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Provides access to XMLHttpRequest functionality.


Security Concerns

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.

Threat Model

The 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.

Possible Attenuations

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 http:, 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.




Creates an 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.


xhr: XMLHttpRequest
The XMLHttpRequest to allow third-party cookies for.

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 Contributors to this page: freaktechnik, wbamberg
 Last updated by: freaktechnik,