Linux Distros Unpatched Vulnerability : CVE-2021-26720

#%NASL_MIN_LEVEL 80900
##
# (C) Tenable, Inc.
##

include('compat.inc');

if (description)
{
  script_id(259350);
  script_version("1.2");
  script_set_attribute(attribute:"plugin_modification_date", value:"2026/07/11");

  script_cve_id("CVE-2021-26720");

  script_name(english:"Linux Distros Unpatched Vulnerability : CVE-2021-26720");

  script_set_attribute(attribute:"synopsis", value:
"The Linux/Unix host has one or more packages installed with a vulnerability that the vendor indicates will not be
patched.");
  script_set_attribute(attribute:"description", value:
"The Linux/Unix host has one or more packages installed that are impacted by a vulnerability without a vendor supplied
patch available.

  - avahi-daemon-check-dns.sh in the Debian avahi package through 0.8-4 is executed as root via
    /etc/network/if-up.d/avahi-daemon, and allows a local attacker to cause a denial of service or create
    arbitrary empty files via a symlink attack on files under /run/avahi-daemon. NOTE: this only affects the
    packaging for Debian GNU/Linux (used indirectly by SUSE), not the upstream Avahi product. (CVE-2021-26720)

Note that Nessus relies on the presence of the package as reported by the vendor.");
  script_set_attribute(attribute:"see_also", value:"https://ubuntu.com/security/CVE-2021-26720");
  script_set_attribute(attribute:"solution", value:
"There is no known solution at this time.");
  script_set_attribute(attribute:"agent", value:"unix");
  script_set_cvss_base_vector("CVSS2#AV:L/AC:L/Au:N/C:P/I:P/A:P");
  script_set_cvss_temporal_vector("CVSS2#E:U/RL:U/RC:C");
  script_set_cvss3_base_vector("CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H");
  script_set_cvss3_temporal_vector("CVSS:3.0/E:U/RL:U/RC:C");
  script_set_attribute(attribute:"cvss_score_source", value:"CVE-2021-26720");

  script_set_attribute(attribute:"exploitability_ease", value:"No known exploits are available");
  script_set_attribute(attribute:"exploit_available", value:"false");
  script_set_attribute(attribute:"vendor_unpatched", value:"true");

  script_set_attribute(attribute:"vuln_publication_date", value:"2021/02/17");
  script_set_attribute(attribute:"plugin_publication_date", value:"2025/08/30");

  script_set_attribute(attribute:"plugin_type", value:"local");
  script_set_attribute(attribute:"cpe", value:"cpe:/o:canonical:ubuntu_linux:14.04:-:lts");
  script_set_attribute(attribute:"cpe", value:"cpe:/o:canonical:ubuntu_linux:16.04:-:lts");
  script_set_attribute(attribute:"cpe", value:"cpe:/o:canonical:ubuntu_linux:18.04:-:lts");
  script_set_attribute(attribute:"cpe", value:"p-cpe:/a:canonical:ubuntu_linux:avahi");
  script_set_attribute(attribute:"generated_plugin", value:"current");
  script_end_attributes();

  script_category(ACT_GATHER_INFO);
  script_family(english:"Misc.");

  script_copyright(english:"This script is Copyright (C) 2025-2026 and is owned by Tenable, Inc. or an Affiliate thereof.");

  script_dependencies("ssh_get_info2.nasl", "set_linux_os_id.nasl");
  script_require_keys("Host/cpu", "Host/local_checks_enabled", "global_settings/vendor_unpatched", "Host/OS/identifier");
  script_require_ports("Host/OS/Ubuntu Linux-14.04", "Host/OS/Ubuntu Linux-16.04", "Host/OS/Ubuntu Linux-18.04");

  exit(0);
}

if (!get_kb_item("global_settings/vendor_unpatched")) exit(0, "Unpatched Vulnerabilities Detection not active.");
if (!get_kb_item("Host/local_checks_enabled")) audit(AUDIT_LOCAL_CHECKS_NOT_ENABLED);
if (empty_or_null(get_one_kb_item("Host/Debian/dpkg-l"))) audit(AUDIT_PACKAGE_LIST_MISSING);

include('linux_unpatched.inc');

