使用helmet.contentSecurityPolicy预防xss攻击

来源：https://helmetjs.github.io/docs/csp/

Content Security PolicyIn short: the CSP module sets the 

Content-Security-Policy

 header which can help protect against malicious injection of JavaScript, CSS, plugins, and more.

The attack

Hackers can do lots of bad things if they can put things onto your webpages.The nastiest attack is probably cross-site scripting (XSS), which is when a hacker puts malicious JavaScript onto your page. If I can run JavaScript on your page, I can do a lot of bad things, from stealing authentication cookies to logging every user action.There are other things attackers can do, even if they can’t execute JavaScript. For example, if I could put a tiny, transparent 1x1 image on your site, I could get a pretty good idea of how much traffic your site gets. If I could get a vulnerable browser plugin like Flash to run, I could exploit its flaws and do things you don’t want!There isn’t one specific attack that the CSP module prevents. The main thing is this: you don’t want anyone putting anything on your webpages that you don’t expect.Read more:Cross-site scripting on Wikipedia
Cross-site Scripting on OWASP
“How does a tracking pixel work?” on Quora

The header

One of the tricky things about these injection attacks is that the browser doesn’t know what’s good and what’s bad. How can it tell the difference between a legitimate JavaScript file and a malicious one? In many cases, it can’t…unless you’ve defined a Content Security Policy.Most modern browsers support a header called 

Content-Security-Policy

, which is effectively a whitelist of things that are allowed to be on your page. You can whitelist JavaScript, CSS, images, plugins, and much more. Things are opt-in, so you’re saying “this stuff is allowed” instead of “this stuff is not allowed”.Let’s say you’ve got a website that links to no external resources at all—just your stuff. You could set a header that looks like this:

Content-Security-Policy: default-src 'self'

This effectively tells the browser “only load things that are from my own domain”. If you’re running example.com and a user tries to load 

https://example.com/my-javascript.js

, it’ll work just fine. But if a user tries to load 

http://evil.com/evil.js

, it won’t load at all!Now, let’s say you want to also allow CSS from Bootstrap’s CDN. You could set a CSP that looks like this:

Content-Security-Policy: default-src 'self'; style-src 'self' maxcdn.bootstrapcdn.com

Now we’ve whitelisted 

'self'

 and 

maxcdn.bootstrapcdn.com

. The user will be able to load CSS from there, but nothing else. The user won’t even be able to load JavaScript or images from that URL, either—only stylesheets.There are a lot of nuances to CSP: what you can and can’t whitelist, browser support for various features, and alternate headers. Refer to the stuff below for more information.Read more:An introduction to Content Security Policy on HTML5 Rocks
Content Security Policy Reference
Can I Use Content Security Policy 1.0
Can I Use Content Security Policy 2.0

The code

Helmet’s 

csp

 module helps set Content Security Policies.You can use this module as part of Helmet:

// Make sure you run "npm install helmet" to get the Helmet package.
var helmet = require('helmet')

app.use(helmet.contentSecurityPolicy({
directives: {
defaultSrc: ["'self'"],
styleSrc: ["'self'", 'maxcdn.bootstrapcdn.com']
}
}))

You can also use it as a standalone module:

// Make sure you run "npm install helmet-csp" to get the csp package.
var csp = require('helmet-csp')

app.use(csp({
directives: {
defaultSrc: ["'self'"],
styleSrc: ["'self'", 'maxcdn.bootstrapcdn.com']
}
}))

This header is not included in the default Helmet bundle.

Directives

All of your CSP directives (like 

default-src

, 

style-src

) are placed under the 

directives

option.

app.use(csp({
directives: {
defaultSrc: ["'self'", 'default.com'],
scriptSrc: ["'self'", "'unsafe-inline'"],
sandbox: ['allow-forms', 'allow-scripts'],
reportUri: '/report-violation',
objectSrc: ["'none'"],
upgradeInsecureRequests: true,
workerSrc: false  // This is not set.
}
}))

