CISA added CVE-2021-3493 to its Known Exploited Vulnerabilities (KEV) Catalog after evidence showed it was being exploited. The Linux kernel flaw can let a local, unprivileged user gain root privileges. SecurityWeek linked its use to Shikitega malware, which also used the separate PwnKit vulnerability, CVE-2021-4034, in an infection chain. The practical response is to install the security update for each affected Ubuntu release and investigate systems that may have been compromised; installing a patch alone does not establish that a host is clean.
What is CVE-2021-3493?
CVE-2021-3493 is a local privilege-escalation vulnerability in the Linux kernel’s OverlayFS implementation. An attacker needs an existing low-privilege foothold on a system; this is not a remote, unauthenticated network flaw. If exploited, it can allow that user to gain root-level privileges.
Ubuntu’s Security Team explains that the kernel did not properly validate the setting of file capabilities on files in an underlying filesystem with respect to user namespaces. The privilege escalation involved unprivileged user namespaces together with an Ubuntu kernel patch that allowed unprivileged overlay mounts. The issue therefore should not be treated as affecting every Linux distribution: the reported scope centered on Ubuntu kernels carrying the relevant behavior. Check Ubuntu’s advisory for the status of a specific release and package track: Ubuntu CVE-2021-3493 advisory.
Why did CISA issue the warning?
CISA’s KEV Catalog is a prioritization resource for vulnerabilities with evidence of exploitation in the wild. CISA directs federal agencies covered by Binding Operational Directive 22-01 to remediate catalog entries on its schedule and also urges organizations outside the federal government to prioritize timely remediation.
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SecurityWeek reported on October 21, 2022 that CISA had added CVE-2021-3493 to the catalog. The coverage connected exploitation to Shikitega, a stealthy Linux malware family targeting Linux endpoints and IoT devices. The reported infection chain used CVE-2021-3493 alongside CVE-2021-4034, known as PwnKit, for privilege escalation and could download a cryptocurrency miner. The report does not establish an incident-wide infection count or total number of affected devices.
Is an Ubuntu system vulnerable?
Do not infer exposure from the word “Linux” alone or rely on a kernel version copied from an old article. Ubuntu’s advisory lists CVE-2021-3493 as high priority and gives it a CVSS 3 score of 8.8 in its 2026 update. It identifies fixed package builds including linux 5.4.0-72.80 for Ubuntu 20.04 and linux 4.15.0-142.146 for Ubuntu 18.04, as well as corresponding fixed builds for other affected Ubuntu package tracks. These are advisory examples, not universal current targets for every installation.
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Check the current Ubuntu notice against the release and kernel package actually installed on each machine. This matters for cloud images, appliances, endpoints, and IoT devices as well as conventional servers: a fleet may contain multiple Ubuntu releases or package tracks, and an old image may not reflect the current security status.
How to respond
- Inventory Ubuntu systems. Include cloud instances and images, appliances, employee endpoints, and IoT devices. Identify each system’s Ubuntu release and installed kernel package.
- Compare installed packages with Ubuntu’s current advisory. Use the entry for CVE-2021-3493 and the relevant release’s package track rather than treating a version cited in older reporting as the right target today.
- Install the vendor security update. Use the normal package-management and change-control process for the system. Reboot where the distribution requires it so the updated kernel is running.
- Verify deployment across the fleet. Confirm that each in-scope system received the applicable update and, where required, rebooted into the updated kernel. Track exceptions and failed updates instead of assuming a rollout succeeded.
- Review security telemetry. Examine authentication events, process activity, persistence mechanisms, and outbound network connections for signs of local privilege escalation, Shikitega components, or cryptocurrency-mining activity.
- Handle suspected compromise as an incident. Isolate the host under your incident-response procedures, preserve relevant evidence, and rotate credentials that may have been exposed. Return it to service only after validation.
What patching does—and does not—confirm
Installing the fixed kernel package addresses the known vulnerability on that system, but it cannot undo exploitation that happened before the update. A host that may have been accessed through the flaw still needs investigation for malware, persistence, stolen credentials, and other unauthorized changes. Treat patch verification and compromise assessment as separate tasks.
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For organizational context, see CISA’s Known Exploited Vulnerabilities Catalog and the Ubuntu security notice for the package status relevant to your release.
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