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Full vulnerability report · 2021
CVE-2021-25216High confidence

A second vulnerability in BIND's GSSAPI security policy negotiation can be targeted by a buffer overflow attack

ISC · BIND9

8.1HighCVSS 3.1
Recommended action
Patch only the product branches with a verified fix

High technical severity; prioritise exposed affected systems while verifying vendor guidance. Verified remediation exists for at least one product or source, but 1 structured product or package state remain unresolved. Apply remediation only to the exact product branch confirmed by its source.

Fix availability varies by product
R
Operational reassessment

Published severity in operational context

Open reassessment dashboard →
Published severityHighOperational priority:Critical, raised one band.upgradedsince 6 Oct 2026

Evidence used

  • No CISA KEV confirmation is currently recorded.
  • The selected CVSS metric records a network-reachable, unauthenticated path with no user interaction.
  • EPSS is 82.37% for the current model date.

Compensating controls

  • Validate the affected product branch and deploy the verified fixed release.
  • Restrict the affected network interface to trusted sources where business-safe.
  • Monitor vendor guidance and exploitation sources for a material change.

Verification

  1. Confirm that the asset runs ISC BIND9 and falls inside the recorded affected range.
  2. Verify the installed build against the product-specific fixed version after deployment.
  3. Validate exposure, authentication requirements and compensating controls in the actual environment.
  4. Reopen this reassessment when CVSS, KEV, EPSS, exploit evidence or remediation changes.
Mitigation target: Within 3 daysRemediation target: Within 90 days

This automated reassessment organises public evidence. It does not know asset exposure, business impact or control effectiveness and does not replace CVSS or a human risk decision.

Cross-source reconciliation

Remediation availability differs by product scope

Verified remediation exists for at least one product or source, but 1 structured product or package state remain unresolved. Apply remediation only to the exact product branch confirmed by its source.

Distribution package intelligence

Release-specific package status

Alpine, Debian findings are scoped to the named distribution, release and source package. An absent finding does not mean a package is unaffected.

7 package states
Repository candidate not checked

A published vendor fix does not prove that a matching update is enabled and installable on a particular asset. Confirm the local package candidate before scheduling remediation.

Distribution releaseSource packageVendor stateFixed versionEvidence
Alpine v3.23v3.23 · mainbindVendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected.9.16.15-r0Alpine Security Database ↗Source updated 3 Oct 2026
Alpine v3.22v3.22 · mainbindVendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected.9.16.15-r0Alpine Security Database ↗Source updated 3 Oct 2026
Alpine v3.21v3.21 · mainbindVendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected.9.16.15-r0Alpine Security Database ↗Source updated 3 Oct 2026
Debian trixietrixie · sourcebind9Vendor fix publishedDebian records a fixed source-package version for this release.1:9.16.15-1Debian Security Tracker ↗Source updated 6 Oct 2026
Debian bookwormbookworm · sourcebind9Vendor fix publishedDebian records a fixed source-package version for this release.1:9.16.15-1Debian Security Tracker ↗Source updated 6 Oct 2026
Debian forkyforky · sourcebind9Vendor fix publishedDebian records a fixed source-package version for this release.1:9.16.15-1Debian Security Tracker ↗Source updated 6 Oct 2026
Debian sidsid · sourcebind9Vendor fix publishedDebian records a fixed source-package version for this release.1:9.16.15-1Debian Security Tracker ↗Source updated 6 Oct 2026
Direct vendor intelligence

Authoritative vendor CSAF and VEX advisories

Structured product status and remediation from the issuing vendor. Product-state explanations are always visible; large lists can be searched or downloaded.

1 current
SSA-389290 · CSAF 2.0 · revision 1 · finalSiemens ProductCERTSSA-389290: Third-Party Component Vulnerabilities in SINEC INS
1 known affected

The vendor explicitly identifies these products as affected by this CVE.

  • SINEC INS
Summary
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.
Remediation
Update to V1.0.1.1 or later version
Optional official sources

National CERT insights
?CERT means Computer Emergency Response Team; CSIRT is the closely related term Computer Security Incident Response Team.

