Ubuntu 14.04 — pygments — multiple vulnerabilities (3 CVEs) — patch and remediation guide

📖 ~4 min read  •  Source: Ubuntu Security Notice USN-4897-2

Related CVEs: CVE-2021-20270 CVE-2021-27291 CVE-2015-8557

Upstream summary: USN-4897-1 fixed several vulnerabilities in Pygments. This update provides
the corresponding update for Ubuntu 14.04 LTS.

Original advisory details:

Ben Caller discovered that Pygments incorrectly handled parsing certain
files. If a user or automated system were tricked into parsing a specially
crafted file, a remote attacker could cause Pygments to hang or consume
resources, resulting in a denial of service. (CVE-2021-27291)

It was discovered that Pygments incorrectl

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

Nothing executes pygments on its own; its code runs inside whatever links against or imports it, so the exposed surface on an Ubuntu 14.04 host is the union of every process that has loaded it — web workers, cron jobs, mail handlers, backup agents. The package manager replaces the files on disk, but processes already running keep the old build mapped, and virtualenvs, vendored copies and container images each carry their own copy, so a host can report itself patched while still running vulnerable code. pygments is a library — code loaded by other programs rather than a daemon — so there is no unit to restart and systemctl status pygments will simply report no such unit; restart its consumers instead.

Environment & Reproduction

Reproduction targets Ubuntu 14.04 (trusty). Confirm release and installed package:

lsb_release -a
cat /etc/os-release
dpkg -l pygments | tail -2
apt-cache policy pygments
uname -r

Exercise the workload that uses pygments while collecting:

sudo tail -200 /var/log/syslog | grep -i pygments
sudo tail -200 /var/log/syslog
sudo tail -200 /var/log/apt/history.log
sudo tail -200 /var/log/kern.log | grep -i apparmor

Root Cause Analysis

Root cause is documented in Ubuntu Security Notice USN-4897-2. Canonical security maintainers shipped fixes in the corresponding pygments update for Ubuntu 14.04; running an outdated build leaves the host exposed to the failure modes described in the advisory. On this release the fix typically arrives via the Ubuntu Pro ESM (esm-infra / esm-apps) channels rather than the standard archive. Correlate apt history with the journal:

grep -A2 -B2 pygments /var/log/apt/history.log
zgrep -A2 -B2 pygments /var/log/apt/history.log.*.gz 2>/dev/null
cat /proc/sys/kernel/tainted   # non-zero = tainted kernel / out-of-tree modules

Quick Triage

Run these on Ubuntu 14.04 to capture the current state of pygments:

dpkg -l pygments | tail -1                  # installed version
dpkg -V pygments                             # verify shipped files
sudo apt update && apt list --upgradable 2>/dev/null | grep -i security
systemctl daemon-reload  # only if the upgrade shipped changed unit files
systemctl daemon-reexec  # REQUIRED after a systemd / libsystemd upgrade: re-execs PID 1 against the new library without rebooting (needrestart flags this as 'systemd-manager')
needrestart -r i  # (pkg: needrestart) interactive: prompts per service. The safe default on a production box
needrestart -r a  # non-interactive: restarts EVERY flagged unit. Maintenance window only -- it will bounce databases and web servers without asking
sudo ufw status verbose 2>/dev/null | head -20
sudo aa-status 2>/dev/null | head -20
# If pygments ships a service unit (unit/job name often differs from pkg name, e.g.
# bind9→named, apache2→apache2, postgresql-NN→postgresql@NN-main):
initctl list 2>/dev/null | grep pygments

On trusty the standard archive no longer ships security fixes. Verify Ubuntu Pro ESM coverage:

# `pro` CLI not available on this release; check the older `ubuntu-advantage-tools`:
sudo ua status --format=json 2>/dev/null | head
apt-cache policy | grep -i esm

Step-by-Step Diagnosis

1. List failing services.

   initctl list | grep -v running

2. Tail the journal / syslog for pygments.

   sudo tail -f /var/log/upstart/pygments.log
   sudo tail -f /var/log/syslog

3. Inspect UFW (Uncomplicated Firewall) state.

   sudo ufw status numbered
   sudo ufw show added
   sudo iptables -L -n -v | head -30

4. Surface AppArmor denials and switch the profile to complain mode if needed.

   sudo journalctl -k 2>/dev/null | grep -i 'apparmor="DENIED"' | tail -30
   sudo aa-status
   # /etc/apparmor.d/usr.bin.pygments or usr.sbin.pygments — inspect first
   sudo aa-complain /etc/apparmor.d/usr.bin.pygments 2>/dev/null || true

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

   sudo dpkg -V pygments
   sudo debsums -c pygments 2>/dev/null
   sudo apt install --reinstall -y pygments

6. Correlate findings with /var/log/apt/history.log, /var/log/dpkg.log, and Ubuntu Security Notice USN-4897-2 to pin the change that introduced the regression.

