Fake login screens

Fergal Daly fergal at esatclear.ie
Sat Feb 14 20:56:41 UTC 2009

2009/2/14 Peteris Krisjanis <pecisk at gmail.com>:
> 2009/2/14 Vincenzo Ciancia <ciancia at di.unipi.it>:
>> On 14/02/2009 Felipe Figueiredo wrote:
>>> As others said, more than once in this thread, the change is
>>> reversible.
>>> There will be a package to install so you don't have to edit your
>>> xorg.conf.
>> I will keep myself informed but I expected that ubuntu-devel-discuss was
>> also a place to discuss the ubuntu development, involving high-impact
>> changes. My mistake, so I will keep myself informed.
>> However, it seems to me that nobody is getting the point about fake
>> login screens: if I am an *user* of somebody else's network, how can I
>> protect myself from another *user* faking a login screen, used as the
>> only running X application, and stealing my password?
> You have evidence that such scenario could happen or even is happened?
> Or you just speculate? Anything can be faked in this world, specially
> on computers.

It's known as the secure attention key.


and yes this is a real attack with lots of history. When I was in
University some of my friend were competing to collect the largest
number of passwords,


>> Under some windows versions, I can use ctrl+alt+delete. I bet the mac
>> has something similar,
> Nope, it doesn't (as far as I know, and I have worked with OS X as
> sysadmin for five years). And Windows Ctrl+Alt+Delete have absolutely
> different meaning than anti-faking measure.
>> and Xorg traditionally had ctrl+alt+backspace
>> (even though, it also kills the session as a nice side effect). Now, you
>> have to consider that even an experienced system administrator may not
>> notice the change when he will install next ubuntu on the client
>> machines of a computing lab, or even worse when upgrading to it. Fancy
>> an unexperienced system administrator as there are many.
> Well, unexperienced system administrator would allow box to contain
> trojan to get your password anyway. Believe me, faking login screens
> is not a way someone would steal your password, unless there is no
> other way.
>> I will surely write my own fake gdm as an exercise just in case I become
>> an user of such an admin :) Because of statistics, you know, if I carry
>> a bomb there can't be another bomb on my plane.
> Strawman argument.
>> If the solution is "currently, ubuntu jaunty is vulnerable to this
>> problem", let's just admit it and make it public in the release notes at
>> least. So that people will know and avoid leaving the default
>> configuration on clients.
> No, Jaunty simply won't have C-A-B feature enabled by default. Simple
> as that. Release notes doesn't have such speculation as "OMG, visual
> interface have changed, someone could use it to steal information from
> people".
>> Personally I would love that the power button returned to gdm, and that
>> gdm created a new X session (like for the "guest login" use case) for
>> every login, without disappearing, and occupying a fixed tty (the one
>> the power button would return to). In that case, gdm could also offer a
>> pre-loaded and not-swappable emergency shell that administrator may
>> access. However, this *really* needs a blueprint so for now is there any
>> other solution?
> Yes, this *really* need blueprint just for a reason - it is how
> world-shattering changes are introduced into Ubuntu. Disabling C-A-B
> by default was blueprint for two years. This is how decision making
> happens.
> Don't get me wrong - I know that changing features is painful process
> of some of us, but as far as I have experienced with Ubuntu, it is
> always pays back in long term. Introduction of compiz broken a lot of
> setups, but Hardy released with nice desktop effects tested for some
> time. NetworkManager 0.7 was introduced as main network configuration
> tool. Sure, I was annoyed, even angry. But I took time to test it and
> understand it and now I admit that it is a future.
> There is a blueprint already for dealing with C-A-B without disabling
> it and I hope it will find a way into Jaunty+1. And that is how system
> should work.
> Cheers,
> Peter.
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