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Actively exploited

CVE-2021-3493 Linux Kernel Privilege Escalation Vulnerability

The overlayfs stacking file system in Linux kernel does not properly validate the application of file capabilities against user namespaces, which could lead to 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 20 Oct 2022.
  • Rated High severity (CVSS 8.8, scored by Vendor).
  • It has been in the catalogue for 1,478 days. Systems still unpatched now have almost certainly been scanned for it.
The Vulnerability

What It Is

The overlayfs implementation in the linux kernel did not properly validate with respect to user namespaces the setting of file capabilities on files in an underlying file system. Due to the combination of unprivileged user namespaces along with a patch carried in the Ubuntu kernel to allow unprivileged overlay mounts, an attacker could use this to gain elevated privileges.

Affected Versions

ProductVersions
Ubuntu linux kernel5.8 kernel to before 5.8.0-50.56
5.4 kernel to before 5.4.0-72.80
4.15 kernel to before 4.15.0-142.146
4.4 kernel to before 4.4.0-209.241

From the public CVE record, as supplied by the vendor. Always confirm against the vendor advisory.

From Watch to Action

What To Do Now

  1. 1

    Find it

    Search your asset inventory and external attack surface for Linux Kernel. 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 10 Nov 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 Linux Flaws

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All 32 exploited Linux vulnerabilities

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Evidence

Sources

  1. CISA Known Exploited Vulnerabilities catalogue
  2. NVD: CVE-2021-3493
  3. CVE record: CVE-2021-3493
  4. FIRST EPSS
  5. git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=7c03e2cda4a584cadc398e8f6641ca
  6. ubuntu.com/security/notices/USN-4917-1
  7. openwall.com/lists/oss-security/2021/04/16/1
  8. packetstormsecurity.com/files/162434/Kernel-Live-Patch-Security-Notice-LSN-0076-1.html
  9. packetstormsecurity.com/files/162866/Ubuntu-OverlayFS-Local-Privilege-Escalation.html
  10. packetstormsecurity.com/files/165151/Ubuntu-Overlayfs-Local-Privilege-Escalation.html