# What is CVE-2021-22991?

> CVE-2021-22991 is a critical-severity flaw (CVSS 9.8) in Big-IP Access Policy Manager from F5, published on 31 March 2021. It is on the list of flaws known to be exploited, so criminals are already using it. The latest forecast puts its chance of use in the next 30 days at 61%. If you run an affected version, patch it first.

Canonical page: https://www.precursorintelligence.com/what-is/cve-2021-22991

## At a glance

- **Verdict:** Fix this now. Attackers are using this flaw today. If you run an affected version, it goes to the top of your list.
- **Precursor Intelligence risk score:** 90 out of 100 (Critical); action: Emergency patch. Kept up to date as the evidence changes.
- **Severity:** Critical (CVSS 9.8 out of 10)
- **Known to be exploited:** Yes, it is on the government list of flaws known to be exploited
- **Chance of use in the next 30 days:** 61% (latest exploit-likelihood forecast)
- **Published:** 31 March 2021
- **Record last updated:** 27 October 2025
- **Main product:** Big-IP Access Policy Manager from F5
- **Affected products:** 14

## What is CVE-2021-22991? In plain terms

It can be attacked from anywhere on the internet without logging in, and nobody has to click anything. If it works, the attacker can read data, change data and take it offline.

### The official description

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.

### What kind of flaw is it?

- CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer

### How an attack on it works

- Where an attacker can be: Anywhere on the internet
- How hard it is to pull off: Easy: no special conditions
- Login needed first: No login needed
- Does someone have to click: No, nobody has to do anything
- What an attacker gets: Can read data, change data and take it offline

Severity vector: `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`

## Is CVE-2021-22991 being exploited? Yes

Attackers are already using it. It is on the government list of flaws known to be exploited, and the latest forecast puts the chance of it being used in the next 30 days at 61%.

Exploit likelihood over the last 7 days: 61%, 61%, 61%, 61%, 61%, 61%, 61%.

### Timeline

- 31 March 2021: Published
- 11 June 2025: Exploit likelihood jumped
- 27 October 2025: Record last updated
- 22 November 2025: Exploit likelihood fell

### How Precursor Intelligence ranks it

CVE-2021-22991 scores **90 out of 100 (Critical)**, with the action "Emergency patch".

Every published flaw gets a risk score from 0 to 100, kept up to date as the evidence changes. Four things go into the score, from the one that counts most to the ones that count less:

- Evidence it is being exploited: biggest factor
- How severe it is: strong factor
- How widely exposed it is: smaller factor
- Threat activity around it: smaller factor

85 and above is Critical. Anything confirmed as exploited is always marked for an emergency patch, whatever its score. When this flaw matches software you track, it lands in your list with its score and the reasons behind it.

## Who is affected by CVE-2021-22991?

The published record lists 14 products. Versions are as published: "12.1.0 to before 12.1.5.3" means from 12.1.0 up to, but not including, 12.1.5.3.

