Bug 1424984 (CVE-2017-2624) - CVE-2017-2624 xorg-x11-server: timing attack against MIT Cookie
Summary: CVE-2017-2624 xorg-x11-server: timing attack against MIT Cookie
Alias: CVE-2017-2624
Product: Security Response
Classification: Other
Component: vulnerability
Version: unspecified
Hardware: All
OS: Linux
Target Milestone: ---
Assignee: Red Hat Product Security
QA Contact:
Depends On: 1427559
Blocks: 1424993
TreeView+ depends on / blocked
Reported: 2017-02-20 09:19 UTC by Martin Prpič
Modified: 2019-09-29 14:07 UTC (History)
4 users (show)

Fixed In Version:
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Clone Of:
Last Closed: 2017-03-01 00:37:17 UTC

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Description Martin Prpič 2017-02-20 09:19:31 UTC
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].

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

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

Comment 1 Martin Prpič 2017-02-20 09:19:34 UTC

Name: Eric Sesterhenn (X41 D-Sec GmbH)

Comment 2 Doran Moppert 2017-02-27 06:29:58 UTC

Red Hat Product Security has rated this issue as having Low security
impact. This issue is not currently planned to be addressed in future
updates. For additional information, refer to the Issue Severity
Classification: https://access.redhat.com/security/updates/classification/.

Comment 5 Andrej Nemec 2017-02-28 15:32:39 UTC
Created xorg-x11-server tracking bugs for this issue:

Affects: fedora-all [bug 1427559]

Comment 6 Andrej Nemec 2017-02-28 15:32:54 UTC
External References:


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