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Published on January 27th, 2015 📆 | 6040 Views ⚑

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GHOST glibc Remote Code Execution Vulnerability Affects All Linux Systems


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A critical vulnerability has been found in glibc, the GNU C library, that affects all Linux systems dating back to 2000. Attackers can use this flaw to execute code and remotely gain control of Linux machines.

The issue stems from a heap-based buffer overflow found in the __nss_hostname_digits_dots() function in glibc. That particular function is used by the _gethostbyname function calls.

“A remote attacker able to make an application call either of these functions could use this flaw to execute arbitrary code with the permissions of the user running the application,” said an advisory from Linux distributor Red Hat.

The vulnerability, CVE-2015-0235, has already been nicknamed GHOST because of its relation to the _gethostbyname function. Researchers at Qualys discovered the flaw, and say it goes back to glibc version 2.2 in Linux systems published in November 2000.

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According to Qualys, there is a mitigation for this issue that was published May 21, 2013 between patch glibc-2.17 versions and glibc-2.18.

“Unfortunately, it was not recognized as a security threat; as a result, most stable and long-term-support distributions were left exposed (and still are): Debian 7 (wheezy), Red Hat Enterprise Linux 6 & 7, CentOS 6 & 7, Ubuntu 12.04, for example,” said an advisory from Qualys posted to the OSS-Security mailing list.

Respective Linux distributions will be releasing patches; Red Hat has released an update for Red Hat Enterprise Linux v.5 server. Novell has a list of SUSE Linux Enterprise Server builds affected by the vulnerability. Debian has already released an update of its software addressing the vulnerability.

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“It’s everywhere, which is kind of the urgency we have here. This has been in glibc for a long time. It was fixed recently, but it was not marked as a security issue, so things that are fairly new should be OK,” said Josh Bressers, a member of the Red Hat security response team. “From a threat level, what it comes down to is a handful of stuff that’s probably dangerous that uses this function.”

Unlike past Internet-wide bugs such as Bash, patching glibc may not be the chore it was with Bash since so many components made silent Bash calls.

“In this instance, you just apply the glibc update, and restart any services that are vulnerable,” Bressers said. “It’s not confusing like Shellshock was.”

Qualys, in its advisory, not only shares extremely in-depth technical information on the vulnerability, but also includes a section explaining exploitation of the Exim SMTP mail server. The advisory demonstrates how to bypass NX, or No-eXecute protection as well as glibc malloc hardening, Qualys said.

Qualys also said that in addition to the 2013 patch, other factors mitigate the impact of the vulnerability, including the fact that the gethostbyname functions are obsolete because of IPv6 and newer applications using a different call. While the flaw is also exploitable locally, this scenario too is mitigated because many programs rely on gethostbyname only if another preliminary call fails and a secondary call succeeds in order to reach the overflow. The advisory said this is “impossible” and those programs are safe.

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There are mitigations against remote exploitation too, Qualys said. Servers, for example, use gethostbyname to perform full-circle reverse DNS checks. “These programs are generally safe because the hostname passed to gethostbyname() has normally been pre-validated by DNS software,” the advisory.

“It’s not looking like a huge remote problem, right now,” Bressers said.

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