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Noroxi
NVD / CVE Program· CNA Go

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

Noroxi assessment

Priority: Monitor (action score 39/100)

  • The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
  • Probability of exploitation in the next 30 days: 1.8% (FIRST EPSS); higher than 78.7% of all records.
  • Anyone who can reach it over the internet can trigger it. A low-privileged account is enough. No user action is required. No special conditions are required; it is repeatable.
  • Fixed version: Go standard library net/http 1.25.13, 1.26.6, 1.27.0-rc.3; Go standard library net/http/internal/http2 1.25.13, 1.26.6, 1.27.0-rc.3; golang.org/x/net golang.org/x/net/idna 0.55.0.

Low priority for now. Upgrade to Go standard library net/http 1.25.13, 1.26.6, 1.27.0-rc.3; Go standard library net/http/internal/http2 1.25.13, 1.26.6, 1.27.0-rc.3; golang.org/x/net golang.org/x/net/idna 0.55.0 or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

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".

CriticalCVSS 9.6 · v3.1—No exploit Fix available
Published
May 22, 2026
Updated
Sep 17, 2026
EPSS
1.8% · 78th percentile

Report tools

JSON

Action score

39

Monitor

Low priority for now.

CVSS
38 / 40 · 9.6 / 10
CISA KEV
0 / 30 · Not listed
EPSS
1 / 30 · 1.8%

CISA SSVC decision

Exploitation
none
Automatable
no
Technical impact
total

Vulnrichment: CISA's decision-tree inputs.

Get told when this record changes

  • Not in KEV: no record of exploitation in the wild
  • No public exploit code on record
  • A fix has been released

If any of these changes, we'll tell you in the daily email digest.

Follow CVE-2026-39821

When you'll hear from us

  • when it enters CISA KEV
  • when exploit code appears or is weaponized
  • when the record is updated (including a fixed version)
  • when EPSS rises 20 points in a week

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

VendorProduct
golangnet

Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

  • golang netbefore 0.55.0

Versions reported by the vendor

Affected version ranges reported by the assigning authority (Go). Independent of NVD's CPE analysis and usually ahead of it.

  • Go standard library net/http

    • before 1.25.13affected · semver
    • 1.26.0-0 and later · before 1.26.6affected · semver
    • 1.27.0-0 and later · before 1.27.0-rc.3affected · semver

  • Go standard library net/http/internal/http2

    • before 1.25.13affected · semver
    • 1.26.0-0 and later · before 1.26.6affected · semver
    • 1.27.0-0 and later · before 1.27.0-rc.3affected · semver

