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HTML5 Security Cheat Sheet

Revision as of 11:55, 9 October 2011 by Will Stranathan (talk | contribs) (Fixed some formatting, grammar. Removed incomplete/question sections.)

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Browser Securability Chart

There are a few sites charting browser capabilities as they relate to the HTML 5 / CSS 3 standard. I have not seen any that mention security. There may not be a need for it, but the sandbox attribute of iframe's will be ignored in browsers except those HTML 5 compliant browsers which support it. If there are differences in implementations, there will be differences in security configuration / settings.

Cross Origin Resource Sharing

  • Validate URLs passed to, 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 the 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.
  • While the RFC recommends a pre-flight request with the OPTIONS verb, current implementations might not perform this request, so it's important that "ordinary" (GET and POST) requests perform any access control necessary.

Local Storage (a.k.a. Offline Storage, Web Storage)

  • Underlying storage mechanism may vary from one user agent to the next. In other words, any authentication your application requires can be bypassed by a user with local privileges to the machine on which the data is stored. Therefore, it's recommended not to store any sensitive information in local storage.


  • Underlying storage mechanism may vary from one user agent to the next. In other words, any authentication your application requires can be bypassed by a user with local privileges to the machine on which the data is stored. Therefore, it's recommended not to store any sensitive information in local storage.


  • 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 authorization 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.


  • The Geolocation RFC recommends that the user agent ask the user's permission before calculating location, but whether or how this decision is remembered varies from browser to browser. Some user agents require the user to visit the page again in order to turn off the ability to get the user's location without asking, so for privacy reasons, it's recommended to require user input before calling getCurrentPosition or watchPosition.

Use the sandbox attribute for untrusted content (iFrame)


Web Messaging

Web Messaging provides a means of messaging between documents from different origins in a way which is generally safer than JSON-P, however, there are still some recommendations to keep in mind:

  • When posting a message, explicitly state the expected origin as the second argument to postMessage rather than * in order to prevent sending the message to an unknown origin after a redirect or some other means of the target window's origin changing.
  • The receiving page should always:
    • Check the origin attribute of the sender to verify the data is originating from the expected location, and
    • Perform input validation on the data attribute of the event to ensure it's in the desired format

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 <video> tag is unsupported, or it may mean additional scripting code from various sources that should be code reviewed.


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 known - e.g. querying global history using :visited (

Related Cheat Sheets

OWASP Cheat Sheets Project Homepage

Authors and Primary Editors

Mark Roxbury - mark.roxberry [at]

Krzysztof Kotowicz - krzysztof [at]

Will Stranathan - will [at]