# What is CVE-2021-25216?

> CVE-2021-25216 is a critical-severity flaw (CVSS 9.8) in Bind from Isc, published on 29 April 2021. It is not on the list of flaws known to be exploited today. The latest forecast puts its chance of use in the next 30 days at 82%. If you run an affected version, plan in the vendor’s fix.

Canonical page: https://www.precursorintelligence.com/what-is/cve-2021-25216

## At a glance

- **Verdict:** Fix this soon. Not confirmed as exploited yet, but serious or likely enough that it should not wait for a quiet month.
- **Precursor Intelligence risk score:** 76 out of 100 (High); action: Patch now. Kept up to date as the evidence changes.
- **Severity:** Critical (CVSS 9.8 out of 10)
- **Known to be exploited:** No, not on the list of flaws known to be exploited today
- **Chance of use in the next 30 days:** 82% (latest exploit-likelihood forecast)
- **Published:** 29 April 2021
- **Record last updated:** 3 May 2022
- **Main product:** Bind from Isc
- **Affected products:** 23

## What is CVE-2021-25216? 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

In BIND 9.5.0 -> 9.11.29, 9.12.0 -> 9.16.13, and versions BIND 9.11.3-S1 -> 9.11.29-S1 and 9.16.8-S1 -> 9.16.13-S1 of BIND Supported Preview Edition, as well as release versions 9.17.0 -> 9.17.1 of the BIND 9.17 development branch, BIND servers are vulnerable if they are running an affected version and are configured to use GSS-TSIG features. In a configuration which uses BIND's default settings the vulnerable code path is not exposed, but a server can be rendered vulnerable by explicitly setting values for the tkey-gssapi-keytab or tkey-gssapi-credential configuration options. Although the default configuration is not vulnerable, GSS-TSIG is frequently used in networks where BIND is integrated with Samba, as well as in mixed-server environments that combine BIND servers with Active Directory domain controllers. For servers that meet these conditions, the ISC SPNEGO implementation is vulnerable to various attacks, depending on the CPU architecture for which BIND was built: For named binaries compiled for 64-bit platforms, this flaw can be used to trigger a buffer over-read, leading to a server crash. For named binaries compiled for 32-bit platforms, this flaw can be used to trigger a server crash due to a buffer overflow and possibly also to achieve remote code execution. We have determined that standard SPNEGO implementations are available in the MIT and Heimdal Kerberos libraries, which support a broad range of operating systems, rendering the ISC implementation unnecessary and obsolete. Therefore, to reduce the attack surface for BIND users, we will be removing the ISC SPNEGO implementation in the April releases of BIND 9.11 and 9.16 (it had already been dropped from BIND 9.17). We would not normally remove something from a stable ESV (Extended Support Version) of BIND, but since system libraries can replace the ISC SPNEGO implementation, we have made an exception in this case for reasons of stability and security.

### What kind of flaw is it?

- CWE-125: Out-of-bounds Read

### 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-2021-25216 being exploited? Not that we know of

It is not on the list of flaws known to be exploited today. The latest forecast puts the chance of it being used in the next 30 days at 82%. That can change at any time, which is why we keep watching.

Exploit likelihood over the last 7 days: 82%, 82%, 82%, 82%, 82%, 82%, 82%.

### Timeline

- 29 April 2021: Published
- 3 May 2022: Record last updated
- 11 June 2025: Exploit likelihood jumped
- 5 July 2025: Exploit likelihood fell

### How Precursor Intelligence ranks it

CVE-2021-25216 scores **76 out of 100 (High)**, with the action "Patch now".

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-2021-25216?

The published record lists 23 products. Versions are as published: "9.0.0 to before 9.11.31" means from 9.0.0 up to, but not including, 9.11.31.

