An openssl1.0 security update has been released for Debian GNU/Linux 9. Guido Vranken discovered an overflow bug in the x64_64 Montgomery squaring procedure used in exponentiation with 512-bit moduli.

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Debian Security Advisory DSA-4594-1 security@debian.org
  https://www.debian.org/security/ Moritz Muehlenhoff
December 27, 2019   https://www.debian.org/security/faq
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Package : openssl1.0
CVE ID : CVE-2019-1551

Guido Vranken discovered an overflow bug in the x64_64 Montgomery
squaring procedure used in exponentiation with 512-bit moduli.

For the oldstable distribution (stretch), this problem has been fixed
in version 1.0.2u-1~deb9u1.

We recommend that you upgrade your openssl1.0 packages.

For the detailed security status of openssl1.0 please refer to
its security tracker page at:
  https://security-tracker.debian.org/tracker/openssl1.0

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