From the upstream announcement [1]:

A vulnerability has been found in the X11R6 font server code in the handling of the SetEventMask request in xfs - if an invalid mask is specified, it passes that bad mask to the SendErrToClient() function for returning to the client in the error message. Unfortunately, SendErrToClient() expects a pointer, but was being passed the raw mask value, causing it to use the mask as a pointer, either segfaulting (if that happened to be an invalid pointer), or returning whatever data happened to be accessible in the 4 bytes at the address specified by the mask value.

X11R6 through X11R6.6 appear to be vulnerable. The fix shown below was introduced by XFree86 in their 3.3.3 release as part of the commit for: Prototype/ansification cleanup for Xserver/XIE, xfs, fontlib, mkfontdir, and fix some bugs found along the way (#2103, Thomas Dickey).

X11R6.7 incorporated the XFree86 changes, and thus X11R6.7 and later releases from X.Org are not vulnerable.

[1] http://lists.freedesktop.org/archives/xorg-announce/2012-July/002040.html

Statement:

Not vulnerable. This issue did not affect the versions of xorg-x11-xfs as shipped with Red Hat Enterprise Linux 5. It does not affect Red Hat Enterprise Linux 6 as it no longer uses or provides the XFS font server.

X.Org and XFree86, including libX11, xdm, xf86dga, xinit, xload, xtrans, and xterm, does not check the return values for setuid and seteuid calls when attempting to drop privileges, which might allow local users to gain privileges by causing those calls to fail, such as by exceeding a ulimit.

Buffer overflow in the X render (Xrender) extension in X.org X server 6.8.0 up to allows attackers to cause a denial of service (crash), as demonstrated by the (1) XRenderCompositeTriStrip and (2) XRenderCompositeTriFan requests in the rendertest from XCB xcb/xcb-demo, which leads to an incorrect memory allocation due to a typo in an expression that uses a "&" instead of a "" operator. NOTE: the subject line of the original announcement used an incorrect CVE number for this issue.

X.Org server (xorg-server) 1.0.0 and later, X11R6.9.0, and X11R7.0 inadvertently treats the address of the geteuid function as if it is the return value of a call to geteuid, which allows local users to bypass intended restrictions and (1) execute arbitrary code via the -modulepath command line option or (2) overwrite arbitrary files via -logfile.

scan.c for LibXPM may allow attackers to execute arbitrary code via a negative bitmapunit value that leads to a buffer overflow.

Multiple vulnerabilities in libXpm for 6.8.1 and earlier, as used in XFree86 and other packages, include (1) multiple integer overflows, (2) out-of-bounds memory accesses, (3) directory traversal, (4) shell metacharacter, (5) endless loops, and (6) memory leaks, which could allow remote attackers to obtain sensitive information, cause a denial of service (application crash), or execute arbitrary code via a certain XPM image file. NOTE: it is highly likely that this candidate will be SPLIT into other candidates in the future, per CVE's content decisions.

Multiple integer overflows in (1) the xpmParseColors function in parse.c, (2) XpmCreateImageFromXpmImage, (3) CreateXImage, (4) ParsePixels, and (5) ParseAndPutPixels for libXpm before 6.8.1 may allow remote attackers to execute arbitrary code via a malformed XPM image file.

Multiple stack-based buffer overflows in (1) xpmParseColors in parse.c, (2) ParseAndPutPixels in create.c, and (3) ParsePixels in parse.c for libXpm before 6.8.1 allow remote attackers to execute arbitrary code via a malformed XPM image file.

XDM in XFree86 opens a chooserFd TCP socket even when DisplayManager.requestPort is 0, which could allow remote attackers to connect to the port, in violation of the intended restrictions.