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Actively exploited

CVE-2021-3156 Sudo Heap-Based Buffer Overflow Vulnerability

Sudo contains an off-by-one error that can result in a heap-based buffer overflow, which allows for privilege escalation.

From the CISA KEV catalogue version 2026.11.06, with Cybergen analysis. Updated 6 Nov 2026.

Our Assessment

Should You Worry?

Patch urgentlyCybergen assessment
  • CISA confirmed exploitation in the wild and added it to the Known Exploited Vulnerabilities catalogue on 6 Apr 2022.
  • Rated High severity (CVSS 7.8, scored by CISA).
  • It has been in the catalogue for 1,675 days. Systems still unpatched now have almost certainly been scanned for it.
The Vulnerability

What It Is

Sudo before 1.9.5p2 contains an off-by-one error that can result in a heap-based buffer overflow, which allows privilege escalation to root via "sudoedit -s" and a command-line argument that ends with a single backslash character.

From Watch to Action

What To Do Now

  1. 1

    Find it

    Search your asset inventory and external attack surface for Sudo. Include cloud images, appliances and anything a supplier runs for you.

    Attack Surface Management
  2. 2

    Fix it

    Apply updates per vendor instructions.

    Vulnerability Management
  3. 3

    Hunt for compromise

    If an affected system was reachable before you patched, assume it may have been hit. Check logs, new accounts, web shells and outbound connections from the date of exposure.

    Incident Response
  4. 4

    Prove it is fixed

    Rescan or test the system after patching. Upgrades that fail silently and forgotten secondary instances are common.

    Penetration Testing

Vendor Advisories

Regulation

The Compliance Clock

Cyber EssentialsRated high, so if it affects devices in your Cyber Essentials scope, the update must be installed within 14 days of the vendor releasing it.NIS2If exploitation causes a significant incident, essential and important entities owe their CSIRT an early warning within 24 hours and a notification within 72 hours.DORAFinancial entities must classify ICT incidents and send an initial notification for a major incident within strict deadlines, with exploited flaws a common cause.US federal (BOD 22-01)Federal civilian agencies had until 27 Apr 2022 to remediate. A sensible benchmark for everyone else.

General guidance, not legal advice. Your obligations depend on your sector, scope and jurisdiction.

Stay Ahead

More Exploited Sudo Flaws

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CVE-2025-32463Sudo Inclusion of Functionality from Untrusted Control Sphere VulnerabilityAdded 29 Sep 2025

All 2 exploited Sudo vulnerabilities

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Evidence

Sources

  1. CISA Known Exploited Vulnerabilities catalogue
  2. NVD: CVE-2021-3156
  3. CVE record: CVE-2021-3156
  4. FIRST EPSS
  5. lists.debian.org/debian-lts-announce/2021/01/msg00022.html
  6. seclists.org/fulldisclosure/2021/Jan/79
  7. openwall.com/lists/oss-security/2021/01/26/3
  8. security.gentoo.org/glsa/202101-33
  9. debian.org/security/2021/dsa-4839
  10. lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/LHXK6ICO5AY
  11. lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/CALA5FTXIQB
  12. openwall.com/lists/oss-security/2021/01/27/1