On July 7, the folks at rPath sent out a security update
for a pair of
kernel vulnerabilities. The update reads, in part:
Previous versions of the kernel package are vulnerable to two
denial of service attacks. The first allows any local user to fill
up file systems by causing core dumps to write to directories to
which they do not have write access permissions.
The bug in question is designated CVE-2006-2451; it was fixed in the 184.108.40.206 kernel release. All
kernels since 2.6.13 are vulnerable, but one cannot just rely on the
nominal version number: Red Hat helpfully backported this bug into
the 2.6.9 kernel shipped with RHEL4.
Reading the description above, some system administrators may feel that
there is no particular urgency in applying this update. The risk that a
rogue user would fill up a disk with core dump files may seem small, so an
update fixing the problem - and which requires a system reboot to be
effective - can maybe be deferred for a while. After all, the Linux kernel
core dump code takes pains to avoid overwriting files with core dumps, so
the real potential for harm is small. It's a denial of service bug.
Except that it's not. All that is required is to create a program
containing a string in the format understood by cron, send it over
to /etc/cron.d, and use the bug to create a core dump there.
Eventually cron will wander along, helpfully pick the line it
understands out of the surrounding binary junk, and execute (as root) the
commands found there. It is a simple and straightforward local root
exploit; an example implementation has been posted to the full-disclosure
Paul Starzetz has posted a complaint about
the characterization of a fully-exploitable vulnerability as a denial of
service problem; he has seen this done with other vulnerabilities as well.
He is right. "Denial of service" makes the vulnerability seem less severe,
especially if it is only exploitable locally. Those words may cause
vulnerabilities to remain open longer by inspiring inaction on both the
administrator and distributor sides. If a bug can be exploited for
privilege escalation, it should not be described as a denial of service
To its credit, Red Hat (which is where the bug was discovered) notes that
the bug could be exploited to gain root privileges. Ubuntu, which closed the vulnerability four days
later, says "This could be exploited to drain available disk space on
system partitions, or, under some circumstances, to execute arbitrary code
with full root privileges." This advisory could use an edit as
well: "under some circumstances" makes the exploit seem unlikely or
difficult. A more accurate wording would be "if the attacker wants."
Lest it seem that rPath and Ubuntu are receiving too much grief: as of this
writing, five days after disclosure, rPath, Ubuntu, and Red Hat are the
only distributors to have fixed this problem. They have done the
most important part: making an update available. All other
distributors who have shipped kernels based on 2.6.13 or later remain
vulnerable to a trivial local root exploit. Might this slow response be
caused, in part, by the perception that this is a mere local denial of
As a community, we feel that we have the best security support out there.
Vulnerabilities are not hidden, and fixes come promptly. In cases like
this one, however, we have let our users down. Presenting an easily
exploitable root vulnerability as a denial of service problem is just the
sort of obfuscation that we normally try to avoid. And the fact that a
number of distributions remain vulnerable is a failure to live up to our
own promises. We can - and must - do better than that.
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