Security readout for executives and security teams
Plain-English summary
This issue affects BIG-IP appliances and can let unauthenticated network requests crash the Traffic Management Microkernel, causing service outage. Sources also note theoretical URL-based access control bypass or remote code execution in certain situations. Because CISA lists it as known exploited, exposed affected systems need urgent attention.
Executive priority
Treat this as an urgent infrastructure risk. BIG-IP often sits in front of critical applications, and successful exploitation can cause outage; CISA KEV status increases remediation priority for any exposed affected deployment.
Technical view
TMM URI normalization may incorrectly handle undisclosed requests to a virtual server, triggering a buffer overflow classified as CWE-119. Affected BIG-IP versions are 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.
Likely exposure
Exposure is likely where affected BIG-IP virtual servers are reachable by untrusted networks. Internet-facing load balancers, application delivery controllers, and access-control front ends are the highest-priority assets to review.
Exploitation context
CISA KEV inclusion supports active exploitation in the wild. The public source bundle confirms unauthenticated network attack conditions and DoS impact, with theoretical access-control bypass or RCE in certain situations, but does not provide exploit details.
Researcher notes
The sources name TMM URI normalization and buffer overflow behavior but withhold request details. Avoid assuming confirmed RCE from the available evidence; the source states RCE and URL-based access-control bypass are theoretical in certain situations.
Mitigation direction
- Upgrade BIG-IP to the fixed version for the deployed branch.
- Prioritize internet-facing BIG-IP systems and critical application front ends.
- Check F5 advisory K56715231 for current vendor guidance and branch-specific instructions.
- Assess End of Software Development versions separately; the CVE says they were not evaluated.
- Restrict exposure of affected virtual servers where immediate upgrade is delayed.
Validation and detection
- Inventory BIG-IP versions and compare them to the affected version ranges.
- Identify virtual servers reachable from the internet or untrusted networks.
- Confirm remediated systems run fixed releases or vendor-approved newer versions.
- Review change records for upgrades after the 2021 advisory date.
- Check vulnerability management tools for CVE-2021-22991 detection coverage.
Public sources used
Michael Williams reviewed this cited source version on .
Conservative CVE-to-ATT&CK context
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ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CWE-119: Exact CWE lookup
Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupExecution behavior lookup
The CVE wording references code or command execution, so execution technique review may help defensive triage. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.
Open ATT&CK lookupCVE-2021-22991 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- Critical
- CVSS
- 9.8 (3.1)
- Known Exploited
- Yes
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CISA KEV status
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H3.95.9Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
9.8CriticalVector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Source materials
- CVE List V5 sourceCVE List V5
- https://support.f5.com/csp/article/K56715231CVE reference · x_refsource_MISC
- https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-22991CVE reference · government-resource
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Improper Restriction of Operations within the Bounds of a Memory Buffer
Improper Restriction of Operations within the Bounds of a Memory Buffer represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.