- [Bind](https://www.precursorintelligence.com/product/isc/bind) by [Isc](https://www.precursorintelligence.com/vendors/isc) (Application): 9.0.0 to before 9.11.31, 9.12.0 to before 9.16.15, 9.17.0 to before 9.17.12
- [Debian Linux](https://www.precursorintelligence.com/product/debian/debian_linux) by [Debian](https://www.precursorintelligence.com/vendors/debian) (Operating system): 9.0, 10.0
- [Active Iq Unified Manager](https://www.precursorintelligence.com/product/netapp/active_iq_unified_manager) by [Netapp](https://www.precursorintelligence.com/vendors/netapp) (Application): version not specified
- [Aff 500f](https://www.precursorintelligence.com/product/netapp/aff_500f) by [Netapp](https://www.precursorintelligence.com/vendors/netapp) (Hardware): version not specified
- [Aff 500f Firmware](https://www.precursorintelligence.com/product/netapp/aff_500f_firmware) by [Netapp](https://www.precursorintelligence.com/vendors/netapp) (Operating system): version not specified
- [Aff A250](https://www.precursorintelligence.com/product/netapp/aff_a250) by [Netapp](https://www.precursorintelligence.com/vendors/netapp) (Hardware): version not specified
- [Aff A250 Firmware](https://www.precursorintelligence.com/product/netapp/aff_a250_firmware) by [Netapp](https://www.precursorintelligence.com/vendors/netapp) (Operating system): version not specified
- [Cloud Backup](https://www.precursorintelligence.com/product/netapp/cloud_backup) by [Netapp](https://www.precursorintelligence.com/vendors/netapp) (Application): version not specified
- [H300e](https://www.precursorintelligence.com/product/netapp/h300e) by [Netapp](https://www.precursorintelligence.com/vendors/netapp) (Hardware): version not specified
- [H300e Firmware](https://www.precursorintelligence.com/product/netapp/h300e_firmware) by [Netapp](https://www.precursorintelligence.com/vendors/netapp) (Operating system): version not specified
- [H300s](https://www.precursorintelligence.com/product/netapp/h300s) by [Netapp](https://www.precursorintelligence.com/vendors/netapp) (Hardware): version not specified
- [H300s Firmware](https://www.precursorintelligence.com/product/netapp/h300s_firmware) by [Netapp](https://www.precursorintelligence.com/vendors/netapp) (Operating system): version not specified
- [H410s](https://www.precursorintelligence.com/product/netapp/h410s) by [Netapp](https://www.precursorintelligence.com/vendors/netapp) (Hardware): version not specified
- [H410s Firmware](https://www.precursorintelligence.com/product/netapp/h410s_firmware) by [Netapp](https://www.precursorintelligence.com/vendors/netapp) (Operating system): version not specified
- [H500e](https://www.precursorintelligence.com/product/netapp/h500e) by [Netapp](https://www.precursorintelligence.com/vendors/netapp) (Hardware): version not specified
- [H500e Firmware](https://www.precursorintelligence.com/product/netapp/h500e_firmware) by [Netapp](https://www.precursorintelligence.com/vendors/netapp) (Operating system): version not specified
- [H500s](https://www.precursorintelligence.com/product/netapp/h500s) by [Netapp](https://www.precursorintelligence.com/vendors/netapp) (Hardware): version not specified
- [H500s Firmware](https://www.precursorintelligence.com/product/netapp/h500s_firmware) by [Netapp](https://www.precursorintelligence.com/vendors/netapp) (Operating system): version not specified
- [H700e](https://www.precursorintelligence.com/product/netapp/h700e) by [Netapp](https://www.precursorintelligence.com/vendors/netapp) (Hardware): version not specified
- [H700e Firmware](https://www.precursorintelligence.com/product/netapp/h700e_firmware) by [Netapp](https://www.precursorintelligence.com/vendors/netapp) (Operating system): version not specified
- [H700s](https://www.precursorintelligence.com/product/netapp/h700s) by [Netapp](https://www.precursorintelligence.com/vendors/netapp) (Hardware): version not specified
- [H700s Firmware](https://www.precursorintelligence.com/product/netapp/h700s_firmware) by [Netapp](https://www.precursorintelligence.com/vendors/netapp) (Operating system): version not specified
- [Sinec Infrastructure Network Services](https://www.precursorintelligence.com/product/siemens/sinec_infrastructure_network_services) by [Siemens](https://www.precursorintelligence.com/vendors/siemens) (Application): before 1.0.1.1

## How do I fix CVE-2021-25216?

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. **Keep an eye on it.** Track the product and we tell you as soon as we spot this flaw on the list of exploited flaws, or a jump in its exploit likelihood.

### References

- [cert-portal.siemens.com](https://cert-portal.siemens.com/productcert/pdf/ssa-389290.pdf) (Patch, Third-party advisory)
- [openwall.com](http://www.openwall.com/lists/oss-security/2021/04/29/1) (Mailing list, Third-party advisory)
- [openwall.com](http://www.openwall.com/lists/oss-security/2021/04/29/2) (Mailing list, Third-party advisory)
- [openwall.com](http://www.openwall.com/lists/oss-security/2021/04/29/3) (Mailing list, Third-party advisory)
- [openwall.com](http://www.openwall.com/lists/oss-security/2021/04/29/4) (Mailing list, Third-party advisory)
- [kb.isc.org](https://kb.isc.org/v1/docs/cve-2021-25215) (Not applicable)
- [lists.debian.org](https://lists.debian.org/debian-lts-announce/2021/05/msg00001.html) (Mailing list, Third-party advisory)
- [security.netapp.com](https://security.netapp.com/advisory/ntap-20210521-0006/) (Third-party advisory)
- [debian.org](https://www.debian.org/security/2021/dsa-4909) (Third-party advisory)
- [zerodayinitiative.com](https://www.zerodayinitiative.com/advisories/ZDI-21-657/) (Third-party advisory, Vulnerability database)

## Questions

### What is CVE-2021-25216?

CVE-2021-25216 is a critical-severity flaw in Bind from Isc, published on 29 April 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-2021-25216?

It scores 9.8 out of 10 on the CVSS severity scale, which is critical. Our risk score for CVE-2021-25216 is 76 out of 100, which is High. Its action is "Patch now". Not confirmed as exploited yet, but serious or likely enough that it should not wait for a quiet month.

### Is CVE-2021-25216 being exploited?

It is not on the list of flaws known to be exploited today. The latest forecast puts the chance of it being used in the next 30 days at 82%. That can change at any time, which is why we keep watching.

### Which versions does CVE-2021-25216 affect?

For Bind from Isc: 9.0.0 to before 9.11.31, 9.12.0 to before 9.16.15, 9.17.0 to before 9.17.12. The record lists 23 products in all, each with its versions, under "Who is affected" on this page.

### How do I fix CVE-2021-25216?

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.

## Does CVE-2021-25216 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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