胜任任何一种机制的编码.

  • 忽略 cookie 的其他缺点，例如非浏览器应用程序等.对于这场战斗，我们只关心基于浏览器的 javascript 应用程序.

  • 在非cookie的方式中，使用header还是request body来传输token并不重要；如果您使用本地存储与会话存储也没关系 - 忽略那里的任何安全差异.是的，我知道从技术上讲 Cookie 使用标头，请忽略它.

简而言之，我们只对比较 browser-handles-tokens 与 your-javascript-handles-tokens 以及比较 XSS 和 CSRF 安全风险感兴趣.

参赛选手

在红色角落，Auth0:本地存储胜过 Cookie，因为 XSS 比 CSRF 更容易修复

在蓝色角落，Stormpath:Cookies 胜过标头，因为实际上 CSRF 比 XSS 更容易修复.

(以下两个论点的详细摘录)

选择的武器

XSS 和 CSRF(我们将交替使用 CSRF 和 XSRF:C 似乎在文档中更受欢迎，X 在代码中更受欢迎)

这是我对攻击类型的超级简化总结:

假设您的无状态、经过 JWT 身份验证的 javascript 浏览器应用程序用于在线银行业务，而攻击者Evil Corp"想要提交一个 AJAX REST 调用，通过冒充您的用户将资金转移到他们的帐户.

XSS(跨站脚本)

(正如 Stormpath 指出的那样，有很多攻击向量 - 我会选择一个)

Evil Corp 购买了用于输入密码的漂亮文本字段小部件的 github 帐户权限.他们知道您的银行网站使用它，因此当您输入密码并按 Enter 时，他们会更新它以提交 AJAX 请求以将资金转移到他们的帐户.您的构建系统愚蠢地提取更新并投入生产.

CSRF(跨站请求伪造)

Evil Corp 知道您的银行网站在 cookie 中使用 JWT 来验证交易，因此他们编写了一个 Web 应用程序来提交 AJAX 请求以将资金转移到他们的帐户.他们将其托管在他们自己的 evil.com 网站上，并在您碰巧在另一个选项卡中登录您的银行网站时，通过电子邮件(网络钓鱼)或其他方式引诱您到那里.浏览器从 evil.com 提交请求，但附加了您的 JWT，因为它会转到正确的站点:银行.

标准防御

对 XSS 的防御是要非常小心您网站中的代码，这样您就永远不会让浏览器处理用户输入的内容而不对其进行清理(删除 javascript 和 html)以及所有 3rd 方库(Evil 的文本字段小部件)在使用前经过审查.正如 Stormpath 正确指出的那样，这很难，几乎是不可能的.

针对 CSRF 的防御是使用一种双重提交 cookie 的形式.这意味着我们的服务器会创建一个令牌(安全的随机字符串)并将其以可读 cookie 的形式发送到我们的 Javascript 浏览器应用程序(按照惯例称为XSRF-TOKEN")，然后我们的 Javascript 将其以标题或正文的形式发送回每个请求.

实际上，double-sumbit-cookie 只是对抗 CSRF 的一种防御，但其他一些需要有状态的服务器会话，并且没有其他(我认为！)提供更好的保护.也可以通过将令牌放入 JWT 并在服务器端将其与标头或正文中的令牌进行比较来实现无状态.

但这种防御的真正破坏 CSRF 的质量是同源策略意味着只有我们的应用从 我们的域 加载的 javascript 才能读取该 cookie.因此，即使 evilcorp.com 上的 javascript 可以将我们的 cookie 连同其请求一起发送，它也无法嵌入我们的 XSRF-TOKEN，因为它一开始就无法读取它地点.

真的简化 CSRF:

  • CSRF attacks 起作用，因为附加 cookie 的浏览器仅依赖于请求的 destination.
  • CSRF defences 起作用，因为 Javascript 对 cookie 的访问取决于 Javascript 的 origin.

Auth0 的论据

<块引用>

处理 XSS 比 XSRF 更容易 Cookie 具有以下功能:允许从服务器端设置一个 HttpOnly 标志，因此它们只能是在服务器上访问，而不是从 JavaScript 访问.这很有用，因为它保护该 cookie 的内容以通过注入访问客户端代码 (XSS).由于令牌存储在本地/会话中存储或客户端 cookie，它们容易受到 XSS 攻击让攻击者访问令牌.这是一个合理的担忧，并且出于这个原因，你应该保持你的令牌过期时间低.