Choose official national sources for this report. Each advisory shows its original language. Your selection is remembered on this device and included in shared links.

Official European source

ENISA European Vulnerability Database

Official EUVD identifiers, advisory evidence and known-exploited context. Missing fields are not treated as evidence of low risk.

1 current
ENISA EUVD identifier

EUVD-2021-12127

No EUVD known-exploited evidence

ENISA has published the identifier mapping but no EUVD description has been stored yet.

EUVD state
Present in the current official mapping
Known exploitation
Not present in the current ENISA EUVD known-exploited dataset. This is not proof of no exploitation.
ENISA score
Not supplied in the stored EUVD record
Advisory evidence
No linked advisory details stored yet
Recommended actionPatch only the product branches with a verified fix

High technical severity; prioritise exposed affected systems while verifying vendor guidance. Verified remediation exists for at least one product or source, but 1 structured product or package state remain unresolved. Apply remediation only to the exact product branch confirmed by its source.

Fix availability varies by product
01

What, why and how

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

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.

Why

The product reads data past the end, or before the beginning, of the intended buffer.

How

An attacker operating through a network path may attempt exploitation without authentication or user interaction. If successful, the issue may execute code or commands in the affected security context.

What

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.

Why

The product reads data past the end, or before the beginning, of the intended buffer.

How

An attacker operating through a network path may attempt exploitation without authentication or user interaction. If successful, the issue may execute code or commands in the affected security context.

02

Exploit reality and attack path

CVSS severity, EPSS forecast probability, public exploit material and CISA-confirmed exploitation are separate signals.

Observed exploitation
?Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.
No confirmed evidence

No CISA KEV match was present at the last successful refresh. This means no confirmation from that source, not proof of no exploitation.

Public PoC / exploit material
?Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.
None recorded

No exploit-tagged reference or CISA SSVC proof-of-concept state is currently recorded. Research may still exist outside the structured feeds.

Likely attack path
a network path → Out-of-bounds Read → execute code or commands in the affected security context
Attack surface
Network
Privileges required
None: unauthenticated exploitation is possible
User interaction
None
Attack complexity
High: exploitation depends on specific conditions
Security boundary
Unchanged: impact remains within the vulnerable component's security authority
Weakness
?CWE means Common Weakness Enumeration: a standard category for the underlying weakness.
CWE-125 ↗

CWE-125: Out-of-bounds Read. The product reads data past the end, or before the beginning, of the intended buffer.

CVSS vector
?CVSS means Common Vulnerability Scoring System. The vector records the metric values used to calculate technical severity.
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H

Common Vulnerability Scoring System 3.1: the compact vector below is decoded into plain language.

AVNetworkAttack vector: The vulnerable component can be reached over a network.ACHighAttack complexity: Successful exploitation depends on specific conditions outside the attacker's direct control.PRNonePrivileges required: The attacker does not need an account or existing privileges.UINoneUser interaction: No action by another user is required.SUnchangedScope: The security impact remains within the vulnerable component's authority.CHighConfidentiality impact: A successful attack can cause a major loss.IHighIntegrity impact: A successful attack can cause a major loss.AHighAvailability impact: A successful attack can cause a major loss.
Post-exploitation / living off the land
After compromise, an attacker may use built-in shells, scripting engines, scheduled tasks and native network utilities for discovery, persistence or movement. This is a plausible LoTL path, not evidence that it has occurred for every attack.
NetworkUnauthenticatedRemote code executionDenial of serviceCWE-125
A

Official authority intelligence

Only matched European and national findings are included. Language selectors and unavailable sources are omitted.

BSI · German · WID-SEC-2025-2027Internet Systems Consortium BIND: Mehrere Schwachstellen ermöglichen Denial of Service

Ein entfernter, anonymer Angreifer kann mehrere Schwachstellen in Internet Systems Consortium BIND ausnutzen, um einen Denial of Service Angriff durchzuführen.

