HTML5 Security Cheat Sheet

= Introduction =

= HTML 5 =

Browser Securability Chart
There are a few sites charting browser capabilities as they related to the HTML 5 / CSS 3 standard. I have not seen any that mention security. There may not be a need for it, but e.g. 'sandbox' will be ignored in down browsers, but which HTML 5 compliant browsers support it. If there are differences in implementations, my assumption is that there will be differences in security configuration / settings.

Cross Origin Resource Sharing

 * Validate URLs passed to XMLHttpRequest.open, current browsers allow these URLS to be cross domain.
 * Ensure that URLs responding with Access-Control-Allow-Origin: * do not include any sensitive content or information that might aid attacker in further attacks. Use Access-Control-Allow-Origin header only on chosen URLs that need to be accessed cross-domain. Don't use that header for the whole domain.
 * Take special care when using Access-Control-Allow-Credentials: true response header. Whitelist the allowed Origins and never echo back the Origin request header in Access-Control-Allow-Origin.
 * Allow only selected, trusted domains in Access-Control-Allow-Origin header. Prefer whitelisting domains over blacklisting or allowing any domain (either through * wildcard or echoing the Origin header content).
 * Keep in mind that CORS does not prevent the requested data from going to an un-authenticated location - it's still important for the server to perform usual CSRF prevention.

WebSockets

 * Drop backward compatibility in implemented client/servers and use only protocol versions above hybi-00. Popular Hixie-76 version and olders are outdated and insecure.
 * While it is relatively easy to tunnel TCP services through WebSockets (e.g. VNC, FTP), doing so enables access to these tunneled services for the in-browser attacker in case of a Cross-Site-Scripting attack. These services might also be called directly from a malicious page or program.
 * The protocol doesn't handle authorisation and/or authentication. Application-level protocols should handle that separately in case sensitive data is being transferred.
 * Endpoints exposed through ws:/ protocol are easily reversible to plaintext. Only wss:// (WebSockets over SSH) should be used for protection against Man-In-The-Middle attacks
 * Spoofing the client is possible outside browser, so WebSockets server should be able to handle incorrect/malicious input. Always validate input coming from the remote site, as it might have been altered.
 * When implementing servers, check the Origin: header in Websockets handshake. Though it might be spoofed outside browser, browsers always add the Origin of the page which initiated Websockets connection.
 * As WebSockets client in browser is accessible through Javascript calls, all Websockets communication can be spoofed or hijacked through Cross-Site-Scripting. Always validate data coming through WebSockets connection.

Use the "sandbox" attribute for untrusted content (iFrame)
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Content Deliverability
CDN or src links to foreign domains = know your content

Progressive Enhancements and Graceful Degradation Risks
The best practice now is to determine the capabilities that a browser supports and augment with some type of substitute for capabilities that are not directly supported. This may mean an onion-like element, e.g. falling through to a Flash Player if the &lt;video&gt; tag is unsupported, or it may mean additional scripting code from various sources that should be code reviewed.

= CSS 3 =

I haven't seen any specific to CSS 3 and it's been a while since I worried about url / !import. I think privacy leaks are the most well know - e.g. querying global history using :visited (https://bugzilla.mozilla.org/show_bug.cgi?id=147777)

= Javascript and Javascript Frameworks =

Do we have cheatsheets for Javascript (e.g. use closures, protect the global namespace) or any of the frameworks like JQuery, script.aculo.us, Prototype, Mootools

= Related Cheat Sheets =

= Authors and Primary Editors =

Mark Roxbury - mark.roxberry [at] owasp.org

Krzysztof Kotowicz - krzysztof [at] kotowicz.net