CVE-2019-16928: Critical Buffer Overflow Flaw in Exim is Remotely Exploitable

Scott Caveza

CVE-2019-16928, a critical heap-based buffer overflow vulnerability in Exim email servers, could allow remote attackers to crash Exim or potentially execute arbitrary code.

Background

Exim Internet Mailer, the popular message transfer agent (MTA) for Unix hosts found on nearly 5 million systems, is back in the news. Earlier this month, CVE-2019-15846, a critical remote code execution (RCE) flaw, was patched in Exim 4.92.2. In June, we blogged about CVE-2019-10149, another RCE, which saw exploit attempts within a week of public disclosure.

On September 28, Exim maintainers published an advance notice concerning a new vulnerability in Exim 4.92 up to and including 4.92.2. From our analysis of Shodan results, over 3.5 million systems may be affected.

Analysis

CVE-2019-16928 is a heap-based buffer overflow vulnerability due to a flaw in string_vformat() found in string.c. As noted in the bug report, the flaw was a simple coding error where the length of the string was not properly accounted for, leading to a buffer overflow condition. The flaw can be exploited by an unauthenticated remote attacker who could use a large crafted Extended HELO (EHLO) string to crash the Exim process that receives the message. This could potentially be further exploited to execute arbitrary code on the host. The flaw was found internally by the QAX A-Team, who submitted the patch. However, the bug is trivial to exploit, and it’s likely attackers will begin actively probing for and attacking vulnerable Exim MTA systems in the near future.

Proof of concept

As part of the patch, a proof of concept (PoC) is available which can be used to exploit the flaw and cause a denial of service (DoS) condition in an affected Exim server.

Solution

The Exim team released version 4.92.3 on September 29 to address CVE-2019-16928. Administrators are encouraged to upgrade as soon as possible. No mitigations exist at this time.

Identifying affected systems

A list of Tenable plugins to identify this vulnerability will appear here as they’re released.

Get more information

Join Tenable's Security Response Team on the Tenable Community.

Learn more about Tenable, the first Cyber Exposure platform for holistic management of your modern attack surface.

Get a free 60-day trial of Tenable.io Vulnerability Management.

Read more >

Published on Sep 30, 2019

People also viewed

Business Cost Analyst - Cloud Infrastructure

San Jose California United States E Santa Clara St., San Jose, California, United States, 95113 Engineering Engineering
Your Role:Tenable is seeking a Business Cost Analyst of Cloud Infrastructure with strong MS Excel experience to assess, analyze and compile current costs and potentially determine initiatives to reduce and improve our cloud infrastructure cost for...

Professional Services Engagement Manager

Singapore Singapore Singapore North Bridge Road, Parkview Square, Singapore, 188788 Professional Services Professional Services
Your Role:The Professional Services Engagement Manager assists in the professional services business development and oversees the delivery of projects. The Professional Services Engagement Manager’s roles include tactical project management oversi...

UX Designer

Columbia Maryland United States Columbia Gateway Drive, Columbia, Maryland, United States, 21046 Engineering Engineering
Your Role:Tenable is looking for an extraordinary Senior UX Designer to join our team. Our group is chartered with creating the next generation of security products while at the same time pioneering unprecedented user experience in the digital sec...

Commercial Territory Manager - West

Columbia Maryland United States Columbia Gateway Drive, Columbia, Maryland, United States, 21046 Sales Sales
Your Role:The Commercial Territory Manager will meet and exceed quarterly sales quota by developing new opportunities within specific geographical territory.  Researching and identifying potential accounts; outbound cold calling to soliciting new ...

Senior Software Engineer (Java & Kotlin)

San Jose California United States E Santa Clara St., San Jose, California, United States, 95113 Engineering Engineering
Your Role:Tenable is looking for a Senior Software Engineer to join the Lumin product development team. As a Sr. Software Engineer you will drive projects end-to-end, collaborate on product requirements with Product Managers, architect and impleme...

Public Sector Channel Manager - Distribution

Reston Virginia United States Reston, Virginia, United States Channel Sales Sales
Your Role:The Public Sector Channel Manager-Distribution is responsible for establishing and managing relationships with the Distribution Partner(s).  The CM-Disty will act as a sales liaison between distributor and Tenable Channel Manager personn...

We have big plans for continued global growth, and we’re looking for people who are creative, flexible and dedicated to helping us build something great – something that matters.