A proftpd-dfsg regression update has been released for Debian GNU/Linux 9 LTS to address an issue introduced by the previous update.

DLA 2338-2: proftpd-dfsg regression update

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Debian LTS Advisory DLA-2338-2 debian-lts@lists.debian.org
  https://www.debian.org/lts/security/ Markus Koschany
August 25, 2020   https://wiki.debian.org/LTS
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Package : proftpd-dfsg
Version : 1.3.5e+r1.3.5b-4+deb9u2
Debian Bug : 968967

The update of proftpd-dfsg released as DLA-2338-1 incorrectly
destroyed the memory pool in function sftp_kex_handle in
contrib/mod_sftp/kex.c which may cause a segmentation fault and thus
prevent sftp connections.

For Debian 9 stretch, this problem has been fixed in version
1.3.5e+r1.3.5b-4+deb9u2.

We recommend that you upgrade your proftpd-dfsg packages.

For the detailed security status of proftpd-dfsg please refer to
its security tracker page at:
  https://security-tracker.debian.org/tracker/proftpd-dfsg

Further information about Debian LTS security advisories, how to apply
these updates to your system and frequently asked questions can be
found at:   https://wiki.debian.org/LTS