CVE-2021-3493
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.
- CVSS
- 8.8Highscored by the publisher (CNA)
- Exploited in the wild
- Yes, in CISA KEV since 20 Oct 2022CISA deadline for US federal agencies: 10 Nov 2022public exploit code
- EPSS
- 49.2%chance of exploitation in the next 30 daystop 2% of scored CVEsFIRST score of 11 Nov 2026
- Fix
- Patch availablesee the advisories in the references
- Published
- 17 Apr 2021analysed here 12 Nov 2026
Do first. Apply updates per vendor instructions. (CISA's required action for this KEV entry)
Summary
Triage
- Attack surface
- Server-side
- Exploitable
- At runtime
- Attack vector
- local
- Can a WAF block it?
- No. See why
The summary, impact and action text on this page are written by our AI analysis pipeline, or taken from the NVD description when a CVE was imported from NVD. They are not the vendor's wording: check them against the sources. A longer AI executive summary and technical analysis are generated for signed-in accounts on the Max plan.
Affected products
| Product | Affected versions | Vendor says fixed in |
|---|---|---|
| Ubuntu linux kernel | >= 5.8 kernel and < 5.8.0-50.56>= 5.4 kernel and < 5.4.0-72.80>= 4.15 kernel and < 4.15.0-142.146>= 4.4 kernel and < 4.4.0-209.241Vendor says fixed in: not stated |
As published in the CVE Program record. A version outside these ranges is not a statement that it is unaffected — vendors sometimes understate a range, and distribution-backported builds carry upstream numbers that do not reflect what was patched into them.
Products named in our analysis
- canonical:ubuntu_linux
Fix and mitigation
WAF and other mitigations
Can a WAF block it? No. The attack needs local access to the machine, so a WAF is not on its path.
Also consider: patch · per-resource ACL
Based on the attack vector (local or physical access). We check Cloudflare, AWS, Google Cloud Armor and Azure; for Akamai, Imperva, Fastly, F5, ask your vendor.
Remediation steps (AI-written)
Step-by-step remediation is generated for accounts on the Max plan, for CVEs that match a technology in their stack. Add this technology to your stack to see the fix checklist.
Exploitation evidence
- CISA KEV catalog
- Listed since 20 Oct 2022.
- CISA due date
- 10 Nov 2022, the remediation deadline for US federal agencies
- CISA required action
- Apply updates per vendor instructions.
- Public exploit code
- Yes: 1 Metasploit module · 21 GitHub proofs of concept
- CISA SSVC
- exploitation active, automatable no, technical impact totalAssessed by CISA 7 Feb 2025 (SSVC 2.0.3). Exploitation is none, poc (public proof of concept) or active.
Metasploit
- exploit/linux/local/cve_2021_3493_overlayfsexploit, rank great
GitHub proofs of concept
- briskets/CVE-2021-3493445 stars
- inspiringz/CVE-2021-349342 stars
- oneoy/CVE-2021-34933 stars
- cerodah/overlayFS-CVE-2021-34932 stars
and 17 more repositories
Listed so you can judge urgency, not as a recommendation to run anything. GitHub “proofs of concept” are unvetted: some are fake or carry malware. Exploit-DB marks the entries its staff verified.
Timeline
- 17 Apr 2021Published
- 19 Apr 2021First public exploit or proof of concept2 days later
- 20 Oct 2022Added to CISA KEV (exploited in the wild)549 days (about 2 years) later
- 10 Nov 2022CISA remediation due date (US federal agencies)21 days later
- 7 Feb 2025CISA SSVC assessment: exploitation active821 days (about 2 years) later
- 21 Oct 2025CVE record last updated256 days (about 9 months) later
- 12 Nov 2026Analysed by vulneraibility.com387 days (about 1 year) later
CVSS vector
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H
| Metric | Value | What it means |
|---|---|---|
| Attack vector | LocalLocal access required (shell, logged-in user). | Local access required (shell, logged-in user). |
| Attack complexity | LowRepeatable attack, no special conditions. | Repeatable attack, no special conditions. |
| Privileges required | LowThe attacker needs basic user privileges. | The attacker needs basic user privileges. |
| User interaction | NoneNo victim action needed. | No victim action needed. |
| Scope | ChangedImpact crosses a security boundary into other components. | Impact crosses a security boundary into other components. |
| Confidentiality | HighTotal data disclosure possible. | Total data disclosure possible. |
| Integrity | HighComplete modification possible. | Complete modification possible. |
| Availability | HighFull outage possible. | Full outage possible. |
Common questions
Is CVE-2021-3493 being exploited?
Yes. CVE-2021-3493 appears in CISA's Known Exploited Vulnerabilities catalog, which means exploitation in the wild has been confirmed and reported. Treat it as active rather than theoretical, and patch it ahead of higher-scoring vulnerabilities that carry no exploitation evidence.
How severe is CVE-2021-3493?
CVE-2021-3493 is rated High with a CVSS score of 8.8. Severity describes how bad exploitation would be, not how likely it is: pair it with exploitation evidence before deciding what to patch first.
Is there a patch for CVE-2021-3493?
Yes. A fix has been recorded for CVE-2021-3493. The vendor advisory is the authority on the exact fixed version — apply it from there rather than from a summary.
What does CVE-2021-3493 affect?
CVE-2021-3493 affects canonical:ubuntu_linux. Confirm the exact affected versions against the vendor advisory before deciding you are exposed.
Exploitation status reflects CISA's KEV catalog as we last synced it. Check the catalog directly.
References
7 references in the CVE record, grouped by the tags the record gives them.
Other links (7)
- ubuntu.com /security/notices/USN-4917-1
- git.kernel.org /pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=7c03e2cda4a584cadc398e8f
- openwall.com /lists/oss-security/2021/04/16/1
- packetstormsecurity.com /files/162434/Kernel-Live-Patch-Security-Notice-LSN-0076-1.html
- packetstormsecurity.com /files/162866/Ubuntu-OverlayFS-Local-Privilege-Escalation.html
- packetstormsecurity.com /files/165151/Ubuntu-Overlayfs-Local-Privilege-Escalation.html
Show 1 more
- cisa.gov /known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-3493
Where the data on this page comes from
- CVE List (CVE.org): description, affected versions, references; CVE Terms of Use
- NVD: CVSS and CPE data
- CISA KEV catalog: exploitation, date added, due date
- CISA Vulnrichment (ADP): SSVC decision points
- FIRST EPSS: exploit probability and percentile
- Metasploit Framework: exploit and scanner modules
- PoC-in-GitHub: GitHub repositories that claim a proof of concept
Summary, impact and triage tags: written by our AI analysis pipeline from the sources above. Check them against the sources.
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