SLES 16 — dnsmasq — multiple vulnerabilities (20 CVEs) — patch and remediation guide

📖 ~4 min read  •  Source: SUSE advisory SUSE-CU-2021:233-1 (see also SUSE bugzilla)

Related CVEs: CVE-2020-25681 CVE-2020-25682 CVE-2020-25683 CVE-2020-25684 CVE-2020-25685 CVE-2020-25686 CVE-2020-25687 CVE-2023-49441  +12 more

Upstream summary: A flaw was found in dnsmasq before version 2.83. A heap-based buffer overflow was discovered in the way RRSets are sorted before validating with DNSSEC data. An attacker on the network, who can forge DNS replies such as that they are accepted as valid, could use this flaw to cause a buffer overflow with arbitrary data in a heap memory segment, possibly executing code on the machine. The highest threat from this vulnerability is to data confidentiality and integrity as well as

Table of contents

1. Symptom & Impact
2. Environment & Reproduction
3. Root Cause Analysis
4. Quick Triage
5. Step-by-Step Diagnosis
6. Solution – Primary Fix
7. Solution – Alternative Approaches
8. Verification & Acceptance Criteria
9. Rollback Plan
10. Prevention & Hardening
11. Related Errors & Cross-Refs
12. References & Further Reading

Symptom & Impact

On SLES 16 hosts that have dnsmasq installed, administrators report behaviour consistent with SUSE advisory SUSE-CU-2021:233-1: zypper patch-check lists open patches, services backed by dnsmasq fail or restart unexpectedly, SELinux denials (avc) appear in ausearch — and for security-rated advisories the host is exposed to the vulnerability set above. Impact ranges from a single service-restart loop to wider availability incidents whenever dnsmasq sits on the serving path.

Environment & Reproduction

Reproduction targets SLES 16. Confirm release, registration, and installed package:

cat /etc/os-release
SUSEConnect --status-text
SUSEConnect --list-extensions 2>/dev/null | head -30
rpm -q dnsmasq
zypper info dnsmasq | head -20

Trigger the workflow that exposes dnsmasq — multiple vulnerabilities (20 CVEs) — patch and remediation guide while collecting:

sudo journalctl -u dnsmasq -b --no-pager | tail -200
sudo journalctl -xe --no-pager | tail -200
sudo tail -200 /var/log/zypp/history
sudo tail -200 /var/log/audit/audit.log
# For SUSE support, bundle evidence with supportconfig:
sudo supportconfig -R /var/tmp -B dnsmasq

Root Cause Analysis

Root cause is documented in SUSE advisory SUSE-CU-2021:233-1. SUSE security maintainers shipped fixes in the corresponding dnsmasq update for SLES 16; running an outdated build leaves the host exposed to the failure modes described in the advisory. Correlate zypper history with system logs:

sudo zypper history | grep dnsmasq
sudo zypper history --since='-7 days' | tail -40
sudo ausearch -m AVC,USER_AVC -ts today | tail -100
cat /proc/sys/kernel/tainted   # non-zero = tainted kernel / out-of-tree modules

Quick Triage

Run these on SLES 16 to capture the current state of dnsmasq:

rpm -q dnsmasq                              # installed NVR
rpm -V dnsmasq                              # verify shipped files
sudo zypper patch-check                    # open patches
sudo zypper lp -r SUSE-SLE-Server-16-* 2>/dev/null | head
systemctl --failed --no-pager
sudo firewall-cmd --list-all
getenforce                                  # SELinux mode
# If dnsmasq ships a systemd unit (unit name may differ from pkg name, e.g.
# bind→named, postgresql-server→postgresql, php-fpm→php-fpm):
systemctl list-unit-files | grep -i dnsmasq | head

Step-by-Step Diagnosis

1. List failed systemd units.

   systemctl --failed --no-pager

2. Tail the journal for dnsmasq and the system bus.

   sudo journalctl -u dnsmasq -f --no-pager
   sudo journalctl -xe -f --no-pager

3. Inspect firewall posture (firewalld is the default on SLES 15+).

   sudo firewall-cmd --list-all-zones --permanent
   sudo nft list ruleset 2>/dev/null | head -50

4. Surface SELinux denials and author a local policy module if needed.

   sudo ausearch -m AVC,USER_AVC -ts today
   sudo ausearch -m AVC -ts today | audit2allow -a -M /tmp/local-fix
   sudo semodule -i /tmp/local-fix.pp

5. Verify dnsmasq integrity and reinstall if anything is altered.

   sudo rpm -V dnsmasq
   sudo zypper verify
   sudo zypper install --force dnsmasq

6. Correlate findings with /var/log/zypp/history, zypper history, and SUSE advisory SUSE-CU-2021:233-1 to pin the change that introduced dnsmasq — multiple vulnerabilities (20 CVEs) — patch and remediation guide.

