Vulnary
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Critical· 9.6

CVE-2026-39821

The Go net package’s IDNA handling incorrectly accepts Punycode labels that decode to plain ASCII, allowing a malicious name to bypass security checks. This flaw can let attackers impersonate legitimate hostnames and gain unauthorized access.

publishedMay 22, 2026
last modifiedAug 4, 2026
sourcesNVD
severity · cvss
9.6
critical · how bad it is
exploitation · epss
<1%
48th percentile · chance of exploitation in 30 days
(ai-assisted) A model wrote this summary from the official data, so double-check it against the source before you act on it. Read the official advisory →
auto-deletes from the system
counting…on Sep 1, 2026

No official fix yet. If none appears within 45 days of first tracking, this entry is removed automatically.

01

Who is affected

Go developers using the standard net package, especially those that perform hostname privilege checks.

02

Real-world impact

An attacker can craft a Punycode hostname that looks like a trusted domain, trick a program into granting access, and then use the Unicode form to access resources it should not reach.

03

Why this severity

The CVSS score is high because the flaw is network‑accessible, requires low effort, and can be exploited without user interaction. It changes the scope of the vulnerability, and it compromises both confidentiality and integrity of the system.

04

What to do about it

no official fix yet

No official fix or mitigation is documented in the sources yet. Monitor the vendor advisory and apply the patch as soon as it is released.

No fix documented in sources

05

Timeline

  1. May 22, 2026 · May 22, 2026
    Published
    Disclosed and added to the National Vulnerability Database.
  2. Aug 4, 2026 · 7h ago
    Advisory updated
    The NVD record was last revised.
06

How it’s attacked

Attack vectorNetwork (remote)
Attack complexityLow
Privileges requiredLow
User interactionNone needed
ScopeChanged
Confidentiality impactHigh
Integrity impactHigh
Availability impactNone
07

References & advisories

NVD advisory ↗Search advisories ↗
(ai-assisted) A model wrote this summary from the official data, so double-check it against the source before you act on it. Read the official advisory →