Official advisory ↗
Canadian Centre for Cyber Security · English · AV21-193ISC BIND security advisory

On 28 April 2021 ISC published Security Advisories to address multiple vulnerabilities in the following product. Exploitation could allow an unauthenticated remote actor to trigger an error which could result in a denial of service and remote code execution.

Official advisory ↗
Cyber Security Agency of Singapore · English · CSA-SB-20210505Security Bulletin 05 May 2021

The Cyber Security Agency of Singapore included this CVE in its official Security Bulletin 05 May 2021, published on 5 May 2021. Open the linked bulletin for the product, severity and reference information published in that issue.

Official advisory ↗
CERT-FR · French · CERTFR-2022-AVI-339Vulnérabilité dans Synology DNS Server

Une vulnérabilité a été découverte dans Synology DNS Server. Elle permet à un attaquant de provoquer un déni de service à distance et une atteinte à la confidentialité des données.

Official advisory ↗
CERT-FR · French · CERTFR-2022-AVI-216Multiples vulnérabilités dans les produits Siemens

d?id=CVE-2021-23840 Référence CVE CVE-2021-25173 https://www.cve.org/CVERecord?id=CVE-2021-25173 Référence CVE CVE-2021-25174 https://www.cve.org/CVERecord?id=CVE-2021-25174 Référence CVE CVE-2021-25175 https://www.cve.org/CVERecord?id=CVE-2021-25175 Référence CVE CVE-2021-25176 https://www.cve.org/CVERecord?id=CVE-2021-25176 Référence CVE CVE-2021-25177 https://www.cve.org/CVERecord?id=CVE-2021-25177 Référence CVE CVE-2021-25178 https://www.cve.org/CVERecord?id=CVE-2021-25178 Référence CVE CVE-2021-25214 https://www.cve.org/CVERecord?id=CVE-2021-25214 Référence CVE CVE-2021-25215 https://www.cve.org/CVERecord?id=CVE-2021-25215 Référence CVE CVE-2021-25216 https://www.cve.org/CVERecord?id=CVE-2021-25216 Référence CVE CVE-2021-25217 https://www.cve.org/CVERecord?id=CVE-2021-25217 Référence CVE CVE-2021-25219 https://www.cve.org/CVERecord?id=CVE-2021-25219 Référence CVE CVE-2021-27290 https://www.cve.org/CVERecord?id=CVE-2021-27290 Référence CVE CVE-2021-31344 https://www.cve.org/CVERecord?id=CVE-2021-31344 Référence CVE CVE-2021-31346 https://www.cve.org/CVERecord?id=CVE-2021-31346 Référence CVE CVE-2021-31784 https://www.cve.org/CVERecord?id=CVE-2021-31784 Référence CVE CVE-2021-31889 https://www.cve.org/CVERecord?id=CVE-2021-31889 Référence CVE CVE-2021-31890 https://www.cve.org/CVERecord?id=

Official advisory ↗
CERT-FR · French · CERTFR-2021-AVI-325Multiples vulnérabilités dans BIND

De multiples vulnérabilités ont été découvertes dans BIND. Elles permettent à un attaquant de provoquer une exécution de code arbitraire à distance, un déni de service à distance, une atteinte à l'intégrité des données et une atteinte à la confidentialité des données.

Official advisory ↗
JVN iPedia · Japanese · JVNDB-2021-011159BIND の GSS-TSIG における境界外読み取りに関する脆弱性

BIND の GSS-TSIG には、tkey-gssapi-keytab または tkey-gssapi-credential オプションの値を明示的に設定している場合、境界外読み取りに関する脆弱性が存在します。

Official advisory ↗
JVN iPedia · Japanese · JVNDB-2021-006294BIND の named における到達可能なアサーションに関する脆弱性

BIND の named には、欠陥のトリガとなるレコードのクエリを受信した場合、アサーションチェックに失敗するため、到達可能なアサーションに関する脆弱性が存在します。

Official advisory ↗
KISA KrCERT/CC · Korean · KNVD-5288BIND DNS 취약점 보안 업데이트 권고

BIND DNS 서버에서 버퍼오버플로우로 인해 발생하는 서비스 거부 취약점(CVE-2021-25216) [1]

Official advisory ↗
03

Patch and workaround

Operational remediation based on structured source evidence.