- [Big-IP Access Policy Manager](https://www.precursorintelligence.com/product/f5/big-ip_access_policy_manager) by [F5](https://www.precursorintelligence.com/vendors/f5) (Application): 12.1.0 to before 12.1.5.3, 13.1.0 to before 13.1.3.6, 14.1.0 to before 14.1.4, 15.1.0 to before 15.1.2.1, 16.0.0 to before 16.0.1.1
- [Big-IP Advanced Firewall Manager](https://www.precursorintelligence.com/product/f5/big-ip_advanced_firewall_manager) by [F5](https://www.precursorintelligence.com/vendors/f5) (Application): 12.1.0 to before 12.1.5.3, 13.1.0 to before 13.1.3.6, 14.1.0 to before 14.1.4, 15.1.0 to before 15.1.2.1, 16.0.0 to before 16.0.1.1
- [Big-IP Advanced Web Application Firewall](https://www.precursorintelligence.com/product/f5/big-ip_advanced_web_application_firewall) by [F5](https://www.precursorintelligence.com/vendors/f5) (Application): 12.1.0 to before 12.1.5.3, 13.1.0 to before 13.1.3.6, 14.1.0 to before 14.1.4, 15.1.0 to before 15.1.2.1, 16.0.0 to before 16.0.1.1
- [Big-IP Analytics](https://www.precursorintelligence.com/product/f5/big-ip_analytics) by [F5](https://www.precursorintelligence.com/vendors/f5) (Application): 12.1.0 to before 12.1.5.3, 13.1.0 to before 13.1.3.6, 14.1.0 to before 14.1.4, 15.1.0 to before 15.1.2.1, 16.0.0 to before 16.0.1.1
- [Big-IP Application Acceleration Manager](https://www.precursorintelligence.com/product/f5/big-ip_application_acceleration_manager) by [F5](https://www.precursorintelligence.com/vendors/f5) (Application): 12.1.0 to before 12.1.5.3, 13.1.0 to before 13.1.3.6, 14.1.0 to before 14.1.4, 15.1.0 to before 15.1.2.1, 16.0.0 to before 16.0.1.1
- [Big-IP Application Security Manager](https://www.precursorintelligence.com/product/f5/big-ip_application_security_manager) by [F5](https://www.precursorintelligence.com/vendors/f5) (Application): 12.1.0 to before 12.1.5.3, 13.1.0 to before 13.1.3.6, 14.1.0 to before 14.1.4, 15.1.0 to before 15.1.2.1, 16.0.0 to before 16.0.1.1
- [Big-IP DDOS Hybrid Defender](https://www.precursorintelligence.com/product/f5/big-ip_ddos_hybrid_defender) by [F5](https://www.precursorintelligence.com/vendors/f5) (Application): 12.1.0 to before 12.1.5.3, 13.1.0 to before 13.1.3.6, 14.1.0 to before 14.1.4, 15.1.0 to before 15.1.2.1, 16.0.0 to before 16.0.1.1
- [Big-IP Domain Name System](https://www.precursorintelligence.com/product/f5/big-ip_domain_name_system) by [F5](https://www.precursorintelligence.com/vendors/f5) (Application): 12.1.0 to before 12.1.5.3, 13.1.0 to before 13.1.3.6, 14.1.0 to before 14.1.4, 15.1.0 to before 15.1.2.1, 16.0.0 to before 16.0.1.1
- [Big-IP Fraud Protection Service](https://www.precursorintelligence.com/product/f5/big-ip_fraud_protection_service) by [F5](https://www.precursorintelligence.com/vendors/f5) (Application): 12.1.0 to before 12.1.5.3, 13.1.0 to before 13.1.3.6, 14.1.0 to before 14.1.4, 15.1.0 to before 15.1.2.1, 16.0.0 to before 16.0.1.1
- [Big-IP Global Traffic Manager](https://www.precursorintelligence.com/product/f5/big-ip_global_traffic_manager) by [F5](https://www.precursorintelligence.com/vendors/f5) (Application): 12.1.0 to before 12.1.5.3, 13.1.0 to before 13.1.3.6, 14.1.0 to before 14.1.4, 15.1.0 to before 15.1.2.1, 16.0.0 to before 16.0.1.1
- [Big-IP Link Controller](https://www.precursorintelligence.com/product/f5/big-ip_link_controller) by [F5](https://www.precursorintelligence.com/vendors/f5) (Application): 12.1.0 to before 12.1.5.3, 13.1.0 to before 13.1.3.6, 14.1.0 to before 14.1.4, 15.1.0 to before 15.1.2.1, 16.0.0 to before 16.0.1.1
- [Big-IP Local Traffic Manager](https://www.precursorintelligence.com/product/f5/big-ip_local_traffic_manager) by [F5](https://www.precursorintelligence.com/vendors/f5) (Application): 12.1.0 to before 12.1.5.3, 13.1.0 to before 13.1.3.6, 14.1.0 to before 14.1.4, 15.1.0 to before 15.1.2.1, 16.0.0 to before 16.0.1.1
- [Big-IP Policy Enforcement Manager](https://www.precursorintelligence.com/product/f5/big-ip_policy_enforcement_manager) by [F5](https://www.precursorintelligence.com/vendors/f5) (Application): 12.1.0 to before 12.1.5.3, 13.1.0 to before 13.1.3.6, 14.1.0 to before 14.1.4, 15.1.0 to before 15.1.2.1, 16.0.0 to before 16.0.1.1
- [SSL Orchestrator](https://www.precursorintelligence.com/product/f5/ssl_orchestrator) by [F5](https://www.precursorintelligence.com/vendors/f5) (Application): 12.1.0 to before 12.1.5.3, 13.1.0 to before 13.1.3.6, 14.1.0 to before 14.1.4, 15.1.0 to before 15.1.2.1, 16.0.0 to before 16.0.1.1

## How do I fix CVE-2021-22991?

1. **Check whether you run it.** Compare your versions with the affected products above. Not sure what you run? Precursor Intelligence can work it out from your public website, or you can pick it from a list.
2. **Apply the vendor’s fix.** Install the update named in the vendor’s advisory. We’ve put the advisory first in the list.
3. **If you can’t patch today.** Follow any mitigation the vendor publishes, and limit who can reach the affected system from the internet until the update is in.
4. **Check it wasn’t used before you patched.** Because criminals are already using this flaw, look for signs of a break-in on affected systems using the vendor’s guidance.

### References

- [support.f5.com](https://support.f5.com/csp/article/K56715231) (Vendor advisory)
- [cisa.gov](https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2021-22991) (Government resource)

## Questions

### What is CVE-2021-22991?

CVE-2021-22991 is a critical-severity flaw in Big-IP Access Policy Manager from F5, published on 31 March 2021. It can be attacked from anywhere on the internet without logging in, and nobody has to click anything. If it works, the attacker can read data, change data and take it offline.

### How serious is CVE-2021-22991?

It scores 9.8 out of 10 on the CVSS severity scale, which is critical. Our risk score for CVE-2021-22991 is 90 out of 100, which is Critical. Its action is "Emergency patch". Attackers are using this flaw today. If you run an affected version, it goes to the top of your list.

### Is CVE-2021-22991 being exploited?

Attackers are already using it. It is on the government list of flaws known to be exploited, and the latest forecast puts the chance of it being used in the next 30 days at 61%.

### Which versions does CVE-2021-22991 affect?

For Big-IP Access Policy Manager from F5: 12.1.0 to before 12.1.5.3, 13.1.0 to before 13.1.3.6, 14.1.0 to before 14.1.4, 15.1.0 to before 15.1.2.1, 16.0.0 to before 16.0.1.1. The record lists 14 products in all, each with its versions, under "Who is affected" on this page.

### How do I fix CVE-2021-22991?

Install the update the vendor names in its advisory, linked first under "How do I fix" on this page. If you can't patch today, follow the vendor's mitigation and limit who can reach the affected system from the internet. Because criminals are already using it, also check affected systems for signs of a break-in.

## Does CVE-2021-22991 affect you?

Precursor Intelligence keeps checking for you:

- **Tell us what you run.** Pick your software from a list, or let us work it out from your website.
- **We match it to every flaw.** Over 400,000 published flaws, checked against your list.
- **Ranked by real-world risk.** Every flaw gets a 0 to 100 risk score, kept up to date, led by evidence criminals are using it.
- **Told as soon as we know.** An alert by email, in the app or by webhook to your own tools.

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