What is the Stack Clash?

The Stack Clash is a vulnerability in the memory management of several operating systems. It affects Linux, OpenBSD, NetBSD, FreeBSD and Solaris, on i386 and amd64. It can be exploited by attackers to corrupt memory and execute arbitrary code.

Qualys researchers discovered this vulnerability and developed seven exploits and seven proofs of concept for this weakness, then worked closely with vendors to develop patches. As a result we are releasing this advisory today as a coordinated effort, and patches for all distributions are available June 19, 2017. We strongly recommend that users place a high priority on patching these vulnerabilities immediately.

What is the Stack Clash vulnerability, precisely?

Each program running on a computer uses a special memory region called the stack. This memory region is special because it grows automatically when the program needs more stack memory. But if it grows too much and gets too close to another memory region, the program may confuse the stack with the other memory region. An attacker can exploit this confusion to overwrite the stack with the other memory region, or the other way around.

Why is it called the Stack Clash?

The first step in exploiting this vulnerability is to collide, or clash, the stack with another memory region. Hence the name: the Stack Clash.

Is it a new vulnerability?

The idea of clashing the stack with another memory region is not new: it was exploited a first time in 2005 and a second time in 2010. After the 2010 exploit, Linux introduced a protection against such exploits: the so-called stack guard-page. Today, we show that stack clashes are widespread and exploitable despite the stack guard-page protection.

Is the Stack Clash one or several vulnerabilities?

Our primary Stack Clash vulnerability is CVE-2017-1000364 and demonstrates that a stack guard-page of a few kilobytes is insufficient. But during our research we discovered more vulnerabilities: some are secondary and directly related to the primary Stack Clash vulnerability (for example, CVE-2017-1000365), and some are exploitable independently (for example, CVE-2017-1000367).

Am I affected by the Stack Clash?

If you are using Linux, OpenBSD, NetBSD, FreeBSD, or Solaris, on i386 or amd64, you are affected. Other operating systems and architectures may be vulnerable too, but we have not researched any of them yet: please refer to your vendor’s official statement about the Stack Clash for more information.

What are the risks posed by the Stack Clash?

The exploits and proofs of concept that we developed in the course of our research are all Local Privilege Escalations: an attacker who has any kind of access to an affected system can exploit the Stack Clash vulnerability and obtain full root privileges.

Is it exploitable remotely?

Our research has mainly focused on local exploitation: as of this writing on June 19, 2017, we do not know of any remotely exploitable application. However, remote exploitation of the Stack Clash is not excluded; although local exploitation will always be easier, and remote exploitation will be very application-specific. The one remote application that we did investigate (the Exim mail server) turned out to be unexploitable by sheer luck.

How can I protect my system from the Stack Clash?

The easiest and safest way to protect your system is to update it: we have been working with the affected vendors since the beginning of May, and by the time you read this, their patches and updates will be available.

What if I can’t (or don’t want to) update or reboot my system?

As a temporary workaround, you may set the hard RLIMIT_STACK and RLIMIT_AS of your local users and remote services to some reasonably low values. Use this workaround at your own risk, however: most likely your limits will not be low enough to resist all attacks (for example, in some cases our Sudo stack-clash exploit allocates merely 137MB of heap memory, and almost no stack memory); or your limits will be too low and will break legitimate applications.

Where can I find the Stack Clash exploits?

As of June 28, 2017, we have published the accompanying exploits.

Where can I get more information?

Please refer to the Stack Clash security advisory for the full technical details.

OR:

Refer to the vendor advisories, which we are listing here as they become available:

SUSE

https://www.novell.com/ support/kb/doc.php?id=7020973

Red Hat

https://access.redhat.com/ security/vulnerabilities/ stackguard

Debian

https://www.debian.org/ security/2017/dsa-3886

https://www.debian.org/ security/2017/dsa-3887

https://www.debian.org/ security/2017/dsa-3888

https://www.debian.org/ security/2017/dsa-3889

Ubuntu

https://www.ubuntu.com/usn/

OpenBSD

https://ftp.openbsd.org/pub/ OpenBSD/patches/6.1/common/ 008_exec_subr.patch.sig

Oracle Solaris

http://www.oracle.com/ technetwork/security-advisory/ alert-cve-2017-3629-3757403. html

I want to write my own Stack Clash exploit, where do I start?

You should try to implement the local-root exploit against Exim on i386 Debian: it is by far the easiest and most representative Stack Clash exploit.

Is the Sudo vulnerability Qualys published on May 30 related to Stack Clash?

https://www.qualys.com/2017/06/14/cve-2017-1000367/cve-2017-1000367.txt https://www.qualys.com/2017/06/14/cve-2017-1000367/linux_sudo_cve-2017-1000367.c

If CVE-2017-1000367 is combined with the Stack Clash, any local user (not just Sudoers) can exploit Sudo to obtain full root privileges on any vulnerable Linux system (not just SELinux systems). Because CVE-2017-1000367 was exploitable independently of the Stack Clash, we (and the affected vendors) decided to not wait for the June 19 Coordinated Release Date and published it on May 30.