CVE-2026-39821: The ToASCII and ToUnicode functions incorrectly accept Punycode-encoded labels that decode to an ASCII-only la

Q: How do I fix CVE-2026-39821?

net: upgrade to 0.55.0. Exact commands for each OS are on this page.

Q: Is CVE-2026-39821 being exploited?

CVE-2026-39821 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2026-39821 affect?

CVE-2026-39821 affects net. Severity is critical (CVSS 9.6). 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

Q: How urgent is CVE-2026-39821?

Priority 53/100. Critical severity

This month

CRITICAL 9.6UPGRADE

CVE-2026-39821 is a critical-severity flaw (CVSS 9.6) affecting net. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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. For example, a program which performs privilege checks on the ASCII hostname may reject "example.com" but permit "xn--example-.com". If that pro

Why this priority

Critical severity

A fixed release or distro patch exists.

Risk facts

CVSS score	9.6 (Critical)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	Low
User interaction	None
EPSS exploit chance	1.8% (top 21%)
CISA exploited list	No
Weakness	CWE-1289
Published	2026-05-22
Last modified	2026-09-17

Publisher (CNA): Go Project · Vendor: golang

Last updated 2026-09-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-05-22 16:16 UTCPublished CVE record published
2026-09-17 12:18 UTCUpdated Record last modified

golang net UPGRADE

Affected: < 0.55.0

Fixed in: 0.55.0

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy net || rpm -q net || net --version

2. UPGRADE to 0.55.0 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade net

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade net

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

Advisories and coverage

Official advisories

AlmaLinux Errata: [ALSA-2026:37435] Important: golang security, bug fix, and enhancement update 2026-08-07
AlmaLinux Errata: [ALSA-2026:35829] Important: grafana-pcp security update 2026-07-08
AlmaLinux Errata: [ALSA-2026:34359] Important: opentelemetry-collector security update 2026-07-07
AlmaLinux Errata: [ALSA-2026:30854] Important: git-lfs security update 2026-06-30

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-39821?

net: upgrade to 0.55.0. Exact commands for each OS are on this page.

Is CVE-2026-39821 being exploited?

CVE-2026-39821 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-39821 affect?

CVE-2026-39821 affects net. Severity is critical (CVSS 9.6). 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

How urgent is CVE-2026-39821?

Priority 53/100. Critical severity

Software pages

Go (Golang) vulnerabilities

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References

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