CVE-2026-39821 — Invoking failure to reject ASCII-only Punycode-encoded labels in golang.org/x/net/idna

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".

Published May 22, 2026Updated Sep 17, 2026Sources: cvelistV5, Go

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the scoring sources call it.

Exploitation

Unreported

no source claims exploitation

EPSS

chance of exploitation in 30 days

Affects

go standard library

149 products listed

CVSS base

9.6

CRITICAL

CISA SSVC assessment

Three decision points CISA publishes for the CVEs it assesses · SSVC 2.0.3. A stakeholder decision, not a severity score.

CISA

Exploitation

None

none · proof-of-concept · active

Automatable

can an attacker script all four kill-chain steps

Technical impact

Total

partial · total control of the vulnerable component

Affected scope

The catalog records vendors and products as separate lists, not pairs, so which product belongs to which vendor is not something this page can say.

Vendors (3)

go standard library golang org x net redhat

Products (149)

net httpnet http internal http2golang org x net idnared hat ansible automation platform 2 5 for rhel 8red hat ansible automation platform 2 5 for rhel 9red hat ansible automation platform 2 6 for rhel 10red hat ansible automation platform 2 6 for rhel 9red hat ansible automation platform 2 7 for rhel 10red hat ansible automation platform 2 7 for rhel 9red hat enterprise linux 10red hat enterprise linux 10 0 extended update supportred hat enterprise linux 7 extended lifecycle supportred hat enterprise linux 8red hat enterprise linux 9red hat enterprise linux 9 2 update services for sap solutionsred hat enterprise linux 9 4 update services for sap solutionsred hat enterprise linux 9 6 extended update supportred hat openshift container platform 4 16red hat openshift container platform 4 20red hat openshift container platform 4 21red hat openshift container platform 4 22red hat openstack platform 16 2red hat satellite 6 16 for rhel 8red hat satellite 6 16 for rhel 9red hat satellite 6 17 for rhel 9red hat satellite 6 18 for rhel 9red hat satellite 6 19 for rhel 9rhem 1 0 for rhel 9rhem 1 1 for rhel 10rhem 1 1 for rhel 9streams for apache kafka 3 2 1cert manager support for red hat openshift release 1 19cluster observability operator 1 5 0devworkspace operator 0 42logging for red hat openshift 6logging for red hat openshift 6 2logging subsystem for red hat openshift 6 4logging subsystem for red hat openshift 6 5multicluster engine for kubernetes 2 11multicluster engine for kubernetes 2 6multicluster engine for kubernetes 2 8multicluster engine for kubernetes 2 9multicluster global hub 1 4 5multicluster global hub 1 6 5multicluster global hub 1 7 0openshift api for data protection 1 3openshift api for data protection 1 5openshift api for data protection 1 6red hat advanced cluster management for kubernetes 2 11red hat advanced cluster management for kubernetes 2 13red hat advanced cluster security 4 9red hat advanced cluster security for kubernetes 4 10red hat advanced cluster security for kubernetes 4 11red hat ansible automation platform 2 6red hat ansible automation platform 2 7red hat edge manager 1 0red hat edge manager 1 1red hat enterprise linux ai 3 4red hat hardened imagesred hat lightspeed formerly insights for runtimes 1 0red hat migration toolkit 1 8red hat migration toolkit for applications 8 1red hat migration toolkit for applications 8 2red hat multicluster global hub 1 5 3red hat openshift ai 2 25red hat openshift ai 3 4red hat openshift builds 1 7 3red hat openshift builds 1 8 1red hat openshift data foundation 4 18red hat openshift data foundation 4 22red hat openshift dev spaces 3 29red hat openshift distributed tracing 3 10 2red hat openshift gitops 1 19red hat openshift gitops 1 20red hat openshift service mesh 2 6red hat openshift service mesh 3 0red hat openshift service mesh 3 1red hat openshift service mesh 3 2red hat openshift service mesh 3 3red hat openshift workload availability 0 12red hat openshift workload availability 0 13red hat openshift workload availability 0 3red hat openshift workload availability 0 7red hat openshift workload availability 0 8red hat openshift workload availability 5 7red hat quay 3 10red hat quay 3 12red hat quay 3 15red hat quay 3 16red hat quay 3 17red hat quay 3 9red hat satellite 6 18red hat satellite 6 19red hat trusted artifact signer 1 4red hat web terminal 1 11red hat web terminal 1 12red hat web terminal 1 13red hat web terminal 1 14red hat web terminal 1 15red hat web terminal 1 16assisted installer for red hat openshift container platform 2compliance operatorconfidential compute attestationcryostat 4deployment validation operatorexternaldns operatorexternal secrets operator for red hat openshiftfence agents remediation operatorfile integrity operatorgatekeeper 3logical volume manager storagemachine deletion remediation operatormultiarch tuning operatormulticluster engine for kubernetesnetwork observability operatornode healthcheck operatoropenshift api for data protectionopenshift developer tools and servicesopenshift lightspeedopenshift pipelinesopenshift serverlesspower monitoring for red hat openshiftred hat 3scale api management platform 2red hat advanced cluster management for kubernetes 2red hat ansible automation platform 2red hat ceph storage 5red hat ceph storage 6red hat ceph storage 8red hat ceph storage 9red hat certification program for red hat enterprise linux 9red hat connectivity link 1red hat developer hubred hat enterprise linux 7red hat enterprise linux ai rhel ai 3red hat openshift ai rhoaired hat openshift cluster manager clired hat openshift container platform 4red hat openshift distributed tracing 3red hat openshift for windows containersred hat openshift on awsred hat openshift virtualization 4red hat openstack platform 17 1red hat openstack platform 18 0red hat satellite 6red hat service interconnect 1red hat service interconnect 2security profiles operatorzero trust workload identity managerzero trust workload identity manager tech preview

Every base score collected

Sources score independently and disagree; each row says who scored it and under which version.

Score	Version	Severity	Expl.	Impact	Source
9.6	CVSS 3.1	CRITICAL	—	—	cvelistV5

Weakness & attack patterns

CWE-1289

References

25 on the record

go.dev/cl/767220
reference
go.dev/issue/78760
reference
groups.google.com/g/golang-announce/c/iI-mYSI0lu8
reference
groups.google.com/g/golang-announce/c/94pEornpRlI
reference
pkg.go.dev/vuln/GO-2026-5026
reference
access.redhat.com/security/cve/CVE-2026-39821
vdb-entry, x_refsource_REDHAT
bugzilla.redhat.com/show_bug.cgi?id=2480756
issue-tracking, x_refsource_REDHAT
security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-39821.json
x_sadp-csaf-vex

Elsewhere on this site

go standard libraryevery CVE for this vendor
golang org x netevery CVE for this vendor
redhatevery CVE for this vendor
CWE-1289other pages naming this weakness

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
No exploit packet captures, log samples, or incident case IDs supplied.