Exim Remote Code Execution Vulnerability (CVE-2020-28020) Threat Alert

Exim Remote Code Execution Vulnerability (CVE-2020-28020) Threat Alert

agosto 30, 2021 | Jie Ji


In May, Qualys publicly disclosed 21 security vulnerabilities in the Exim server, announcing that these vulnerabilities affected all Exim versions released after 2004 and most of them can be exploited in default configurations. Recently, NSFOCUS found that certain vulnerability details and PoCs were publicly available. Among the vulnerabilities, the most severe one is the integer overflow vulnerability (CVE-2020-28020) in Exim. This vulnerability resides in the receive_msg function and allows attackers to cause an integer overflow by bypassing the email header length restriction via \n. An unauthenticated attacker could exploit this vulnerability to cause a denial of service or remote code execution. Currently, vulnerability details and the related PoC are already made publicly available and users of this software are advised to take protective measures as soon as possible.

Exim is a mail transfer agent (MTA), which is responsible for receiving, routing, and delivering email messages. It is mainly built in the UNIX-like systems, like Solaris, AIX, and Linux, to send and receive emails. Thanks to its flexible configurations, Exim usually works with other application software to handle high-volume Internet traffic.

Reference link: https://www.exim.org/static/doc/security/CVE-2020-qualys/CVE-2020-28020-HSIZE.txt

Scope of Impact

Affected Versions

  • Exim < 4.94.2

Unaffected Versions

  • Exim = 4.94.2


Official Fix

Currently, this vulnerability has been fixed in Exim 4.94.2. Affected users are advised to upgrade to version 4.94.2 as soon as possible from the following link: https://ftp.exim.org/pub/exim/exim4/


This advisory is only used to describe a potential risk. NSFOCUS does not provide any commitment or promise on this advisory. NSFOCUS and the author will not bear any liability for any direct and/or indirect consequences and losses caused by transmitting and/or using this advisory. NSFOCUS reserves all the rights to modify and interpret this advisory. Please include this statement paragraph when reproducing or transferring this advisory. Do not modify this advisory, add/delete any information to/from it, or use this advisory for commercial purposes without permission from NSFOCUS.


NSFOCUS, Inc., a global network and cyber security leader, protects enterprises and carriers from advanced cyber attacks. The company’s Intelligent Hybrid Security strategy utilizes both cloud and on-premises security platforms, built on a foundation of real-time global threat intelligence, to provide multi-layered, unified and dynamic protection against advanced cyber attacks.

NSFOCUS works with Fortune Global 500 companies, including four of the world’s five largest financial institutions, organizations in insurance, retail, healthcare, critical infrastructure industries as well as government agencies. NSFOCUS has technology and channel partners in more than 60 countries, is a member of both the Microsoft Active Protections Program (MAPP), and the Cloud Security Alliance (CSA). A wholly owned subsidiary of NSFOCUS Technologies Group Co., Ltd., the company has operations in the Americas, Europe, the Middle East and Asia Pacific.