OpenSSH 4.3p2, and probably other versions, allows local users to hijack forwarded X connections by causing ssh to set DISPLAY to :10, even when another process is listening on the associated port, as demonstrated by opening TCP port 6010 (IPv4) and sniffing a cookie sent by Emacs.
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A6085
https://issues.rpath.com/browse/RPL-2397
https://exchange.xforce.ibmcloud.com/vulnerabilities/41438
http://www.vupen.com/english/advisories/2008/2584
http://www.vupen.com/english/advisories/2008/2396
http://www.vupen.com/english/advisories/2008/1630/references
http://www.vupen.com/english/advisories/2008/1624/references
http://www.vupen.com/english/advisories/2008/1526/references
http://www.vupen.com/english/advisories/2008/1448/references
http://www.vupen.com/english/advisories/2008/1124/references
http://www.vupen.com/english/advisories/2008/1123/references
http://www.vupen.com/english/advisories/2008/0994/references
http://www.us-cert.gov/cas/techalerts/TA08-260A.html
http://www.slackware.org/security/viewer.php?l=slackware-security&y=2008&m=slackware-security.540188
http://www.securitytracker.com/id?1019707
http://www.securityfocus.com/bid/28444
http://www.securityfocus.com/archive/1/490054/100/0/threaded
http://www.mandriva.com/security/advisories?name=MDVSA-2008:078
http://www.globus.org/mail_archive/security-announce/2008/04/msg00000.html
http://www.gentoo.org/security/en/glsa/glsa-200804-03.xml
http://www.debian.org/security/2008/dsa-1576
http://wiki.rpath.com/wiki/Advisories:rPSA-2008-0120
http://tools.cisco.com/security/center/content/CiscoSecurityNotice/CVE-2008-1483
http://support.avaya.com/elmodocs2/security/ASA-2008-205.htm
http://support.attachmate.com/techdocs/2374.html
http://sunsolve.sun.com/search/document.do?assetkey=1-77-1019235.1-1
http://sunsolve.sun.com/search/document.do?assetkey=1-26-237444-1
http://sourceforge.net/project/shownotes.php?release_id=590180&group_id=69227
http://security.FreeBSD.org/advisories/FreeBSD-SA-08:05.openssh.asc
http://secunia.com/advisories/31882
http://secunia.com/advisories/31531
http://secunia.com/advisories/30361
http://secunia.com/advisories/30347
http://secunia.com/advisories/30249
http://secunia.com/advisories/30230
http://secunia.com/advisories/30086
http://secunia.com/advisories/29939
http://secunia.com/advisories/29873
http://secunia.com/advisories/29735
http://secunia.com/advisories/29721
http://secunia.com/advisories/29686
http://secunia.com/advisories/29683
http://secunia.com/advisories/29676
http://secunia.com/advisories/29626
http://secunia.com/advisories/29554
http://secunia.com/advisories/29537
http://secunia.com/advisories/29522
http://lists.opensuse.org/opensuse-security-announce/2008-04/msg00007.html
http://lists.apple.com/archives/security-announce//2008/Sep/msg00005.html
http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?objectID=c01462841
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=463011
http://aix.software.ibm.com/aix/efixes/security/ssh_advisory.asc