CVE-2021-22991
CVE-2021-22991 is a security flaw in F5 BIG-IP Traffic Management Microkernel, equipment big companies use to route traffic to their websites and apps. Attackers can run their own code on F5 BIG-IP Traffic Management Microkernel. The US Cybersecurity and Infrastructure Security Agency (CISA) added it to its catalog of flaws known to be used in attacks on 18 Jan 2022.[1]
F5 BIG-IP Traffic Management Microkernel 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. F5 has published a fix or advice.
Description
CISA calls it a buffer overflow in F5 BIG-IP Traffic Management Microkernel. In its words: “The Traffic Management Microkernel of BIG-IP ASM Risk Engine has a buffer overflow vulnerability, leading to a bypassing of URL-based access controls.”[1]
The CVE record adds: “On BIG-IP versions 16.0.x before 16.0.1.1, 15.1.x before 15.1.2.1, 14.1.x before 14.1.4, 13.1.x before 13.1.3.6, and 12.1.x before 12.1.5.3, undisclosed requests to a virtual server may be incorrectly handled by the Traffic Management Microkernel (TMM) URI normalization, which may trigger a buffer overflow, resulting in a DoS attack. In certain situations, it may theoretically allow bypass of URL based access control or remote code execution (RCE). Note: Software versions which have reached End of Software Development (EoSD) are not evaluated.”[2]
The underlying weakness is memory buffer error (CWE-119), in MITRE’s list of common software weaknesses.
It scores 9.8 out of 10 on the Common Vulnerability Scoring System, which rates it critical, scored by NVD using CVSS 3.1.[2]
Affected versions
NVD’s record doesn’t list version ranges for it; F5’s advisory has them[3]. CISA names the product as F5 BIG-IP Traffic Management Microkernel.
Most families won’t have F5 BIG-IP Traffic Management Microkernel; it’s for whoever looks after IT at work.
Exploitation
CISA added it to its catalog of known exploited vulnerabilities on 18 Jan 2022, 293 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 1 Feb 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 62%. That’s in the top 1% of the flaws it scores.[4]
CISA assesses it as automatable: attackers can run it against many targets at once.
CISA doesn’t say who was attacked, or how.
Fix
F5 has published a fix or advice.[3] CISA’s required action: “Apply updates per vendor instructions.”[1]
- Check whether you use it.F5 BIG-IP Traffic Management Microkernel 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, or not in use. CISA gave US government agencies until 1 Feb 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 9.8.[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 62%.[4] |
See also
Other F5 flaws
- CVE-2026-94127, F5 BIG-IP APM: heap-based buffer overflow ·
- CVE-2025-53521, F5 BIG-IP: stack-based buffer overflow ·
- CVE-2023-46748, F5 BIG-IP Configuration Utility: SQL injection ·
- CVE-2023-46747, F5 BIG-IP Configuration Utility: authentication bypass ·
- CVE-2022-1388, F5 BIG-IP: missing authentication ·
Other memory buffer error flaws
- CVE-2026-88779, Citrix NetScaler: improper restriction of operations within the bounds of a memory buffer ·
- CVE-2026-88772, Citrix NetScaler: improper restriction of operations within the bounds of a memory buffer ·
- CVE-2026-88771, Citrix NetScaler: improper input validation ·
- CVE-2026-8452, Citrix NetScaler ADC and NetScaler Gateway: improper restriction of operations within the bounds of a memory buffer ·
- CVE-2009-3459, Adobe Acrobat and Reader: heap-based buffer overflow ·
Added the same day
- CVE-2021-40870, Aviatrix Controller: controller unrestricted upload of file ·
- CVE-2021-33766, Microsoft Exchange Server: information disclosure ·
- CVE-2021-32648, October CMS: improper authentication ·
- CVE-2021-25298, Nagios XI: XI OS command injection ·
- CVE-2021-25297, Nagios XI: XI OS command injection ·
Every F5 flaw on CISA’s list · Everything added in January 2022
References
- ^ Cybersecurity and Infrastructure Security Agency. “F5 BIG-IP Traffic Management Microkernel Buffer Overflow”. Known Exploited Vulnerabilities Catalog, added 18 Jan 2022.
- ^ National Institute of Standards and Technology. “CVE-2021-22991 Detail”. National Vulnerability Database, published 31 Mar 2021.
- ^ F5. Advisories and fixes: support.f5.com.
- ^ FIRST. “Exploit Prediction Scoring System”. EPSS 0.624, percentile 99.2, 31 Oct 2026.
- CVE Program. “CVE-2021-22991”. 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.