但是如果你想想 cookie 的攻击面，主要的攻击面之一是XSRF.现实情况是 XSRF 是最容易被误解的一种攻击和普通开发人员可能甚至不了解风险，因此许多应用程序缺乏抗 XSRF 机制.然而，每个人都知道注射是什么.简单地说，如果你允许在您的网站上输入，然后在不转义的情况下呈现它，您对 XSS 开放.所以根据我们的经验，更容易保护防御 XSS 而不是防御 XSRF.除此之外，anti-XSRF 是不是每个 Web 框架都内置的.另一方面，XSS 很容易通过使用大多数默认情况下可用的转义语法来防止模板引擎.https://auth0.com/blog/2014/01/27/ten-things-you-should-know-about-tokens-and-cookies#xss-xsrf

Stormpath 的论点

<块引用>

Stormpath 建议您将 JWT 存储在网络 cookie 中应用程序，因为它们提供了额外的安全性，并且使用现代 Web 框架防止 CSRF 的简单性.HTML5 Web Storage 易受 XSS 攻击，攻击面较大区域，并且可以在成功攻击时影响所有应用程序用户.https://stormpath.com/博客/where-to-store-your-jwts-cookies-vs-html5-web-storage/

还有:

<块引用>

我看到很多关于 cookie 反对访问的讨论令牌.虽然我们都被存储会话 ID 的系统所困扰在 cookie 中，该 cookie 不安全，因此被盗.那很烂，但这不是使用令牌的理由.这是一个避免的理由非安全、非 https cookie.https://stormpath.com/blog/token-auth-spa/p>

我的看法

Stormpath 支持 cookie 的论点非常有说服力，但其中有一个漏洞，我认为他们没有清楚地解决:

双重提交 CSRF 防御依赖于我的 CSRF 攻击者无法访问我的 cookie 的事实:其中包含 XSRF-TOKEN 的那个.但是，在 XSS 攻击中，该 cookie 是否与本地存储一样容易受到攻击?

XSS 漏洞利用可以在 my 域上运行 javascript，因此它可以读取我的 javascript 可以读取的相同 cookie.(浏览器不知道它不是我的Javascript)

从另一方面来看:本地存储受到同源策略的保护，就像可读的 cookie 一样.如果我使用 Auth0 方法，并且 XSS 攻击者知道如何在本地存储中找到我的 JWT 并使用它.难道同一个攻击者不能使用同一个 XSS 脚本来获取我的 XSRF-TOKEN cookie 并使用它吗?

这两种攻击都要求他们阅读和理解我的 javascript 应用程序，但这在他们的浏览器中.

那么有什么区别呢?一个真的比另一个更安全吗?为什么?

解决方案

放弃所有希望，除非你能抵御 XSS！

或者

根据其他标准选择适合您的方法，因为两者同样安全，同样不安全.

如果您使用 cookie，则绝对应该使用双重提交 cookie 防御或类似的东西，因为它确实可以保护您在没有 XSS 的情况下免受 CSRF 的影响.也就是说，如果你不这样做，你肯定会受到来自其他域的 CSRF 攻击，这些攻击甚至不需要 XSS 漏洞利用即可工作.

但无论哪种方式，您的源代码都是公开可用的(浏览器中的 JavaScript)，因此对于有动机的黑客来说，从本地存储中找到要提取的令牌与读取 XSRF-TOKEN cookie 之间的努力没有显着差异.如果 Evil Corp 可以让一些 JavaScript 在你的域中运行——那就是 XSS——那么你就完蛋了.

您在选择时可能要考虑的非安全相关标准:

  • Cookie 很方便，因为您不必编写 JavaScript 代码来管理令牌 - 只需 XSRF.

  • 如果你想使用它，重定向也会变得更加自动.

  • 本地存储更容易适应非浏览器应用程序 - 从服务器的角度来看，因为如果你用 Java 编写一个不想处理 cookie 的 Android 应用程序，你的服务器不会不需要区分 in 和浏览器，因为它不使用 cookie.