var distro_constraints_array = {
  "Ubuntu Linux-14.04": {
    "package_manager": "dpkg-l",
    "constraints": [
      {
        "release": "14.04",
        "pkgs": [
          {"reference": "avahi-autoipd"},
          {"reference": "avahi-daemon"},
          {"reference": "avahi-dbg"},
          {"reference": "avahi-discover"},
          {"reference": "avahi-dnsconfd"},
          {"reference": "avahi-ui-utils"},
          {"reference": "avahi-utils"},
          {"reference": "libavahi-client-dev"},
          {"reference": "libavahi-client3"},
          {"reference": "libavahi-common-data"},
          {"reference": "libavahi-common-dev"},
          {"reference": "libavahi-common3"},
          {"reference": "libavahi-common3-udeb"},
          {"reference": "libavahi-compat-libdnssd-dev"},
          {"reference": "libavahi-compat-libdnssd1"},
          {"reference": "libavahi-core-dev"},
          {"reference": "libavahi-core7"},
          {"reference": "libavahi-core7-udeb"},
          {"reference": "libavahi-glib-dev"},
          {"reference": "libavahi-glib1"},
          {"reference": "libavahi-gobject-dev"},
          {"reference": "libavahi-gobject0"},
          {"reference": "libavahi-qt4-1"},
          {"reference": "libavahi-qt4-dev"},
          {"reference": "libavahi-ui-dev"},
          {"reference": "libavahi-ui-gtk3-0"},
          {"reference": "libavahi-ui-gtk3-dev"},
          {"reference": "libavahi-ui0"},
          {"reference": "python-avahi"}
        ]
      }
    ]
  },
  "Ubuntu Linux-16.04": {
    "package_manager": "dpkg-l",
    "constraints": [
      {
        "release": "16.04",
        "pkgs": [
          {"reference": "avahi-autoipd"},
          {"reference": "avahi-daemon"},
          {"reference": "avahi-dbg"},
          {"reference": "avahi-discover"},
          {"reference": "avahi-dnsconfd"},
          {"reference": "avahi-ui-utils"},
          {"reference": "avahi-utils"},
          {"reference": "libavahi-client-dev"},
          {"reference": "libavahi-client3"},
          {"reference": "libavahi-common-data"},
          {"reference": "libavahi-common-dev"},
          {"reference": "libavahi-common3"},
          {"reference": "libavahi-common3-udeb"},
          {"reference": "libavahi-compat-libdnssd-dev"},
          {"reference": "libavahi-compat-libdnssd1"},
          {"reference": "libavahi-core-dev"},
          {"reference": "libavahi-core7"},
          {"reference": "libavahi-core7-udeb"},
          {"reference": "libavahi-glib-dev"},
          {"reference": "libavahi-glib1"},
          {"reference": "libavahi-gobject-dev"},
          {"reference": "libavahi-gobject0"},
          {"reference": "libavahi-qt4-1"},
          {"reference": "libavahi-qt4-dev"},
          {"reference": "libavahi-ui-dev"},
          {"reference": "libavahi-ui-gtk3-0"},
          {"reference": "libavahi-ui-gtk3-dev"},
          {"reference": "libavahi-ui0"},
          {"reference": "python-avahi"}
        ]
      }
    ]
  },
  "Ubuntu Linux-18.04": {
    "package_manager": "dpkg-l",
    "constraints": [
      {
        "release": "18.04",
        "pkgs": [
          {"reference": "avahi-autoipd"},
          {"reference": "avahi-daemon"},
          {"reference": "avahi-discover"},
          {"reference": "avahi-dnsconfd"},
          {"reference": "avahi-ui-utils"},
          {"reference": "avahi-utils"},
          {"reference": "gir1.2-avahi-0.6"},
          {"reference": "libavahi-client-dev"},
          {"reference": "libavahi-client3"},
          {"reference": "libavahi-common-data"},
          {"reference": "libavahi-common-dev"},
          {"reference": "libavahi-common3"},
          {"reference": "libavahi-common3-udeb"},
          {"reference": "libavahi-compat-libdnssd-dev"},
          {"reference": "libavahi-compat-libdnssd1"},
          {"reference": "libavahi-core-dev"},
          {"reference": "libavahi-core7"},
          {"reference": "libavahi-core7-udeb"},
          {"reference": "libavahi-glib-dev"},
          {"reference": "libavahi-glib1"},
          {"reference": "libavahi-gobject-dev"},
          {"reference": "libavahi-gobject0"},
          {"reference": "libavahi-ui-gtk3-0"},
          {"reference": "libavahi-ui-gtk3-dev"},
          {"reference": "python-avahi"}
        ]
      }
    ]
  }
};

var distro_constraints_values = linux_unpatched::get_distro_constraints(distro_constraints_arr:distro_constraints_array);
if (empty_or_null(distro_constraints_values)) audit(AUDIT_HOST_NOT, 'affected');
var report = linux_unpatched::check_unpatched_constraints(distro_constraints_values:distro_constraints_values);

if (!empty_or_null(report))
{
  security_report_v4(
      port       : 0,
      severity   : SECURITY_WARNING,
      extra      : report
  );
  exit(0);
}
else
{
  audit(AUDIT_HOST_NOT, 'affected');
}