Cyber Resilience

CVE-2026-3591

Auth Bypass in Isc Bind 9.20.0 – 9.20.21

Auth Bypass
Published
25 March 2026
Modified
21 May 2026
Patch / advisory
View advisory
CVSS Score v3.1 5.4
Click a component to see what it means
Raw vectorCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N
EPSS Score 0.0036 29th percentile
Risk Priority 43 floored blend · peak EPSS

Summary

CVE-2026-3591 is a medium-severity Authentication Bypass by Primary Weakness (CWE-305) vulnerability in Isc Bind. Its CVSS base score is 5.4 (Medium).

Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked at the 29th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

The strongest mitigations our analysis identified map to IA-2 (Identification and Authentication (Organizational Users)) and IA-8 (Identification and Authentication (Non-organizational Users)) — see the control section below for these in your framework.

OWASP Top 10 for Web (2025)

A07:2025 Authentication Failures

EU & UK References

Vulnerability Data

A use-after-return vulnerability exists in the `named` server when handling DNS queries signed with SIG(0). Using a specially-crafted DNS request, an attacker may be able to cause an ACL to improperly (mis)match an IP address. In a default-allow ACL (denying…

more

only specific IP addresses), this may lead to unauthorized access. Default-deny ACLs should fail-secure. This issue affects BIND 9 versions 9.20.0 through 9.20.20, 9.21.0 through 9.21.19, and 9.20.9-S1 through 9.20.20-S1. BIND 9 versions 9.18.0 through 9.18.46 and 9.18.11-S1 through 9.18.46-S1 are NOT affected.

CWE(s)
CWE-305CWE-562

Related Threats

MITRE ATT&CK Enterprise Techniques

T1190 Exploit Public-Facing Application Initial Access
Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.
attack.mitre.org →
T1068 Exploitation for Privilege Escalation Privilege Escalation
Adversaries may exploit software vulnerabilities in an attempt to elevate privileges.
attack.mitre.org →
T1203 Exploitation for Client Execution Execution
Adversaries may exploit software vulnerabilities in client applications to execute code.
attack.mitre.org →
T1210 Exploitation of Remote Services Lateral Movement
Adversaries may exploit remote services to gain unauthorized access to internal systems once inside of a network.
attack.mitre.org →
T1212 Exploitation for Credential Access Credential Access
Adversaries may exploit software vulnerabilities in an attempt to collect credentials.
attack.mitre.org →
Derived from this CVE’s CWE(s) via the direct CWE→ATT&CK cross-walk.

CVEs Like This One

CVE-2026-3593Same product: Isc Bind
CVE-2026-5947Same product: Isc Bind
CVE-2026-5946Same product: Isc Bind
CVE-2026-5950Same product: Isc Bind
CVE-2026-1519Same product: Isc Bind
CVE-2026-3104Same product: Isc Bind
CVE-2026-3119Same product: Isc Bind
CVE-2026-3592Same product: Isc Bind
CVE-2026-3039Same product: Isc Bind
CVE-2023-6516Same product: Isc Bind

Affected Assets

isc
bind
9.20.0 — 9.20.21 · 9.21.0 — 9.21.20

Mitigating Controls

Control response

Prevent
Stop it (NIST 800-53)

—

Detect
Catch it (NIST detect / respond)

—

Harden
Shrink the surface (DISA STIG)
  • 5 hardening rules · 3 OS baselines
Validate
Prove the fix (OWASP ASVS)
  • V6.4.3
  • V7.2.4

Mitigating Controls (NIST 800-53 r5) AI

IA-2 Identification and Authentication (Organizational Users) good match

Requires unique identification and authentication of users, structurally blocking bypass of the mechanism.

IA-8 Identification and Authentication (Non-organizational Users) good match

Requires unique identification and authentication of non-organizational users, blocking bypass paths.

SA-11 Developer Testing and Evaluation good match

Developer testing and static/dynamic analysis directly find functions that return stack-variable addresses.

AC-3 Access Enforcement partial match

Enforces approved authorizations so a bypass weakness cannot be exploited to reach resources.

SA-15 Development Process, Standards, and Tools partial match

Requiring documented development standards and tools can embed coding rules that forbid returning addresses of automatic variables.

SA-8 Security and Privacy Engineering Principles partial match

Secure engineering principles include memory-safety and lifetime rules that structurally disallow returning addresses of stack locals.

Mitigating Controls (NIST CSF 2.0) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.

PR.PS-06 full match
prevents

Secure development practices throughout the SDLC prevent the primary weaknesses that enable authentication bypass.

PR.AA-03 mostly match
prevents

Strong authentication mechanisms and policy enforcement directly reduce bypass opportunities arising from implementation flaws.

ID.RA-01 partial match
prevents

Identifying and recording vulnerabilities catches the primary weaknesses that allow authentication bypass.

Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.

A.8.29 Security testing in development and acceptance mostly match
finds

Security testing can detect authentication bypass conditions before deployment.

A.8.25 Secure development life cycle partial match
prevents

Secure development lifecycle reduces likelihood of introducing bypass flaws during implementation.

A.8.27 Secure system architecture and engineering principles partial match
degrades

Secure architecture and engineering principles discourage unsafe stack usage patterns.

A.8.28 Secure coding partial match
prevents

Secure coding practices help prevent the primary weakness that enables authentication bypass.

A.8.3 Information access restriction partial match
mitigates

Information access restriction complements authentication by limiting what can be reached even if bypass occurs.

A.8.5 Secure authentication partial match
prevents

Secure authentication mechanisms directly address bypass risks in the implemented authentication process.

Hardening callouts derived

Configuration rules from DISA STIG baselines that bear on weaknesses of the type cited by this CVE. Each rule is shown with the relationship its mapping actually records, against the CWE it was authored against. Derived via CVE→CWE over `controls_xwalks` (authoritative rows only; rows rated `none` are excluded).

Ubuntu 22.04 (1 rule)
  • V-260470 Ubuntu 22.04 LTS, when booted, must require authentication upon booting into single-user and maintenance modes. prevents CWE-305
Ubuntu 24.04 (1 rule)
  • V-270675 Ubuntu 24.04 LTS when booted must require authentication upon booting into single-user and maintenance modes. prevents CWE-305

References