无论如何，请自行决定，但请注意您编写的 JavaScript和您使用的第 3 方 JavaScript！

I'm building a JWT-based (JSON Web Token) authentication mechanism for an browser-based Javascript web app, working with a stateless server (no user-sessions!) and I want to know, once and for all, if using storing my JWT token in a cookie will protect my token from XSS attacks, or if there is no protection, so there's no real advantage over using browser local storage in my Javascript app.

I have seen this question asked and answered in SO and in many blogs, but I've never seen an answer that really satisfies me.

This question was originally held on the basis that it solicits opinion - and given my original wording, rightly so. So let me make it clear here and now that I don't want an opinion based on vague notions of developer laziness or such - that's what the ground rules are meant to eliminate. What I want is an evidence-backed Yes/No answer. Either:

  • "Yes, cookies can be protected from XSS and CSRF and here's how" or
  • "No, by protecting your cookies from CSRF, you always open them up to the same kind of XSS attack that made cookies a good idea in the first place"

So I'm going to restate the question, with some simplifying ground-rules, and point out the holes in advance, so that you, the experts, can set me straight.

Ground rules

  • Your app is a javascript browser app - it might be in AngularJS, but it might be custom-built. It communicates with the server via REST calls. Let's say, jQuery $ajax calls.

  • The server is stateless: there is no session management.

  • The app users JWTs as the main authentication token ('access token' in OAuth2 parlance) and validates them on the server using a secret signing key

  • Ignore other important advantages of cookies: browser management, less chance of coding poorly, etc. For this battle, I want to consider the absolute security, and assume we can competently code either mechanism.

  • Ignore other disadvantages of cookies, such as non-browser apps, etc. For this battle we are only concerned with a browser-based javascript app.

  • It doesn't matter whether you use a header or a request body to transmit tokens in the non-cookie approach; nor does it matter if you're using local storage vs session storage - ignore any security differences there. And yes I know that technically Cookies use headers, ignore that.

In a nutshell, we're only interested in comparing browser-handles-tokens vs your-javascript-handles-tokens and the comparative XSS and CSRF security risks.

The contestants

In the red corner, Auth0: Local Storage beats Cookies, because XSS is easier to fix than CSRF

In the blue corner, Stormpath: Cookies beats headers, because actually CSRF is easier to fix than XSS.

(excerpts of both arguments in detail below)

Weapons of choice

XSS and CSRF (we'll use CSRF and XSRF interchangeably: the C seems to be more popular in documentation, the X in code)

Here's my super-simplified summary of the attack types:

Let's assume your stateless, JWT-authenticated, javascript browser app is for online banking and the attacker, "Evil Corp", wants to submit an AJAX REST call that transfers funds to their account by impersonating your users.

XSS (Cross-site scripting)

(As Stormpath points out, there are many attack vectors - I'll pick one)

Evil Corp buys the github account rights for the nifty text field widget you use for password entry. They know your bank site uses it, so they update it to submit AJAX requests to transfer funds to their account when you type in your passord and hit enter. Your build system foolishly pulls the update and puts in production.

CSRF (Cross-Site Request Forgery)

Evil Corp knows your bank site uses JWTs in cookies to authenticate transactions, so they write a web app that submits AJAX requests to transfer funds to their account. They host this on their own evil.com site, and lure you there with an email (phishing) or some other way, when you happen to be logged into your bank site in another tab. The browser submits the request from evil.com, but attaches your JWT becaues it's going to the correct site: the bank.

Standard defences

The defence against XSS is to be very careful about the code in your site so that you never let the browser process something the user types in without sanitizing it (removing javascript and html) and that all the 3rd party libraries (Evil's text field widget) are vetted before being used. As Stormpath rightly points out, this is hard, bordering on impossible.

The defence against CSRF is to use a form of double-submit-cookie. This means our server creates a token (securely random string) and sends it to our Javascript browser app in a readable cookie (call it "XSRF-TOKEN" by convention), and our Javascript sends it back in a header or body with every request.

Actually, double-sumbit-cookie's are only one defence agasint CSRF, but some others require stateful server sessions and no other (I think!) offers any better protection. The statelessness can be achieved by also putting the token in the JWT and comparing it on the server side with the one that comes in the header or body.

