X authority bypass

Question

I'm trying to write an application that runs as a daemon and monitors running X sessions. Right now I'm struggling to find documentation regarding the X security model. Specifically, I'm attempting to connect to running X displays from my daemon process. Calling XOpenDisplay(dispName) doesn't work, I guess because my process doesn't have permission to connect to this display. After a bit of research, it looks like I need to do something with xauth.

In my test environment, the X server is started like this:

/usr/bin/X -br -nolisten tcp :0 vt7 -auth /var/run/xauth/A:0-QBEVDj

That file contains a single entry, that looks like this:

#ffff##:  MIT-MAGIC-COOKIE-1  XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

by adding an entry to ~/.Xauthority with the same hex key, I can connect to the X server. However, this is difficult because I need to programmatically find the auth file the X server is using (the location of which I guess will change from distro to distro, and probably from one boot to the next), then query it, then write a new auth file. If the process is running as a daemon, it might not have a home directory, so how do I know where to write the new entries to?

Ideally, what I'm looking for is a way to bypass the need to have the xauth cookie in ~/.Xauthority, or even to know what the cookie is at all. I realise that this is unlikely - what good is a security model if it's easily bypassed? but I'm hoping someone on this list may have a few good ideas. Is there a way to specify that my process is privileged and thus should automatically be given access to any display on the local machine?

Answer 1

You don't have to use a home directory if you specify an XAUTHORITY environment variable, which specifies the location of the .Xauthority file. Read the xauth man page.

But, in general, it's hard to locate the auth file, for the reasons you mentioned; also, this "fishing for auth tokens" approach would only work for local displays.

With regard to letting root (or some other user) connect to an X server willy-nilly, you'd probably have to patch the source code to do this, and you'd have to use something like getpeereid to obtain the connecting user's uid/gid (this only works on Unix-domain sockets, which I presume would be the type used for local connections, anyway).

Answer 2

Xauth is not the only security mechanism for X

There is also another one (less secure) that just performs IP based authentication (See xhost).

So if you switch your X server to this less secure mode it will trust any connections coming from the defined set of IPs.

This way you do not need to deal with Xauthority at all.