CVE-2021-3493
CVE-2021-3493 is a security flaw in Linux Kernel, the core of Linux, which runs most servers, Android phones and many home routers. Someone who already has a foothold can use Linux Kernel to take full control of the machine. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 20 Oct 2022.[1]
Linux Kernel is business software or equipment. Most families won’t have it, but attackers break into it to get inside companies, and from there to what those companies hold about their customers, patients and staff. Fixed in version 5.8.0-50.56, 5.4.0-72.80, 4.15.0-142.146 and 4.4.0-209.241 of linux kernel.
Description
CISA calls it a privilege escalation in Linux Kernel. In its words: “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.”[1]
The CVE record adds: “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.”[2]
The underlying weakness is missing authorization (CWE-862), in MITRE’s list of common software weaknesses.
It scores 7.8 out of 10 on the Common Vulnerability Scoring System, which rates it high, scored by NVD using CVSS 3.1.[2]
Affected versions
NVD’s record lists these fixed versions: in each line, everything before the version shown is affected.[2]
| Product | Fixed in |
|---|---|
| linux kernel | 5.8.0-50.56 |
| linux kernel | 5.4.0-72.80 |
| linux kernel | 4.15.0-142.146 |
| linux kernel | 4.4.0-209.241 |
Most families won’t have Linux Kernel; it’s for whoever looks after IT at work.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 20 Oct 2022, 551 days after it was made public. CISA lists a flaw only when it has reliable evidence of it being exploited, and gave US federal agencies until 10 Nov 2022 to fix it.[1]
FIRST’s Exploit Prediction Scoring System, which estimates how likely a flaw is to be used in attacks in the next 30 days from what it sees across the internet, puts it at 49%. That’s in the top 2% of the flaws it scores.[4]
CISA assesses it as not automatable: each attack takes some work.
CISA doesn’t say who was attacked, or how.
Fix
Linux has published a fix or advice.[3] CISA’s required action: “Apply updates per vendor instructions.”[1]
- Check whether you use it.Linux Kernel is business software or equipment, so most families won’t have it. If you run a business, or work from home on company equipment, ask whoever looks after your IT.
- Send this page to your IT person.Ask them to confirm it’s fixed (fixed in version 5.8.0-50.56, 5.4.0-72.80, 4.15.0-142.146 and 4.4.0-209.241 of linux kernel), or not in use. CISA gave US government agencies until 10 Nov 2022 to fix it.
- If it can’t be fixed, take it offline.CISA’s advice when there’s no fix is to stop using the product, or take it off the internet, until there is one.
You can’t fix someone else’s server, but you can watch for what follows a break-in like this: a breach notice, a password reset you didn’t ask for, or a scam message that knows things it shouldn’t. HARD watches for those for you and your family.
Timeline
| Date | Event |
|---|---|
| The flaw is made public in the National Vulnerability Database, with a severity score of 7.8.[2] | |
| CISA adds it to its catalog of flaws known to be used in attacks.[1] | |
| CISA’s deadline for US government agencies to fix it.[1] | |
| FIRST puts the odds of it being used in attacks in the next 30 days at 49%.[4] |
See also
Other Linux Kernel flaws
- CVE-2026-46300: missing initialization of a variable ·
- CVE-2026-53266: out-of-bounds write ·
- CVE-2025-39964: race condition ·
- CVE-2025-39682: improper check for unusual or exceptional conditions ·
- CVE-2026-53362: unspecified ·
- CVE-2022-0995: out-of-bounds write ·
Other missing authorization flaws
- CVE-2024-57726, SimpleHelp: missing authorization ·
- CVE-2023-52163, Digiever DS-2105 Pro: missing authorization ·
- CVE-2025-40602, SonicWall SMA1000 appliance: sMA1000 missing authorization ·
- CVE-2025-6205, Dassault Systèmes DELMIA Apriso: missing authorization ·
- CVE-2025-20362, Cisco Secure Firewall Adaptive Security Appliance and Secure Firewall Threat Defense: secure firewall adaptive security (ASA) appliance and secure firewall threat defense (FTD) missing authorization ·
Added the same day
- CVE-2022-41352, Synacor Zimbra Collaboration Suite (ZCS): arbitrary file upload ·
Every Linux flaw on CISA’s list · Everything added in October 2022
References
- ^ Cybersecurity and Infrastructure Security Agency. “Linux Kernel Privilege Escalation Vulnerability”. Known Exploited Vulnerabilities Catalog, added 20 Oct 2022.
- ^ National Institute of Standards and Technology. “CVE-2021-3493 Detail”. National Vulnerability Database, published 17 Apr 2021.
- ^ Linux. Advisories and fixes: git.kernel.org, ubuntu.com.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.492, percentile 98.9, 31 Oct 2026.
- CVE Program. “CVE-2021-3493”. The official CVE record.
This article is built from CISA’s catalog (version 2026.10.30), NVD’s record and FIRST’s scores, kept in HARD’s database and checked every morning. The plain English is HARD’s.