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CAU-2008-0001 - Slowly Closing Door Race Condition

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                    Computer Academic Underground
                          Security Advisory 

Advisory ID:    CAU-2008-0001
Release Date:   04/01/2008
Title:          Slowly Closing Door Race Condition
Application/OS: Physical Structures
Topic:          Physical structures employing exit doors with locks
                are vulnerable to a race condition.
Vendor Status:  Not Notified
Attributes:     Physical, Race Condition
Advisory URL:   http://www.caughq.org/advisories/CAU-2008-0001.txt
Author/Email:   CAU <advisories (at) caughq.org>


Physical structures which employ automatically locking doors to secure 
exit points expose a race condition which may allow unauthorized entry.


Malicious outsiders may be able to enter a structure via an exit point.

Exit points may additionally provide an exit from a secure area of the
structure, allowing an outsider entering through the exit point to gain
direct access to the secure area.

Affected Systems

Physical structures which employ automatically locking doors at exit
points of the structure.

Technical Explanation

An exit's lock[1] generally converts a two-way door into a one-way
door, allowing a person to traverse the door's threshold in one
direction but not in the other.  These types of locks are used to
secure exit points of structures so that people may exit via the door
but not re-enter without disabling the lock through force or

When a person exits the structure through an exit point which is
secured by such a mechanism, a race condition exists wherein a
malicious outsider may be able to reach the door and enter through it
before it closes and locks itself.

Many doors, especially heavier ones, also employ closing mechanisms[2]
which are designed to cause the door to close slowly so as not to slam
the door shut and damage the door frame, or damage any human appendage
which may be in between the door and it's frame.  Such closing
mechanisms can greatly increase the amount of time that the race
condition exists.

Solution & Recommendations

1) Always ensure that personnel exiting an exit door wait outside the
   door until it has completely closed and locked before walking

2) Employ a double door system such as is used in an air-lock where
   the interior door must be secured prior to the exterior door being
   allowed to open.


First identify the exit point that you want to exploit.  Stand at a
safe distance during a high-traffic time and watch for people to use
the exit point.  Time how long it takes for the door to close and
lock itself when someone traverses the exit point.

Next, identify a safe hiding place near the exit point, preferably
in a direction that would be behind a person exiting the door, but
which is within a distance to the exit point which you could traverse
in under the door's closing time at a brisk pace or run.

Finally, hide in this location during a lower traffic time and wait
for someone to utilize the exit point.  After they have exited the
door and are walking away, run to the door and enter before it has
closed and locked.  Extra points are awarded for a spectacular dive
and/or roll to catch the door at the very last second.


[1] http://en.wikipedia.org/wiki/Lock_%28device%29
[2] http://en.wikipedia.org/wiki/Door_closer

Credits & Gr33ts

Theodor Geisel, AHA!, NMRC, Uninformed Journal, dc214 

I)ruid, C²ISSP

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