====================================================================


                             CERT-Renater

                 Note d'Information No. 2018/VULN277
_____________________________________________________________________

DATE                : 07/09/2018

HARDWARE PLATFORM(S): /

OPERATING SYSTEM(S): Systems running libX11 versions prior to 1.1.6,
                          libXcursor versions prior to 1.1.15.

=====================================================================
https://lists.x.org/archives/xorg-devel/2018-August/057443.html
https://lists.x.org/archives/xorg-devel/2018-August/057451.html
_____________________________________________________________________

X.Org security advisory: August 21, 2018

Multiple issues in libX11
=========================

The functions XGetFontPath, XListExtensions and XListFonts from libX11
are vulnerable to three different issues:

Off-by-one writes (CVE-2018-14599).
-----------------------------------

The functions XGetFontPath, XListExtensions, and XListFonts are
vulnerable to an off-by-one override on malicious server responses.

The server replies consist of chunks consisting of a length byte
followed by actual string, which is not NUL-terminated.

While parsing the response, the length byte is overridden with '\0',
thus the memory area can be used as storage of C strings later on. To
be able to NUL-terminate the last string, the buffer is reserved with
an additional byte of space.

For a boundary check, the variable chend (end of ch) was introduced,
pointing at the end of the buffer which ch initially points to.
Unfortunately there is a difference in handling "the end of ch".

While chend points at the first byte that must not be written to,
the for-loop uses chend as the last byte that can be written to.

Therefore, an off-by-one can occur.


Out of boundary write (CVE-2018-14600).
---------------------------------------

The length value is interpreted as signed char on many systems
(depending on default signedness of char), which can lead to an out of
boundary write up to 128 bytes in front of the allocated storage, but
limited to NUL byte(s).

Casting the length value to unsigned char fixes the problem and allows
string values with up to 255 characters.


Crash on invalid reply (CVE-2018-14598).
----------------------------------------

If the server sends a reply in which even the first string would
overflow the transmitted bytes, list[0] (or flist[0]) will be set to
NULL and a count of 0 is returned.

If the resulting list is freed with XFreeExtensionList or
XFreeFontPath later on, the first Xfree call:

    Xfree (list[0]-1)
 turns into
    Xfree (NULL-1)

which will most likely trigger a segmentation fault.

Patches
=======

Patches for these issues have been commited to the libX11 git repository.
libX11 1.6.6 will be released shortly and will include those patches.

https://gitlab.freedesktop.org/xorg/lib/libx11

b469da1430cdcee06e31c6251b83aede072a1ff0  CVE-2018-14599
dbf72805fd9d7b1846fe9a11b46f3994bfc27fea  CVE-2018-14600
e83722768fd5c467ef61fa159e8c6278770b45c2  CVE-2018-14598

Thanks
======

X.Org thanks Tobias Stoeckmann for reporting these issues to our
security team and assisting them in understanding them and evaluating
our fixes.

_____________________________________________________________________

X.Org security advisory: August 22, 2018

Out-of-bounds write in libXcursor prior to 1.1.15
=================================================

libXcursor could write one byte out of bounds when processing Xcursor
theme files.  In certain cases, such as when used in the Firefox web
browser, this could be used as part of an exploit chain to allow
further attacks on an X client process, as reported via Mozilla's
ASan Nightly project.   This issue has been assigned CVE-2015-9262.

Patches
=======

A patch for this issue was committed to the libXcursor git repository
in 2015, and included in the libXcursor 1.1.15 release.

https://gitlab.freedesktop.org/xorg/lib/libxcursor/commit/897213f36baf6926daf6d192c709cf627aa5fd05

Thanks
======

X.Org thanks Shubham Shrivastav of Samsung for reporting this issue to
X.Org originally, and Alex Gaynor of Mozilla for helping us understand
how this could be exploited by an attacker.

--
	-Alan Coopersmith-              alan.coopersmith at oracle.com
	  X.Org Security Response Team - xorg-security at lists.x.org

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