Cyber Resilience

CVE-2026-39821

Golang Net ≤ 0.55.0

Published
22 May 2026
Modified
13 August 2026
Patch / advisory
View advisory
CVSS Score v3.1 9.6
Click a component to see what it means
Raw vectorCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N
EPSS Score 0.0066 48th percentile
Risk Priority 67 floored blend · peak EPSS

Summary

CVE-2026-39821 is a critical-severity Improper Validation of Unsafe Equivalence in Input (CWE-1289) vulnerability in Golang Net. Its CVSS base score is 9.6 (Critical).

Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked at the 48th 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 SI-10 (Information Input Validation) — see the control section below for these in your framework.

EU & UK References

Vulnerability Data

The ToASCII and ToUnicode functions incorrectly accept Punycode-encoded labels that decode to an ASCII-only label. For example, ToUnicode("xn--example-.com") incorrectly returns the name "example.com" rather than an error. This behavior can lead to privilege escalation in programs using the idna package.…

more

For example, a program which performs privilege checks on the ASCII hostname may reject "example.com" but permit "xn--example-.com". If that program subsequently converts the ASCII hostname to Unicode, it will inadvertently permits access to the Unicode name "example.com".

CWE(s)
CWE-1289

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 →
Derived from this CVE’s CWE(s) via the direct CWE→ATT&CK cross-walk.

CVEs Like This One

CVE-2026-42506Same product: Golang Net
CVE-2026-25681Same product: Golang Net
CVE-2026-25680Same product: Golang Net
CVE-2026-27136Same product: Golang Net
CVE-2026-42502Same product: Golang Net
CVE-2026-27139Same vendor: Golang
CVE-2026-33810Same vendor: Golang
CVE-2025-22873Same vendor: Golang
CVE-2026-49942Shared CWE-1289
CVE-2026-1094Shared CWE-1289

Affected Assets

golang
net
≤ 0.55.0

Mitigating Controls

Control response

Prevent
Stop it (NIST 800-53)

—

Detect
Catch it (NIST detect / respond)

—

Harden
Shrink the surface (DISA STIG)

—

Validate
Prove the fix (OWASP ASVS)
  • V5.2.2
  • V9.1.3
  • V10.4.6
  • V2.2.2

Mitigating Controls (NIST 800-53 r5) AI

SI-10 Information Input Validation good match

SI-10 requires validity checks on information inputs, directly stopping unsafe equivalence validation failures for identifiers and references.

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 mostly match
prevents

Secure SDLC practices directly require proper input validation and equivalence checking to prevent this class of flaw.

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 unsafe equivalence flaws but does not prevent them at design time.

A.8.26 Application security requirements partial match
prevents

Application security requirements can mandate input validation rules that prevent unsafe equivalence checks.

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

Secure architecture principles include proper input validation and canonicalization to avoid unsafe equivalence.

A.8.28 Secure coding partial match
prevents

Secure coding standards directly require correct validation of resource identifiers and equivalence checks.

References