  • golang.org/x/net golang.org/x/net/idna

    • before 0.55.0affected · semver

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

EcosystemPackageAffected rangeFix
AlmaLinux:10git-lfsbefore 3.7.1-5.el10_2.53.7.1-5.el10_2.5
AlmaLinux:10go-fdo-serverbefore 1.0.1-2.el10_2.31.0.1-2.el10_2.3
AlmaLinux:10go-fdo-server-manufacturerbefore 1.0.1-2.el10_2.31.0.1-2.el10_2.3
AlmaLinux:10go-fdo-server-ownerbefore 1.0.1-2.el10_2.31.0.1-2.el10_2.3
AlmaLinux:10go-fdo-server-rendezvousbefore 1.0.1-2.el10_2.31.0.1-2.el10_2.3
AlmaLinux:10go-toolsetbefore 1.26.5-1.el10_2.alma.11.26.5-1.el10_2.alma.1
AlmaLinux:10golangbefore 1.26.5-1.el10_2.alma.11.26.5-1.el10_2.alma.1
AlmaLinux:10golang-binbefore 1.26.5-1.el10_2.alma.11.26.5-1.el10_2.alma.1
AlmaLinux:10golang-docsbefore 1.26.5-1.el10_2.alma.11.26.5-1.el10_2.alma.1
AlmaLinux:10golang-miscbefore 1.26.5-1.el10_2.alma.11.26.5-1.el10_2.alma.1
AlmaLinux:10golang-racebefore 1.26.5-1.el10_2.alma.11.26.5-1.el10_2.alma.1
AlmaLinux:10golang-srcbefore 1.26.5-1.el10_2.alma.11.26.5-1.el10_2.alma.1
AlmaLinux:10golang-testsbefore 1.26.5-1.el10_2.alma.11.26.5-1.el10_2.alma.1
AlmaLinux:10grafanabefore 10.2.6-27.el10_210.2.6-27.el10_2
AlmaLinux:10grafana-pcpbefore 5.3.0-7.el10_25.3.0-7.el10_2
AlmaLinux:10grafana-selinuxbefore 10.2.6-27.el10_210.2.6-27.el10_2
AlmaLinux:10opentelemetry-collectorbefore 0.152.1-1.el10_20.152.1-1.el10_2
AlmaLinux:10osbuild-composerbefore 165.1-5.el10_2.alma.1165.1-5.el10_2.alma.1
AlmaLinux:10osbuild-composer-corebefore 165.1-5.el10_2.alma.1165.1-5.el10_2.alma.1
AlmaLinux:10osbuild-composer-workerbefore 165.1-5.el10_2.alma.1165.1-5.el10_2.alma.1
AlmaLinux:10yggdrasilbefore 0.4.9.2-1.el10_20.4.9.2-1.el10_2
AlmaLinux:10yggdrasil-develbefore 0.4.9.2-1.el10_20.4.9.2-1.el10_2
AlmaLinux:8delvebefore 1.26.1-1.module_el8.10.0+4223+bd807c2e1.26.1-1.module_el8.10.0+4223+bd807c2e
AlmaLinux:8git-lfsbefore 3.4.1-11.el8_103.4.1-11.el8_10

+96

Same product

golang: all records

Other highest-scoring records for the same primary product.

  • CVE-2018-17848The html package (aka x/net/html) through 2018-09-25 in Go mishandles <math><template><mn><b></template>, leading to a "panic: runtime error
    31Monitor
  • CVE-2018-17847The html package (aka x/net/html) through 2018-09-25 in Go mishandles <svg><template><desc><t><svg></template>, leading to a "panic: runtime
    31Monitor
  • CVE-2018-17846The html package (aka x/net/html) through 2018-09-25 in Go mishandles <table><math><select><mi><select></table>, leading to an infinite loop
    31Monitor