Status
?Patch availability is based on structured fixed-version fields and authoritative update references. If no fix is verified, check the vendor advisory before making a change.
Fix availability varies by product
Affected
SINEC INS
Fixed
Action
Use the product-specific evidence above. Patch only products with a verified fixed release, and keep every affected or under-investigation state without a matching fix in the remediation queue.
Workaround
No verified workaround is recorded. If business-safe, reduce exposure to the affected interface and allow only trusted sources until authoritative guidance is available.
04

Evidence and provenance

Published 29 Apr 2021 · Last source change 16 Sept 2024, 22:25 UTC · CWE-125 · Out-of-bounds Read

CVE recordCVE.org · 5.1
CVSS sourceCNA
EPSS source
?The date BlackTree first stored a score for this CVE from the daily FIRST EPSS feed.
FIRST · tracked since 2026-08-14
European sourceENISA EUVD · EUVD-2021-12127
Product sourceVendor CSAF · Siemens ProductCERT
Remediation sourceVendor CSAF · Siemens ProductCERT
CWE sourceNIST NVD
NVD statusNVD modified after enrichment

Core structured fields are present and their contributing authorities are shown above.

Material change intelligence

What changed after publication

View recent updates ↗
  1. Affected versionsThe structured affected or fixed version information changed.
    Before
    Open Source Branches 9.5 though 9.11 9.5.0 through versions before 9.11.31; Open Source Branches 9.12 though 9.16 9.12.0 through versions before 9.16.14; Supported Preview Branch 9.11-S 9.11.3-S1 through versions before 9.11.31-S1; Supported Preview Branch 9.16-S 9.16.8-S1 through versions before 9.16.14-S1; Development Branch 9.17 9.17.0 through versions before 9.17.2 · Fixed: An authoritative update reference is available, but the fixed version is not recorded in the structured CVE fields. Check the linked vendor advisory for the applicable release.
    After
    Open Source Branches 9.5 though 9.11 9.5.0 through versions before 9.11.31; Open Source Branches 9.12 though 9.16 9.12.0 through versions before 9.16.14; Supported Preview Branch 9.11-S 9.11.3-S1 through versions before 9.11.31-S1; Supported Preview Branch 9.16-S 9.16.8-S1 through versions before 9.16.14-S1; Development Branch 9.17 9.17.0 through versions before 9.17.2 · Fixed: Update to V1.0.1.1 or later version
    Siemens ProductCERT ↗
Material fields only · duplicate refreshes suppressed · history retained for the configured operational retention period
CVE published
Fixed release or patch reference recorded
Fixed release recorded from official guidance
ENISA EUVD · EUVD-2021-12127↗Patch or update reference↗CVE authority or vendor reference↗Official CVE record (CNA and ADP data)↗NVD record↗
Technical terms and abbreviations used in this report
CVE
Common Vulnerabilities and Exposures: the public identifier for one disclosed vulnerability.
CVSS
Common Vulnerability Scoring System: a technical severity framework; it is not patching priority by itself.
EPSS
Exploit Prediction Scoring System: FIRST's estimate of the probability that exploitation activity will be observed in the next 30 days; it is a forecast, not confirmation.
CWE
Common Weakness Enumeration: the standard category describing the underlying software or hardware weakness.
CNA
CVE Numbering Authority: an organisation authorised to assign and publish CVE records.
CISA ADP
Cybersecurity and Infrastructure Security Agency Authorized Data Publisher: structured enrichment added to a CVE record.
NVD
National Vulnerability Database: NIST's enrichment service for CVE records.
CERT / CSIRT
A computer security incident response team that publishes warnings or coordinates incident response.
PoC
Proof of concept: public material that demonstrates or helps reproduce exploitation.
CSAF
Common Security Advisory Framework: a machine-readable format for security advisories.
LoTL
Living off the land: abuse of legitimate tools or system functions during an attack.
Free version - for non-commercial use only.CVE-2021-25216 · cve.blacktree.nl