CVE-2021-22555
CVE-2021-22555 is a security flaw in Linux Kernel, the core of Linux, which runs most servers, Android phones and many home routers. Attackers can corrupt memory in Linux Kernel, which they use to take it over. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 6 Oct 2025.[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 unspecified of Linux Kernel.
Description
CISA calls it a heap out-of-bounds write in Linux Kernel. In its words: “Linux Kernel contains a heap out-of-bounds write vulnerability that could allow an attacker to gain privileges or cause a DoS (via heap memory corruption) through user name space.”[1]
The underlying weakness is out-of-bounds write (CWE-787), in MITRE’s list of common software weaknesses.[2]
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 one fixed version: everything before it is affected.[2]
| Product | Fixed in |
|---|---|
| Linux Kernel | unspecified |
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 6 Oct 2025, 1,552 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 27 Oct 2025 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 79%. That’s in the top 1% 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 mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.”[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 unspecified of Linux Kernel), or not in use. CISA gave US government agencies until 27 Oct 2025 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 79%.[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 out-of-bounds write flaws
- CVE-2026-86950, Apple devices: out-of-bounds write ·
- CVE-2026-87491, Chrome and other Chromium browsers: out of bounds write ·
- CVE-2026-11645, Chrome and other Chromium browsers: out-of-bounds read and write ·
- CVE-2026-0300, Palo Alto Networks PAN-OS: out-of-bounds write ·
- CVE-2026-3909, Google Skia: out-of-bounds write ·
Added the same day
- CVE-2025-61882, Oracle E-Business Suite: unspecified ·
- CVE-2021-43226, Microsoft Windows: privilege escalation ·
- CVE-2013-3918, Microsoft Windows: out-of-bounds write ·
- CVE-2011-3402, Microsoft Windows: remote code execution ·
- CVE-2010-3962, Microsoft Internet Explorer: uninitialized memory corruption ·
Every Linux flaw on CISA’s list · Everything added in October 2025
References
- ^ Cybersecurity and Infrastructure Security Agency. “Linux Kernel Heap Out-of-Bounds Write Vulnerability”. Known Exploited Vulnerabilities Catalog, added 6 Oct 2025.
- ^ National Institute of Standards and Technology. “CVE-2021-22555 Detail”. National Vulnerability Database, published 7 Jul 2021.
- ^ Linux. Advisories and fixes: git.kernel.org, git.kernel.org, security.netapp.com, github.com.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.787, percentile 99.6, 31 Oct 2026.
- CVE Program. “CVE-2021-22555”. 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.