Published before and after this one in the same product

  • ← EarlierCVE-2026-25681May 22, 2026 · Invoking incorrect handling of character references in DOCTYPE nodes in golang.org/x/net/html
    24Monitor
  • ← EarlierCVE-2026-27136May 22, 2026 · Invoking duplicate attributes can cause XSS in golang.org/x/net/html
    24Monitor
  • Later →CVE-2026-42502May 22, 2026 · Invoking incorrect handling of HTML elements in foreign content in golang.org/x/net/html
    24Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / packageFixed versionSource
Go standard library net/http1.25.13Vendor (CNA)
Go standard library net/http1.26.6Vendor (CNA)
Go standard library net/http1.27.0-rc.3Vendor (CNA)
Go standard library net/http/internal/http21.25.13Vendor (CNA)
Go standard library net/http/internal/http21.26.6Vendor (CNA)
Go standard library net/http/internal/http21.27.0-rc.3Vendor (CNA)
golang.org/x/net golang.org/x/net/idna0.55.0Vendor (CNA)
azl3 application-gateway-kubernetes-ingress 1.7.7-3 on Azure Linux 3.01.7.7-4 · CBL-Mariner ReleasesMicrosoft (MSRC)
azl3 azcopy 10.25.1-4 on Azure Linux 3.010.25.1-5 · CBL-Mariner ReleasesMicrosoft (MSRC)
azl3 azurelinux-image-tools 1.2.0-2 on Azure Linux 3.01.3.0-2 · CBL-Mariner ReleasesMicrosoft (MSRC)
azl3 cert-manager 1.12.15-6 on Azure Linux 3.01.12.15-8 · CBL-Mariner ReleasesMicrosoft (MSRC)
azl3 cf-cli 8.7.11-5 on Azure Linux 3.08.7.11-6 · CBL-Mariner ReleasesMicrosoft (MSRC)
azl3 cloud-provider-kubevirt 0.5.1-3 on Azure Linux 3.00.5.1-4 · CBL-Mariner ReleasesMicrosoft (MSRC)
azl3 containerd2 2.1.6-2 on Azure Linux 3.02.1.6-3 · CBL-Mariner ReleasesMicrosoft (MSRC)
azl3 containerized-data-importer 1.62.0-3 on Azure Linux 3.01.62.0-5 · CBL-Mariner ReleasesMicrosoft (MSRC)
azl3 coredns 1.11.4-15 on Azure Linux 3.01.11.4-17 · CBL-Mariner ReleasesMicrosoft (MSRC)
azl3 cri-tools 1.32.0-4 on Azure Linux 3.01.32.0-6 · CBL-Mariner ReleasesMicrosoft (MSRC)
azl3 docker-buildx 0.14.0-11 on Azure Linux 3.00.14.0-13 · CBL-Mariner ReleasesMicrosoft (MSRC)
azl3 docker-cli 25.0.7-2 on Azure Linux 3.025.0.7-3 · CBL-Mariner ReleasesMicrosoft (MSRC)
azl3 docker-compose 2.27.0-9 on Azure Linux 3.02.27.0-11 · CBL-Mariner ReleasesMicrosoft (MSRC)
azl3 etcd 3.5.28-1 on Azure Linux 3.03.5.30-2 · CBL-Mariner ReleasesMicrosoft (MSRC)
azl3 etcd 3.5.30-2 on Azure Linux 3.03.5.30-2 · CBL-Mariner ReleasesMicrosoft (MSRC)
azl3 flannel 0.24.2-26 on Azure Linux 3.00.24.2-28 · CBL-Mariner ReleasesMicrosoft (MSRC)
azl3 gh 2.62.0-15 on Azure Linux 3.02.62.0-16 · CBL-Mariner ReleasesMicrosoft (MSRC)
azl3 git-lfs 3.6.1-2 on Azure Linux 3.03.6.1-3 · CBL-Mariner ReleasesMicrosoft (MSRC)
azl3 git-lfs 3.6.1-4 on Azure Linux 3.03.6.1-3 · CBL-Mariner ReleasesMicrosoft (MSRC)
azl3 golang 1.25.10-1 on Azure Linux 3.01.25.11-2 · CBL-Mariner ReleasesMicrosoft (MSRC)
azl3 golang 1.26.3-1 on Azure Linux 3.01.26.4-2 · CBL-Mariner ReleasesMicrosoft (MSRC)
azl3 golang 1.27.0-1 on Azure Linux 3.0—Microsoft (MSRC)
azl3 ignition-flatcar 2.22.0-2 on Azure Linux 3.02.22.0-3 · CBL-Mariner ReleasesMicrosoft (MSRC)

+90

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

EPSS, last 120 days+1% in 7 days

FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

No commit or PR link among the references.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

No nightly-computed relations.

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

No known public program.

Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).

Technical details

Attack conditions

  • Anyone who can reach it over the internet can trigger it.
  • A low-privileged account is enough.
  • No user action is required.
  • No special conditions are required; it is repeatable.

If successful

Confidentiality
high · data can be read
Integrity
high · data or configuration can be modified
Availability
none

Impact can extend beyond the vulnerable component (scope changes).

Attack vector
Network
Attack complexity
Low
Privileges required
Low
User interaction
None
Scope
Changed
Confidentiality impact
High
Integrity impact
High
Availability impact
None

Weakness class (CWE)

CWE-1289 · Improper Validation of Unsafe Equivalence in Input

CVSS vector

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N

nvd-secondary

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

MITRE has no CAPEC/ATT&CK mapping for this CWE.

Change log

  1. Fix✗ → ✓

For records you follow, these changes also arrive as notifications. →

References

Show all references (19 more)

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records