Solution – Primary Fix

Apply the corrective zypper transaction referenced by SUSE advisory SUSE-CU-2021:233-1, then reload affected systemd units:

sudo zypper ref                        # refresh repos
sudo zypper -n patch                   # apply ALL open patches (recommended)
# Or target a single package:
sudo zypper -n update dnsmasq
sudo systemctl daemon-reload
# Unit name may differ from pkg name; check first:
systemctl list-unit-files | grep -i dnsmasq | head
sudo systemctl restart dnsmasq
rpm -q dnsmasq                           # confirm new NVR
systemctl is-active dnsmasq 2>/dev/null  # confirm running (if a unit exists)

For kernel / glibc / systemd / openssl advisories a reboot is required (or SLE Live Patching where licensed):

sudo zypper ps -s                      # services using deleted libs
sudo systemctl reboot                  # or: sudo shutdown -r now
# SUSE Live Patching (kgraft / klp) avoids reboot for kernel CVEs:
sudo zypper install -y kernel-livepatch-$(uname -r | tr - _)
klp -v patches                         # active livepatches

Solution – Alternative Approaches

If the primary patch is not viable, choose from these:

• Roll back via Snapper (Btrfs snapshots taken automatically before zypper transactions on SLES 16):

  sudo snapper list
  sudo snapper undochange <pre>..<post>  # diff between two snapshot numbers
  sudo snapper rollback <pre>            # boot the host into the chosen snapshot

• Lock the package so zypper cannot upgrade it:

  sudo zypper al dnsmasq                   # add lock
  zypper ll | grep dnsmasq                 # list locks
  sudo zypper rl dnsmasq                   # remove lock

• Install an older NVR if a regression is suspected:

  zypper se -s dnsmasq                     # show all available versions
  sudo zypper install --oldpackage dnsmasq-<older-NVR>

• Switch SELinux to permissive briefly to confirm policy is the cause, then re-enforce:

  sudo setenforce 0
  # reproduce, capture denials, author a custom module:
  sudo ausearch -m AVC -ts recent | audit2allow -a -M mylocal
  sudo semodule -i mylocal.pp
  sudo setenforce 1

• Where SLE Live Patching is licensed, apply kernel fixes without reboot:

  klp -v patches                         # active livepatches
  sudo zypper install -y kernel-livepatch-$(uname -r | tr - _)

Verification & Acceptance Criteria

All of these should pass after the fix:

rpm -q dnsmasq                                            # expected fixed NVR
sudo zypper patch-check                                  # 0 critical patches outstanding
systemctl is-active dnsmasq 2>/dev/null
sudo journalctl -u dnsmasq --since "5 minutes ago" --no-pager | grep -iE "error|fail" || echo OK
sudo firewall-cmd --list-services
getenforce
sudo zypper ps -s                                        # any services still using deleted libs

The original reproduction for dnsmasq — multiple vulnerabilities (20 CVEs) — patch and remediation guide must not trigger across two consecutive runs.

Rollback Plan

Capture state before any change:

rpm -qa > /root/rpm-pre.txt
sudo zypper history list > /root/zypper-history-pre.txt
# Snapper takes pre/post snapshots automatically on Btrfs root.
sudo snapper create -d 'pre-patch-dnsmasq'   # explicit named snapshot
sudo snapper list | head

To revert if the patch is bad:

# Preferred on Btrfs root — boot the prior snapshot:
sudo snapper rollback <snapshot-id>
sudo systemctl reboot
# Or downgrade just the package:
sudo zypper install --oldpackage dnsmasq-<older-NVR>
sudo systemctl daemon-reload
sudo systemctl restart dnsmasq
# Custom security policy cleanup:
sudo semodule -r mylocal

Prevention & Hardening

Reduce the chance of this recurring on SLES 16:

• Enable automatic patch installation:

  sudo zypper install -y zypper-automatic
  sudo systemctl enable --now zypper-automatic.timer
  # Or use YaST: yast2 online_update_configuration

• Subscribe to sle-security-updates and watch suse.com/support/update.

• Mirror through SUSE Manager or RMT (Repository Mirroring Tool) for controlled rollouts:

  sudo zypper install -y rmt-server rmt-cli
  sudo rmt-cli sync
  sudo rmt-cli products enable SLES/16/x86_64

• Lock sensitive packages so they cannot be auto-upgraded:

  sudo zypper al dnsmasq

• Ensure Snapper is enabled on the root subvolume and pre/post hooks run for every zypper transaction:

  sudo snapper -c root get-config | head
  # Default zypper plugin: /usr/lib/zypp/plugins/commit/snapper.zypp-commit-plugin

• Monitor file integrity with AIDE:

  sudo zypper install -y aide
  sudo aide --init && sudo mv /var/lib/aide/aide.db.new /var/lib/aide/aide.db
  sudo aide --check

• Subscribe to SUSE Live Patching so kernel CVEs can be remediated without reboot:

  sudo SUSEConnect -p sle-module-live-patching/16.0/x86_64
  sudo zypper install -y kernel-livepatch-$(uname -r | tr - _)
  klp -v patches

• SLES 16 ships with SELinux in enforcing mode by default; review and maintain custom modules in /etc/selinux/targeted/ rather than disabling enforcement.

• Apply CIS SUSE Linux Enterprise Server Benchmark hardening.

Related Errors & Cross-Refs

Issues that commonly surface alongside dnsmasq — multiple vulnerabilities (20 CVEs) — patch and remediation guide: zypper lock contention, systemd unit ordering cycles, SELinux AVC bursts, firewalld zone drift, and kernel taint flags. Useful triage:

sudo zypper ps -s
systemd-analyze critical-chain
sudo ausearch -m AVC -ts today | tail
sudo firewall-cmd --get-active-zones
cat /proc/sys/kernel/tainted

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References & Further Reading

Primary reference: SUSE advisory SUSE-CU-2021:233-1 (see also SUSE bugzilla). Manual pages useful on SLES 16:

man zypper
man zypper.conf
man systemctl
man journalctl
man firewall-cmd
man snapper
man semanage
man audit2allow
man SUSEConnect
man klp

Other resources: SUSE Linux Enterprise Server 16 documentation, suse.com/security, SUSE security blog, and per-package notes in /usr/share/doc/packages/dnsmasq/ for components implicated in dnsmasq — multiple vulnerabilities (20 CVEs) — patch and remediation guide.