Critical 9.6Published May 22, 2026

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

Updated under an hour ago (Oct 28, 2026, 2:10 PM ET)

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It affects Go standard library net/http and 59 other products. Add what you run to find out whether that includes you.

No exploitation reported and no EPSS estimate yet.

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Every CVE for Go standard library net/http

Exploited in the last 30 days

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".

CVSS: 9.6 CRITICAL (v3.1, CISA ADP)CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N
Weakness (CWE): CWE-1289
Known exploited: Not in CISA's KEV catalog
EPSS (exploitation in the next 30 days): Not loaded for this CVE
Assigned by: Go
NVD analysis: Modified
Record last updated: Sep 17, 2026
First seen by InfoSec Signals: Oct 9, 2026

Affected products

Affected products

Go standard library net/http go_standard_library:net_http (from the name)	before 1.25.13; from 1.26.0-0 before 1.26.6; 1.27.0-0 builds before 1.27.0-rc.3	CNA record
Go standard library net/http/internal/http2 go_standard_library:net_http_internal_http2 (from the name)	before 1.25.13; from 1.26.0-0 before 1.26.6; 1.27.0-0 builds before 1.27.0-rc.3	CNA record
golang.org/x/net golang.org/x/net/idna golang.org_x_net:idna (from the name)	before 0.55.0	CNA record
Red Hat Red Hat Ansible Automation Platform 2.5 for RHEL 8 redhat:ansible_automation_platform	from 0:1.6.6-1.el8ap not affected	CISA ADP
Red Hat Red Hat Enterprise Linux 10 redhat:enterprise_linux	from 0:3.7.1-5.el10_2.5 not affected	CISA ADP
Red Hat Red Hat Enterprise Linux 10.0 Extended Update Support redhat:enterprise_linux_eus	from 0:1.26.5-1.el10_0 not affected	CISA ADP
Red Hat Red Hat Enterprise Linux 7 Extended Lifecycle Support redhat:rhel_els	from 0:0.11-1.el7_9 not affected	CISA ADP
Red Hat Red Hat Enterprise Linux 9.2 Update Services for SAP Solutions redhat:rhel_e4s	from 1:0.2.2-1.el9_2.4 not affected	CISA ADP
Red Hat Red Hat Enterprise Linux 9.6 Extended Update Support redhat:rhel_eus	from 0:1.26.5-1.el9_6 not affected	CISA ADP
Red Hat Red Hat OpenShift Container Platform 4.16 redhat:openshift	from 0:4.16.69-202608202336.p0.g763478c.assembly.4.16.69.el9 not affected	CISA ADP
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References

References

https://go.dev/cl/767220	issue-tracking
https://go.dev/issue/78760	issue-tracking
https://groups.google.com/g/golang-announce/c/iI-mYSI0lu8	mailing-list
https://groups.google.com/g/golang-announce/c/94pEornpRlI
https://pkg.go.dev/vuln/GO-2026-5026	vendor-advisory
https://access.redhat.com/errata/RHSA-2026:23262
https://access.redhat.com/errata/RHSA-2026:23264
https://access.redhat.com/errata/RHSA-2026:26546
https://access.redhat.com/errata/RHSA-2026:26547
https://access.redhat.com/errata/RHSA-2026:30650
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The CVE record on cve.org NVD's entry

The CVE record is authoritative. Products marked “from the name” were identified from the CNA's free-text names, not a CPE, and are matched less certainly. This product uses the NVD API but is not endorsed or certified by the NVD.