Multiple Vendor X Font Server Multiple Vulnerabilities
8 Oct. 2007
Summary
The X Window System (or X11) is a graphical windowing system used on Unix-like systems. It is based on a client/server model. The X Window System font server (xfs) is used to render fonts for the X server.
Remote exploitation of a multiple vulnerabilities in X.Org Foundation's X Font Server, as included in various vendors' operating system distributions, could allow an attacker to execute arbitrary code.
Vulnerable Systems:
* XFS version X11R7.2-1.0.4 (Previous versions may also be affected)
An integer overflow vulnerability exists within the handlers for the QueryXBitmaps and QueryXExtents protocol requests. Both requests result in a call to the build_range() function. This function takes a 32bit integer from the request, and uses it in an arithmetic operation that calculates the size of a dynamic buffer. This calculation can overflow, which leads to an improperly sized memory allocation. This results in a heap overflow.
Additionally, a heap corruption vulnerability exists within the handlers for the QueryXBitmaps and QueryXExtents protocol requests. Both requests result in a call to the swap_char2b() function. This function takes a 32bit integer from the request, and uses it as the number of bytes to swap in the request buffer. This allows an attacker to swap an arbitrary number of bytes on the heap.
Exploitation of these vulnerabilities could result in the execution of arbitrary code with the privileges of the X Font Server, usually 'xfs'.
On current versions of Solaris, these vulnerabilities are remotely exploitable. The XFS service is turned on by default, and listens on TCP port 7100. On modern Linux systems, these vulnerabilities are only locally exploitable since the server is configured to listen on a UNIX socket only.
Workaround:
On Solaris, stop XFS from listening remotely by disabling it via the service manager.
Vendor Status:
The X.Org team has addressed these vulnerabilities with the release of XFS version 1.0.5. Additionally, a patch for version 1.0.4 has been made available. For more information, consult the X.Org advisory at the following URL: http://lists.freedesktop.org/archives/xorg-announce/2007-October/000416.html
