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No exploitation reportedFix available

CVE-2026-39821

No exploitation reported by the sources we read.

  • Published2026-05-22
  • Fixed in0.55.0
  • WeaknessCWE-1289

Am I affected?

With a domain, we read the version its public headers and pages expose. A distribution build can carry the fix under an older number.

Description

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 program subsequently converts the ASCII hostname to Unicode, it will inadvertently permits access to the Unicode name "example.com".

Timeline

dates in UTC
  1. 2026-05-2216:16CVE publishedNVD

Observed activity

isMalicious corpus · updated 2026-10-23 18:48 UTC
Indicators tagged with this CVE, per day0 indicators over 7 days
Block these indicators
  • Firewall list (EDL, text): https://api.ismalicious.com/edl/cve/CVE-2026-39821?format=txt
  • CSV export: https://api.ismalicious.com/edl/cve/CVE-2026-39821?format=csv
  • TAXII collection of exploited CVEs
  • The EDL and CSV take an API key on the Pro plan.

Versions per branch

NVD · OSV
Golang net
BranchAffectedFixed in
0.55< 0.55.00.55.0
stdlib
BranchAffectedFixed in
1.25< 1.25.131.25.13

as publishedRanges as NVD and OSV publish them; a distribution’s build can be fixed under an older number.

Distribution builds and vendor advisories

Packages from OSV.dev and products from the vendors' CSAF advisories.

version not verifiedVersions not verified: read as OSV.dev and the vendors publish them, not checked against each vendor's release list.

Packages

  • delveAlmaLinux:8all versions · fixed in 1.26.1-1.module_el8.10.0+4223+bd807c2e
  • git-lfsAlmaLinux:10all versions · fixed in 3.7.1-5.el10_2.5
  • git-lfsAlmaLinux:9all versions · fixed in 3.7.1-4.el9_8.1
  • git-lfsAlmaLinux:8all versions · fixed in 3.4.1-11.el8_10
  • go-fdo-serverAlmaLinux:10all versions · fixed in 1.0.1-2.el10_2.3and 3 other packages
  • golangAlmaLinux:10all versions · fixed in 1.26.5-1.el10_2.alma.1and 7 other packages
  • golangAlmaLinux:9all versions · fixed in 1.26.5-1.el9_8and 7 other packages
  • golangAlmaLinux:8all versions · fixed in 1.26.5-1.module_el8.10.0+4223+bd807c2eand 7 other packages

12 more package ranges on OSV.dev

Vendor advisories

  • CVE-2026-39821 Red Hat Product Security2,233 products fixed · 149 known affected

    golang.org/x/net/idna: golang: net/http: golang.org/x/net/idna: Privilege escalation via incorrect Punycode label processing

    rhai/assisted-installer-rhel9 as a component of Assisted Installer for Red Hat OpenShift Container Platform 2 · cert-manager/cert-manager-operator-rhel9 as a component of cert-manager Operator for Red Hat OpenShift · compliance/openshift-compliance-operator-bundle as a component of Compliance Operator · +146 more

  • RHSA-2026:87410 Red Hat Product Security3 products fixed

    Red Hat Security Advisory: RHOAI 2.25.12 - Red Hat OpenShift AI

  • RHSA-2026:79150 Red Hat Product Security10 products fixed

    Red Hat Security Advisory: OpenShift Container Platform 4.22.18 packages and security update

  • RHSA-2026:84734 Red Hat Product Security19 products fixed

    Red Hat Security Advisory: RHOAI 3.4.5 - Red Hat OpenShift AI

  • RHSA-2026:77611 Red Hat Product Security10 products fixed

    Red Hat Security Advisory: OpenShift Container Platform 4.16.73 packages and security update

  • RHSA-2026:79039 Red Hat Product Security4 products fixed

    Red Hat Security Advisory: OpenShift Container Platform 4.18.57 bug fix and security update

175 more advisories

Full record on NVD

Fix

  1. Update Golang net to 0.55.0
  2. Vendor fix Red Hat Product Security RHSA-2026:87410 · 3 productsFor Red Hat OpenShift AI 2.25.12 see the following documentation, which will be updated shortly for this release, for important instructions on how to upgrade your cluster and fully apply this errata update: https://docs.redhat.com/en/documentation/red_hat_openshift_ai/
  3. Vendor fix Red Hat Product Security RHSA-2026:79150 · 10 productsFor OpenShift Container Platform 4.22 see the following documentation, which will be updated shortly for this release, for important instructions on how to upgrade your cluster and fully apply this asynchronous errata update: https://docs.redhat.com/en/documentation/openshift_container_platform/4.…
  4. Vendor fix Red Hat Product Security RHSA-2026:84734 · 19 productsFor Red Hat OpenShift AI 3.4.5 see the following documentation, which will be updated shortly for this release, for important instructions on how to upgrade your cluster and fully apply this errata update: https://docs.redhat.com/en/documentation/red_hat_openshift_ai/
  5. Vendor fix Red Hat Product Security RHSA-2026:77611 · 10 productsFor OpenShift Container Platform 4.16 see the following documentation, which will be updated shortly for this release, for important instructions on how to upgrade your cluster and fully apply this asynchronous errata update: https://docs.redhat.com/en/documentation/openshift_container_platform/4.…
  6. Vendor fix Red Hat Product Security RHSA-2026:79039 · 4 productsFor OpenShift Container Platform 4.18 see the following documentation, which will be updated shortly for this release, for important instructions on how to upgrade your cluster and fully apply this asynchronous errata update: https://docs.redhat.com/en/documentation/openshift_container_platform/4.…
  7. Vendor fix Red Hat Product Security RHSA-2026:77552 · 4 productsFor OpenShift Container Platform 4.17 see the following documentation, which will be updated shortly for this release, for important instructions on how to upgrade your cluster and fully apply this asynchronous errata update: https://docs.redhat.com/en/documentation/openshift_container_platform/4.…

Advisories and patches

Detection

Nuclei template
none so far
ProjectDiscovery
Exploit-DB
no entry
Exploit-DB
Proof-of-concept repositories
none listed
PoC-in-GitHub

Sources

Technical details

CVSS v3 Vector
CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:N
Published
2026-05-22
Last Modified
2026-09-17
NVD status
Modified
MSRC Title
Invoking failure to reject ASCII-only Punycode-encoded labels in golang.org/x/net/idna

Frequently asked questions

What is CVE-2026-39821?

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 program subsequently converts the ASCII hostname to Unicode, it will inadvertently permits access to the Unicode name "example.com".

Is CVE-2026-39821 actively exploited?

Active exploitation of CVE-2026-39821 has not been confirmed. Its EPSS score was 0.7% on 2026-10-22, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2026-39821?

CVE-2026-39821 has a CVSS v3 base score of 8.2 (HIGH severity), with vector string CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:N.

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