21Nails Vulnerabilities Impact 60% of the Internet's Email Servers

The maintainers of the Exim email server software have released updates today to patch a collection of 21 vulnerabilities that can allow threat actors to take over servers using both local and remote attack vectors. The Record reports: Known as 21Nails, the vulnerabilities were discovered by security firm Qualys. The bugs impact Exim, a type of email server known as a mail transfer agent (MTA) that helps email traffic travel across the internet and reach its intended destinations. While there are different MTA clients available, an April 2021 survey shows that Exim has a market share of nearly 60% among all MTA solutions, being widely adopted around the internet. The 21Nails vulnerabilities, if left unpatched, could allow threat actors to take over these systems and then intercept or tamper with email communications passing through the Exim server.

As Qualys explains in its security advisory, the 21Nails vulnerabilities are as bad as it gets. All Exim server versions released in the past 17 years, since 2004, the beginning of the project's Git history, are affected by the 21Nails bugs. This includes 11 vulnerabilities that require local access to the server to exploit, but also 10 bugs that can be exploited remotely across the internet. Security experts recommend that Exim server owners update to Exim version 4.94 to protect their systems against attacks.

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