In late February security researchers announced the discovery of a critical a security vulnerability in the GNU C library (glibc). This is the same library in which the Ghost vulnerability was found last year.
Why Should This Issue Be Considered Important?
This issue is a stack based buffer overflow (defined) vulnerability of critical severity that affects a large number of Linux systems e.g.:
- RedHat Enterprise Linux 6 (glibc version 2.12)
- RedHat Enterprise Linux 7 (glibc version 2.17)
- Debian squeeze (glibc version 2.11)
- Debian wheezy (glibc version 2.13)
- Debian Jessie (glibc version 2.19)
A complete list is available from this US-CERT vulnerability note.
As with the Ghost vulnerability the getaddrinfo() function (defined) is the source of the vulnerability. This newer vulnerability could allow an attacker to gain control over vulnerable systems as they connect to a DNS server under the control of an attacker.
Google researchers in their responsible disclosure (defined) of this vulnerability state that this flaw can exploited using sudo, curl and ssh (among others). Man-in-the-middle attacks (defined) and attacker controlled domain names are also mentioned as a means of exploiting this vulnerability within Google’s security advisory.
Since this is a stack based buffer overflow this overflows can be triggered using oversized UDP (defined) or TCP (defined) responses (larger than 2048 bytes) which are then immediately followed by a response which overflows the stack.
The above overflow could (for instance) be triggered by an attacker by having the target system perform a DNS (defined) lookup for a website domain under the control of the attacker. Further technical details exploiting this flaw are available from this message located on the glibc project mailing list.
Mitigating factors for this vulnerability include ASLR (defined), Moreover an option to use until you can apply the necessary security patch is limiting the response sizes accepted by the DNS resolver locally to 1024 bytes thus preventing the stack be based buffer overflows. Further mitigations are mentioned by Google (see the heading: Issue Summary) in their security advisory and by the glibc developers in their patch announcement for this vulnerability. A further defence in-depth (defined)(PDF) mitigation is provided by SANS Institute’s Johannes B. Ullrich in this forum thread.
How Can I Protect Myself from This Issue?
If your Linux device is found to be vulnerable continue to check for updates until one becomes available that resolves this issue. You can check for updates for your Linux device by using the Package Manager bundled with your Linux distribution (see this link (Debian) and this link (Ubuntu) that should assist you in using the package manager for your distribution of Linux).
Specific information for some of the affected versions of Linux are provided below:
- RedHat (blog post with anFAQ also provided)(while noting the disadvantage of RedHat’s DNS mitigations mentioned in Qualy’s blog post)
Once the update is installed you will need to restart/reboot the Linux device to have the update take effect.
Update: 20th March 2016:
The glibc vulnerability affected VMware’s ESXi version 5.5 and 6.0 products (among the other products listed in this post). In order to address these issues, please refer to VMware’s security advisory to download the necessary updates.
What is a library (when used in the context of computing)?
The general concept of a code library is defined here, only Windows systems use DLLs (defined) and so are not relevant for this discussion of Linux systems.