opensmtpd-extras: An out-of-bounds read could lead to remote code execution (CVE-2020-8794)

OpenSMTPD before 6.6.4 allows remote code execution because of an out-of-bounds read in mta_io in mta_session.c for multi-line replies. Although this vulnerability affects the client side of OpenSMTPD, it is possible to attack a server because the server code launches the client code during bounce handling.

References:

https://www.openwall.com/lists/oss-security/2020/02/24/5
https://nvd.nist.gov/vuln/detail/CVE-2020-8794

Affected branches:

master
3.12-stable

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