A libonig regression update has been released for Debian GNU/Linux 9 LTS to undo CVE-2020-26159 after it was a false-positive.

DLA 2431-2: libonig regression update

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Debian LTS Advisory DLA-2431-2 debian-lts@lists.debian.org
  https://www.debian.org/lts/security/ Markus Koschany
January 30, 2021   https://wiki.debian.org/LTS
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Package : libonig
Version : 6.1.3-2+deb9u2
CVE ID : CVE-2020-26159

It was discovered that CVE-2020-26159 in the Oniguruma regular
expressions library, notably used in PHP mbstring, was a false-positive. In
consequence the patch for CVE-2020-26159 was reverted. For reference, the
original advisory text follows.

CVE-2020-26159

In Oniguruma an attacker able to supply a regular expression for
compilation may be able to overflow a buffer by one byte in
concat_opt_exact_str in src/regcomp.c

For Debian 9 stretch, this problem has been fixed in version
6.1.3-2+deb9u2.

We recommend that you upgrade your libonig packages.

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

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