The following flaw was reported in the Xorg server:

Summary and Impact
------------------
xorg-server/xorg-server-1.19.0/os/mitauth.c:79 uses memcmp() to check
the received MIT cookie against a series of valid cookies. If the cookie
is correct, it is allowed to attach to the Xorg session. Since most
memcmp() implementations return after an invalid
byte is seen, this causes a time difference between a valid and invalid
byte, which in theory could allow an efficient brute force attack[1].

Analysis
--------
X41 was not able to measure a significant difference using the optimized
memcmp() version of a standard Linux system, but for a naive implementation
consisting of a loop comparing the bytes. Since timing attacks against
memcmp() have been successful in the past [2] and fixed elsewhere
[3][4] X41 would consider this an issue. If this would be exploited, it
would allow a local attacker to run code in the Xorg session of another
user.

In order to prevent this, MIT-COOKIES should be removed or a memcmp()
similar to timingsafe_memcmp()[5] used. Other projects (e.g. openssl)
use timing safe memcmp() implementations to compare cookies retrieved
via the network[6].

[1] https://cryptocoding.net/index.php/Coding_rules#Compare_secret_strings_in_constant_time
[2] http://de.slideshare.net/cisoplatform7/defcon-22paulmcmillanattackingtheiotusingtimingattac
[3] http://seb.dbzteam.org/crypto/python-oauth-timing-hmac.pdf
[4] https://bugs.ruby-lang.org/issues/10098
[5] http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/lib/libc/string/timingsafe_memcmp.c
[6] https://github.com/openssl/openssl/blob/master/ssl/t1_lib.c#L1249