But the real CSRF-busting quality of this defence, is that same-origin-policy mean that only the javascript that our app loaded from our domain can read that cookie. So even if the javascript on evilcorp.com can send our cookies with its requests, it can't embed our XSRF-TOKEN because it can't read it in the first place.

To really simplify CSRF:

  • CSRF attacks work because a browser attaching a cookie depends only on the destination of a request.
  • CSRF defences work because Javascript access to a cookie depends on the origin of the Javascript.

Auth0's argument

It's easier to deal with XSS than XSRF Cookies have this feature that allows setting an HttpOnly flag from server side so they can only be accessed on the server and not from JavaScript. This is useful because it protects the content of that cookie to be accessed by injected client-side code (XSS). Since tokens are stored in local/session storage or a client side cookie, they are open to an XSS attack getting the attacker access to the token. This is a valid concern, and for that reason you should keep your tokens expiration low.

But if you think about the attack surface on cookies, one of the main ones is XSRF. The reality is that XSRF is one of the most misunderstood attacks, and the average developer, might not even understand the risk, so lots of applications lack anti-XSRF mechanism. However, everybody understands what injection is. Put simply, if you allow input on your website and then render that without escaping it, you are open to XSS. So based on our experience, it is easier to protect against XSS than protecting against XSRF. Adding to that, anti-XSRF is not built-in on every web framework. XSS on the other hand is easy to prevent by using the escape syntax available by default on most template engines. https://auth0.com/blog/2014/01/27/ten-things-you-should-know-about-tokens-and-cookies#xss-xsrf

Stormpath's argument

Stormpath recommends that you store your JWT in cookies for web applications, because of the additional security they provide, and the simplicity of protecting against CSRF with modern web frameworks. HTML5 Web Storage is vulnerable to XSS, has a larger attack surface area, and can impact all application users on a successful attack. https://stormpath.com/blog/where-to-store-your-jwts-cookies-vs-html5-web-storage/

Also:

I see a lot of discussions where cookies are pitted against access tokens. While we’ve all been burned by systems that store a session ID in a cookie, and that cookie is not secured and thus gets stolen. That sucks, but its not a reason to use tokens. Its a reason to avoid non-secure, non-https cookies. https://stormpath.com/blog/token-auth-spa/

My take

Stormpath's argument in favour of cookies is pretty convincing, but there's a hole in it that I don't see them addressing clearly:

The double-submit CSRF defence relies on the fact that my CSRF attacker cannot access my cookie: the one with the XSRF-TOKEN in it. But isn't that cookie just as vulnerable in an XSS attack as local storage?

An XSS exploit can run javascript on my domain, so it can read the same cookies my javascript can. (The browser doesn't know that it isn't my Javascript)

To look at it from the other side: local storage is protected by the same-origin-policy just as much as a readable cookie. If I'm using the Auth0 approach, and an XSS attacker knows how to find my JWT in local storage and use it. Can't that same attacker use that same XSS script to grab my XSRF-TOKEN cookie and use that?

Both attacks require them to read and understand my javascript app, but that's out there in their browser.

So what's the difference? Is one really more secure than another, and why?

解决方案

Abandon all hope unless you can secure against XSS!

Or

Choose the approach that suits you based on other criteria because both are equally secure, equally insecure.

If you use cookies, you should definitely use the double-submit-cookie defence, or something similar, because it does protect you against CSRF in the absence of XSS. That is, if you don't do this, you're definitely open to CSRF attacks - from other domains - that don't even require XSS exploits to work.

But either way, your source code is publicly available (JavaScript in your browser) so for a motivated hacker, there is no significant difference in effort between finding which token to pull from local storage and reading your XSRF-TOKEN cookie. If Evil Corp can get some JavaScript running in your domain - that's XSS - then you're hosed.

Non-security-related criteria you might want to consider for your choice:

  • Cookies are convenient because you don't have to write JavaScript code to manage the token - only the XSRF.

  • Redirection becomes a little more automatic too, if you want to use it.

  • Local storage is easier to adapt to non-browser apps - from the server perspective that is, because if you write say, an Android app in Java that doesn't want to deal with cookies, your server doesn't need to make any distinction between in and the browser, since it's not using cookies.

Anyway, make up your own mind, but be careful about the JavaScript you write and the 3rd party JavaScript you use!

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