Raw vector
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:HCVSS and EPSS are reproduced from their sources (NVD, FIRST EPSS). Risk Priority is our own derived reading, not an NVD score.
Summary
CVE-2021-3493 is a high-severity Privilege Context Switching Error (CWE-270) vulnerability in Canonical Ubuntu Linux. Its CVSS base score is 8.8 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Junk Data (T1001.001); ranked in the top 1% of CVEs by exploit likelihood; CISA has added it to the Known Exploited Vulnerabilities catalog; a public proof-of-concept is referenced.
Deeper analysis AI-assisted summary
Synthesised by an AI model from the NVD description and linked references — a reading aid, not an authoritative source.
The vulnerability CVE-2021-3493 resides in the overlayfs implementation within the Linux kernel. It arises from insufficient validation of file capability settings on files in an underlying filesystem when unprivileged user namespaces are in use. The flaw is exacerbated by an Ubuntu-specific kernel patch that permits unprivileged overlay mounts, affecting systems running affected Ubuntu kernels.
A local attacker who can create unprivileged user namespaces can exploit the issue to set arbitrary file capabilities. This enables the attacker to gain elevated privileges, with a CVSS score of 8.8 reflecting the high impact on confidentiality, integrity, and availability under a local attack vector.
Ubuntu addressed the vulnerability through USN-4917-1, which includes updated kernel packages that enforce proper capability validation for overlayfs. An upstream kernel commit also resolves the validation gap for user namespaces.
Public exploit code has been published demonstrating local privilege escalation on vulnerable Ubuntu systems.
OWASP Top 10 for Web (2025)
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2021-26815
Vulnerability Data
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…
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carried in the Ubuntu kernel to allow unprivileged overlay mounts, an attacker could use this to gain elevated privileges.
- CWE(s)
- KEV Date Added
- 20 October 2022
Related Threats
MITRE ATT&CK Enterprise Techniques
CVEs Like This One
Affected Assets
Mitigating Controls
Control response
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- 2 hardening rules · 2 OS baselines
V13.2.2
Likely Mitigating Controls AI
Per-CVE control mapping for this CVE has not run yet; the list below is derived from the weakness types (CWEs) cited in the NVD entry.
Periodic review and update of procedures reduces incorrect authorization implementations over time.
Supervision identifies cases where authorization logic incorrectly permits unauthorized actions.
Defining permitted attribute values and auditing modifications reduces the chance of incorrect authorization outcomes due to tampered or missing labels.
The authorization process and usage restrictions help prevent incorrect authorization for remote access types.
Establishing configuration and connection requirements helps ensure correct rather than incorrect authorization for wireless access.
Establishing connection authorization processes for mobile devices helps ensure authorization decisions are correctly implemented rather than incorrect.
Monitoring account use, notifying on changes, and reviewing accounts for compliance corrects incorrect authorization assignments.
Ensures authorization decisions for external system use are correctly implemented and enforced.
Mitigating Controls (NIST CSF 2.0) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.
Enforcing least-privilege authorization policies and reviews directly prevents improper privilege changes across contexts.
Integrating secure-development practices catches and eliminates context-switching privilege errors during design and coding.
Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.
A.5.15 only requires defining a topic-specific access control policy and requirements (with references to other controls); it does not itself enforce rules or stop a privilege context switching error from being present or exploited
A.5.18 governs the provisioning, review, and revocation of access rights granted to an already-authenticated identity; it has no bearing on whether the product itself correctly manages privilege transitions between contexts as described in CWE-270.
Restricting privileged utilities reduces exposure to context-switch privilege errors.
Privileged-access-rights control directly addresses improper privilege elevation during context switches.
Secure-SDLC practices can catch privilege-handling defects early but do not guarantee runtime correctness.
Hardening callouts derived
Configuration rules from DISA STIG baselines that bear on weaknesses of the type cited by this CVE. Each rule is shown with the relationship its mapping actually records, against the CWE it was authored against. Derived via CVE→CWE over `controls_xwalks` (higher-evidence rows only; rows rated `none` are excluded).
Ubuntu 22.04 (1 rule)
- V-260470 Ubuntu 22.04 LTS, when booted, must require authentication upon booting into single-user and maintenance modes. blocks CWE-270