Exim, the mail transfer agent used by more than half the email servers on the Internet, has a vulnerability. The flaw, found in versions from 4.80 through 4.92.1, allows a malicious actor to use an encrypted TLS connection to remotely execute code with root privileges.
The vulnerability, designated CVE-2019-15846, was discovered by researcher Zerons in late July. It takes advantage of the TLS ServerName Indicator (SNI), a feature that allows TLS to serve different certificates for various websites on a single server. A buffer overflow triggered by a relatively simple SNI request followed by a counterfeit client certificate are enough to exploit the vulnerability.
Responsible disclosure procedures were followed and a patch for the vulnerability has been made available in Exim 4.92.2.
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