Solution – Primary Fix

Apply the corrective apt transaction referenced by Ubuntu Security Notice USN-4897-2, then reload the affected service:

sudo apt update
sudo apt -y install --only-upgrade pygments
# upstart uses initctl, not systemctl:
# Service name may differ from pkg name; check first:
initctl list 2>/dev/null | grep pygments
dpkg -l pygments | tail -1            # confirm new version

On trusty the standard archive is past EoL for security; enable Ubuntu Pro ESM to receive the fix:

# Older releases use the `ua` command:
sudo ua attach <token>
sudo ua enable esm-infra
sudo ua enable esm-apps
sudo apt update
sudo apt -y install --only-upgrade pygments

For kernel / glibc / systemd / openssl advisories a reboot (or Livepatch) is required:

sudo apt install -y needrestart
sudo needrestart -r l       # list units that need restart
sudo systemctl reboot       # or: sudo shutdown -r now

Solution – Alternative Approaches

If the primary upgrade is not viable, pick from these:

• Hold the package so apt cannot upgrade it:

  sudo apt-mark hold pygments
  apt-mark showhold | grep pygments
  # Release the hold later with:
  sudo apt-mark unhold pygments

• Pin a known-good version via apt preferences:

  # /etc/apt/preferences.d/pygments.pref
  Package: pygments
  Pin: version <good-version>
  Pin-Priority: 1001

• Downgrade to an older version if a regression is suspected:

  apt-cache madison pygments
  sudo apt install --allow-downgrades -y pygments=<older-version>

• Investigate AppArmor blocking the new binary; switch to complain briefly, capture denials, then re-enforce:

  sudo aa-complain /etc/apparmor.d/usr.bin.pygments 2>/dev/null
  # reproduce the failure
  sudo journalctl -k | grep apparmor | tail
  sudo aa-enforce /etc/apparmor.d/usr.bin.pygments 2>/dev/null

• Take only the security pocket update and defer the full point-release upgrade:

  sudo apt -y install --only-upgrade -t trusty-security pygments

Verification & Acceptance Criteria

All of these should pass after the fix is applied:

dpkg -l pygments | tail -1                                  # expected fixed version
apt list --upgradable 2>/dev/null | grep -i security || echo OK
sudo tail -50 /var/log/syslog | grep pygments || echo OK
sudo ufw status numbered | head
sudo aa-status 2>/dev/null | head -5

The conditions described in the advisory must no longer be reported for pygments across two consecutive runs.

Rollback Plan

Capture state before any change:

apt list --installed 2>/dev/null > /root/apt-pre.txt
dpkg --get-selections > /root/dpkg-pre.txt
# ZFS-on-root (Ubuntu 20.04+ default installer option):
sudo zfs snapshot rpool/ROOT/ubuntu@pre-pygments
# LVM-on-root:
sudo lvcreate -L 4G -s -n root_pre_patch /dev/<vg>/<root-lv>

To revert:

sudo apt install --allow-downgrades -y pygments=<old-version>
# Kernel rollback: pick the prior kernel from the GRUB menu, then:
sudo systemctl reboot
# ZFS rollback (rolls the whole root dataset):
sudo zfs rollback -r rpool/ROOT/ubuntu@pre-pygments

Prevention & Hardening

Reduce the chance of this recurring on Ubuntu 14.04 (trusty):

• Enable scheduled security updates via unattended-upgrades:

  sudo apt install -y unattended-upgrades update-notifier-common
  sudo dpkg-reconfigure -plow unattended-upgrades
  # /etc/apt/apt.conf.d/50unattended-upgrades:
  Unattended-Upgrade::Allowed-Origins { "${distro_id}:${distro_codename}-security"; };

• Install needrestart so services restart automatically after library upgrades:

  sudo apt install -y needrestart
  # /etc/needrestart/needrestart.conf -> $nrconf{restart} = 'a';

• Attach Ubuntu Pro for ESM (mandatory on this past-EoL release) and Livepatch where supported:

  sudo ua attach <token>
  sudo ua enable esm-infra
  sudo ua enable esm-apps

• Subscribe to ubuntu-security-announce and watch ubuntu.com/security/cves.

• Monitor file integrity with debsums and AIDE:

  sudo apt install -y debsums aide
  sudo debsums -ca
  sudo aideinit && sudo mv /var/lib/aide/aide.db.new /var/lib/aide/aide.db
  sudo aide --check

• For estate-wide patching, manage with Canonical Landscape:

  sudo apt install -y landscape-client
  sudo landscape-config

• Keep AppArmor profiles in enforce mode and apply CIS Ubuntu Linux Benchmark hardening.

Related Errors & Cross-Refs

Issues that commonly surface alongside a pygments update: apt lock contention, broken dpkg state, systemd ordering cycles, AppArmor denials, and UFW rule drift. Useful triage:

sudo dpkg --configure -a
sudo apt --fix-broken install
initctl list | head
sudo journalctl -k 2>/dev/null | grep -i apparmor | tail
cat /proc/sys/kernel/tainted

View all ubuntu-14-04 tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

Primary reference: Ubuntu Security Notice USN-4897-2. Manual pages useful on Ubuntu 14.04:

man apt
man apt-get
man apt-mark
man dpkg
man initctl
# journald not present on this release
man ufw
man apparmor
man aa-status
man unattended-upgrades
man ua

Other resources: Ubuntu Security Notices, Ubuntu CVE Tracker, Ubuntu upgrade notes, and per-package notes in /usr/share/doc/pygments/ for components implicated in this advisory.