Red Hat is warning enterprise Linux users to update their installations of XFree86 to fix a number of serious security bugs, some of which could allow attackers to take over a system. Affected operating systems include Enterprise Linux AS 3, Enterprise Linux ES 3 and Enterprise Linux WS 3, Red Hat said in an advisory.

Separately, vendors have patched critical flaws in ImageMagick, Sylpheed and several components of SGI's Advanced Linux Environment.

XFree86 is an implementation of the X Window System providing low-level graphics functionality for graphical user interface systems such as KDE and Gnome. The most serious flaw is an integer overflow in the libXpm library, used by some applications in opening XPM images, Red Hat said. An attacker could use a malicious XPM file to execute code on a user's system.

Red Hat said the bug is only moderately critical, but independent security firm Secunia said in an advisory that the vulnerability is serious because it could allow a remote attacker to gain system access. The latest XFree86 release, issued on 16 March, fixes the libXpm vulnerability and several others.

Red Hat and others are also patching a newly disclosed vulnerability in the Sylpheed email client, which could allow the execution of malicious code when a message is displayed. Attackers could use a message containing an attachment with a MIME-encoded filename to trigger a boundary error, resulting in a buffer overflow, according to researchers. Versions 0.8.0 to 1.0.3 and development versions 1.9.0 to 1.9.4 are affected, said Secunia, which gave the bug a "highly critical" rating. The fix is available from Sylpheed's website and from Linux vendors.

MandrakeSoft and Red Hat have patched flaws in ImageMagick, a widely used open-source image editing suite, that could allow the remote execution of malicious code. The bugs include a format string error within the handling of filenames and a boundary error in the decoding of PSD images, and could be exploited by specially crafted image files, according to security experts.

SGI issued an update for its Advanced Linux Environment fixing a number of bugs, which could allow remote system takeover or denial of service, or could let malicious local users manipulate the contents of some files. The bugs affect the xpdf, squid and kdenetwork components, SGI said.