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Actively exploited in the wild. This CVE is on the CISA Known Exploited Vulnerabilities list — treat remediation as urgent. Federal remediation due by 10 Nov 2022.
Ubuntu LinuxOperating system · Canonical

CVE-2021-3493

HIGH · 7.8 CVSS v3.1 Published 2021-04-17
Fix available
A fix is available. Upgrade to 18.04 / 20.04 or later.
See remediation →
96/100
Remediation priority · Urgent
In the wild Public exploit Zero-click Patch available

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

The overlayfs implementation in the Linux kernel did not properly validate file capabilities on files in underlying filesystems with respect to user namespaces. Combined with Ubuntu's patch allowing unprivileged overlay mounts, this allowed local attackers to gain elevated privileges through privilege escalation.

MitigationApply the Linux kernel security patch to address the improper capability validation in overlayfs; alternatively, consider disabling unprivileged user namespaces and unprivileged overlay mounts if not required by the environment.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Ubuntu LinuxOperating system
Affected:< 18.04>= 18.04.1, < 20.04< 20.10

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Local
Complexity
Low
Privileges
Low
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Confirm the operating system is Ubuntu
    Run: cat /etc/os-release and look for 'Ubuntu' in the NAME or ID field
    Affected if The system is not Ubuntu - this CVE specifically affects Ubuntu Linux kernels
  2. Check the Ubuntu version against affected ranges
    Run: cat /etc/os-release and compare the VERSION_ID or VERSION string against: < 18.04; >= 18.04.1 and < 20.04; < 20.10. For example, 18.04.1 through 18.04.5 and 19.10 are affected
    Affected if The installed Ubuntu version falls within any of the vulnerable ranges (< 18.04; >= 18.04.1 and < 20.04; < 20.10)
  3. Verify if unprivileged user namespaces are enabled
    Run: sysctl kernel.unprivileged_userns_clone (if it exists) or check /proc/sys/kernel/unprivileged_userns_clone. Also check for any AppArmor or seccomp configurations that may restrict user namespace creation
    Affected if Unprivileged user namespaces are enabled (value is 1 or not restricted), which is required for the exploit chain to work
  4. Check if overlayfs is accessible to the user
    Attempt to mount an overlayfs as an unprivileged user: mkdir -p /tmp/overlay_test && mount -t overlay overlay_test -o lowerdir=/tmp,upperdir=/tmp,workdir=/tmp /tmp/overlay_test 2>&1. Alternatively, check if the overlay kernel module is loaded (lsmod | grep overlay)
    Affected if Unprivileged users can mount overlay filesystems, which is part of Ubuntu's patch that enables this vulnerability

The system is affected if it is running a vulnerable Ubuntu version (within the specified ranges) AND unprivileged user namespaces are enabled AND unprivileged overlay mounts are possible.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 18.04 / 20.04 / 20.10 or later
Fixed in 18.0420.0420.10
Vendor patch git.kernel.org →
Interim mitigation

Apply the Linux kernel security patch to address the improper capability validation in overlayfs; alternatively, consider disabling unprivileged user namespaces and unprivileged overlay mounts if not required by the environment.

Recommended fix High confidence

Ubuntu 18.04: Upgrade to linux-image 5.4.0-1041 or later (HWE kernel); Ubuntu 20.04: Upgrade to linux-image 5.4.0-1041 or later; Ubuntu 20.10: Upgrade to linux-image 5.8.0-1044 or later

  1. Run 'sudo apt update' to refresh package lists
  2. Run 'sudo apt upgrade linux-image-generic' (or 'sudo apt-get update && sudo apt-get install linux-image-generic') to install the fixed kernel
  3. Reboot the system to load the updated kernel: 'sudo reboot'
  4. After reboot, verify the kernel version with 'uname -r' and confirm it's at or above the fixed version (e.g., 5.4.0-1041 for Ubuntu 18.04/20.04, 5.8.0-1044 for Ubuntu 20.10)
  5. Optionally, verify the fix is applied by checking that overlayfs does not allow unprivileged file capability changes
Caveat Kernel upgrades require system reboot and may require reboot for a reason; ensure compatibility with any kernel-specific drivers or modules before upgrading

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Ubuntu Linux Exploited in the wild — priority engagement
  • Consultation3.0 h
  • Implementation6.0 h
  • Testing4.0 h
  • Review / QA2.0 h
15.0 hours of engineering $2,640
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-3493 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data