Feeds

Anatomy of a 22-year-old X Window bug: Get root with newly uncovered flaw

Grab a patch today if you share your Unix-flavoured desktop with other people

Beginner's guide to SSL certificates

The X Window System, which today underpins Linux desktops the world over, has been around for more than two decades – and so have its bugs.

Sysadmins have a few days to patch libXfont to remove a newly discovered, 22-year-old privilege-escalation bug in the code before any tiresome users whip out an exploit. The flaw allows someone logged into a vulnerable machine to crash the X server, or possibly execute injected code as a superuser.

Hard on the heels of a Chaos Communication Congress presentation that found “hundreds” of bugs (discussed at the X.org mailing list here), the newly found bug is a textbook stack buffer overflow that dates back to 1991 – and is present in all versions of X11.

The bug is very straightforward, and will impact shared computers, but it is ideal to dissect to reveal how this sort of security blunder happens.

As the X.org advisory states: “A BDF font file containing a longer than expected string could overflow the buffer on the stack. Testing in X servers built with Stack Protector resulted in an immediate crash when reading a user-provided specially crafted font.”

The guilty party is this block of code in bdfReadCharacters() in libXfont/tree/src/bitmap/bdfread.c:

char charName[100];

if (sscanf((char *) line, "STARTCHAR %s", charName) != 1) {
   bdfError("bad character name in BDF file\n");
   goto BAILOUT; /* bottom of function, free and return error */
}

If you can't already see the bug then we'll explain. On-screen fonts can be stored in Glyph Bitmap Distribution Format (BDF) [PDF] files, which start with the following line to declare the format version the font is adhering to:

STARTCHAR 2.1

That's all well and good if the loaded font has a short version number, expressed as a string, which in this case is "2.1". That information is copied into the string variable charName by the sscanf() call in bdfread.c. The problem is, sscanf() is not told to limit the number of bytes read for the version number and will keep copying data from the file until it hits a white-space character.

The charName variable is declared as having a length of 100 bytes, so feeding it a crafted BDF font with a "STARTCHAR" version number longer than that will punch through the boundary of the variable's allotted space in memory and into other data on the stack. This means an attacker could overwrite the memory that controls the processor's instruction pointer on leaving the bdfReadCharacters() function, effectively hijacking the program.

And since the X server is usually run with superuser privileges, the normal user can start running code to take control of the machine if the attack is successful. Much more in-depth explanations on how stack buffer overflows can be exploited, despite some of the protections in place on modern systems, can be found here and here.

The fix for the bug is simple; you simply tell sscanf() to read at most 99 bytes, leaving one for the terminating NULL:

if (sscanf((char *) line, "STARTCHAR %99s", charName) != 1) {

As the X.org announcement states:

As libXfont is used to read user-specified font files in all X servers distributed by X.Org, including the Xorg server which is often run with root privileges or as setuid-root in order to access hardware, this bug may lead to an unprivileged user acquiring root privileges in some systems.

In the December Chaos Communication Congress presentation, Ilja van Sprundel said he'd able to find 120 bugs in a couple of months, “and I'm not close to done”. Van Sprundel had already triggered a major X.org security update in May 2013, with tens of fixes needed because client libraries trusted servers to send valid data, and not sanity-tested what was sent.

The latest bug, discovered using the cppcheck static analyzer, is designated CVE-2013-6462; security updates should be available from all good package managers and repositories. ®

Choosing a cloud hosting partner with confidence

More from The Register

next story
SMASH the Bash bug! Apple and Red Hat scramble for patch batches
'Applying multiple security updates is extremely difficult'
Apple's new iPhone 6 vulnerable to last year's TouchID fingerprint hack
But unsophisticated thieves need not attempt this trick
Hackers thrash Bash Shellshock bug: World races to cover hole
Update your gear now to avoid early attacks hitting the web
Oracle SHELLSHOCKER - data titan lists unpatchables
Database kingpin lists 32 products that can't be patched (yet) as GNU fixes second vuln
Who.is does the Harlem Shake
Blame it on LOLing XSS terroristas
Researchers tell black hats: 'YOU'RE SOOO PREDICTABLE'
Want to register that domain? We're way ahead of you.
Stunned by Shellshock Bash bug? Patch all you can – or be punished
UK data watchdog rolls up its sleeves, polishes truncheon
Ello? ello? ello?: Facebook challenger in DDoS KNOCKOUT
Gets back up again after half an hour though
prev story

Whitepapers

Providing a secure and efficient Helpdesk
A single remote control platform for user support is be key to providing an efficient helpdesk. Retain full control over the way in which screen and keystroke data is transmitted.
Intelligent flash storage arrays
Tegile Intelligent Storage Arrays with IntelliFlash helps IT boost storage utilization and effciency while delivering unmatched storage savings and performance.
Beginner's guide to SSL certificates
De-mystify the technology involved and give you the information you need to make the best decision when considering your online security options.
Security for virtualized datacentres
Legacy security solutions are inefficient due to the architectural differences between physical and virtual environments.
Secure remote control for conventional and virtual desktops
Balancing user privacy and privileged access, in accordance with compliance frameworks and legislation. Evaluating any potential remote control choice.