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We are planning to deprecate the use by Firefox add-ons of the techniques described in this document.
Don't use these techniques to develop new add-ons. Use WebExtensions instead.
If you maintain an add-on which uses the techniques described here, consider migrating it to use WebExtensions instead.
Add-ons developed using these techniques might not work with multiprocess Firefox (e10s), which is already the default in Firefox Nightly and Firefox Developer Edition, and will soon be the default in Beta and Release versions of Firefox. We have documentation on making your add-ons multiprocess-compatible, but it will be more future-proof for you to migrate to WebExtensions.
A wiki page containing resources, migration paths, office hours, and more, is available to help developers transition to the new technologies.
Provides access to
By default, the
XMLHttpRequest object grants full access to any protocol scheme, which means that it can be used to read from (but not write to) the host system's entire filesystem. It also has unfettered access to any local area networks, VPNs, and the internet.
XMLHttpRequest object can be used by an add-on to "phone home" and transmit potentially sensitive user data to third parties.
If access to the filesystem isn't prevented, it could easily be used to access sensitive user data, though this may be inconsequential if the client can't access the network.
If access to local area networks isn't prevented, malicious code could access sensitive data.
If transmission of cookies isn't prevented, malicious code could access sensitive data.
Attenuating access based on a regular expression may be ineffective if it's easy to write a regular expression that looks safe but contains a special character or two that makes it far less secure than it seems at first glance.
Before being exposed to unprivileged code, this object needs to be attenuated in such a way that, at the very least, it can't access the user's filesystem. This can probably be done most securely by white-listing the protocols that can be used in the URL passed to the
open() method, and limiting them to
https:, and possibly a special scheme that can be used to access the add-on's packaged, read-only resources.
Finally, we need to also consider attenuating http/https requests such that they're "sandboxed" and don't communicate potentially sensitive cookie information.
XMLHttpRequest. This is a constructor, so its use should always be preceded by the
new operator. For more information about
XMLHttpRequest objects, see the MDN page on Using XMLHttpRequest and the Security Concerns section in this page.
Force relevant cookies to be sent with this
XMLHttpRequest even if normally they would not be.
XMLHttpRequest to allow third-party cookies for.