Directives can be kebab-cased (like 

script-src

) or camel-cased (like 

scriptSrc

); they are equivalent.The following directives are supported:

base-uri

 or 

baseUri

block-all-mixed-content

 or 

blockAllMixedContent

child-src

 or 

childSrc

connect-src

 or 

connectSrc

default-src

 or 

defaultSrc

font-src

 or 

fontSrc

form-action

 or 

formAction

frame-ancestors

 or 

frameAncestors

frame-src

 or 

frameSrc

img-src

 or 

imgSrc

manifest-src

 or 

manifestSrc

media-src

 or 

mediaSrc

object-src

 or 

objectSrc

plugin-types

 or 

pluginTypes

prefetch-src

 or 

prefetchSrc

report-to

 or 

reportTo

report-uri

 or 

reportUri

require-sri-for

 or 

requireSriFor

sandbox

 or 

sandbox

script-src

 or 

scriptSrc

style-src

 or 

styleSrc

upgrade-insecure-requests

 or 

upgradeInsecureRequests

worker-src

 or 

workerSrc

CSP violations

If you’ve specified a 

reportUri

, browsers will POST any CSP violations to your server. Here’s a simple example of an Express route that handles those reports:

// You need a JSON parser first.
app.use(bodyParser.json({
type: ['json', 'application/csp-report']
}))

app.post('/report-violation', function (req, res) {
if (req.body) {
console.log('CSP Violation: ', req.body)
} else {
console.log('CSP Violation: No data received!')
}

res.status(204).end()
})

Not all browsers send CSP violations in the same way, so this might require a little work.Note: If you’re using a CSRF module like csurf, you might have problems handling these violations without a valid CSRF token. The fix is to put your CSP report route above csurf middleware.This module’s 

reportOnly

 option will switch the header to 

Content-Security-Policy-Report-Only

. This instructs browsers to report violations to the 

reportUri

 (if specified) but it will not block any resources from loading.

app.use(csp({
directives: {
// ...
},
reportOnly: true
})

You may also set this to a function to decide dynamically whether to use 

reportOnly

 mode. You could use this for a dynamic kill switch. This function will be called with the request and response objects and must return a boolean.

app.use(csp({
directives: {
// ...
},
reportOnly: function (req, res) {
if (req.query.cspmode === 'debug') {
return true
} else {
return false
}
}
})

Browser sniffing

By default, this module will look at the incoming 

User-Agent

 header and send different headers depending on the detected browser. For example, Chrome prior to version 25 uses an alternate header called 

X-WebKit-CSP

, and this module handles that. If no browser is detected, this module will set all the headers with the 2.0 spec.To disable this browser sniffing and assume a modern browser, set the 

browserSniff

 option to 

false

.

app.use(csp({
directives: {
// ...
},
browserSniff: false
})

To set all headers, including legacy ones, set the 

setAllHeaders

 option to 

true

. Note that this will change the value of the headers based on 

User-Agent

. You can disable this by using the 

browserSniff: false

 option above.

app.use(csp({
directives: {
// ...
},
setAllHeaders: true
})

Old Android browsers can be very buggy. This is 

false

 by default.

app.use(csp({
directives: {
// ...
},
disableAndroid: true
})

Generating nonces

You can dynamically generate nonces to allow inline 

<script>

 tags to be safely evaluated. Here’s a simple example:

var uuid = require('node-uuid')

app.use(function (req, res, next) {
res.locals.nonce = uuid.v4()
next()
})

app.use(csp({
directives: {
scriptSrc: [
"'self'",
function (req, res) {
return "'nonce-" + res.locals.nonce + "'"  // 'nonce-614d9122-d5b0-4760-aecf-3a5d17cf0ac9'
}
]
}
}))

app.use(function (req, res) {
res.end('<script nonce="' + res.locals.nonce + '">alert(1 + 1);</script>')
})

Using CSP with a CDN

The default behavior of CSP is generate headers tailored for the browser that’s requesting your page. If you have a CDN in front of your application, the CDN may cache the wrong headers, rendering your CSP useless. Make sure to eschew a CDN when using this module or set the 

browserSniff

 option to 

false

.Thanks to the many contributors that helped put Helmet together! Helmet is open source under the MIT License.Icon by Hand-Drawn Goods, licensed under a CC 3.0 license.