The xhr module provides access to XMLHttpRequest functionality, also known as AJAX.


The XMLHttpRequest object is currently fairly limited, and does not yet implement the addEventListener() or removeEventListener() methods. It also doesn't yet implement the upload property.

Furthermore, the XMLHttpRequest object does not currently support the mozBackgroundRequest property. All security UI, such as username/password prompts, are automatically suppressed, so if required authentication information isn't passed to the open() method, the request will fail.

Resource Use

Whenever this module is unloaded, all in-progress requests are immediately aborted.

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

We may also want to consider attenuating further based on domain name and possibly even restricting the protocol to https: only, to reduce risk.

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.

API Reference




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 MDC page on Using XMLHttpRequest and the Limitations section in this page.



Returns the number of XMLHttpRequest objects that are alive (i.e., currently active or about to be).

Returns: integer

The number of live XMLHttpRequest objects.