CVE-2026-39821

CRITICALPre-NVD 9.610.0▲
EchelonGraph scoreMEDIUM confidence

Score 10.0 from GitHub Security Advisory (severity: CRITICAL) published 2026-05-26. CISA-ADP (Vulnrichment) CVSS v3.1 baseline 9.6; sources differ by 0.4.

Triggered by: GitHub Security Advisory CVSS
Sources: cisa-adp, epss, ghsa
Trending — Patch released this week
10.0EG
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS PROB: 1%CVSS: 9.6Exploit: None knownExposed: 0

A fix is available — apply it.

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 v3
9.6
EG Score
10.0(medium)
EG Risk
45(Track)
EG Risk 45/100SSVC: Track

EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).

How it’s computed
Severity100% × 45%
Exploitation1% × 40%
Automatability0% × 15%
Action: Routine — remediate on your standard cadence.
EPSS PROB
1%
EPSS %ILE
50%
KEV
Not listed

Published

May 22, 2026

Last Modified

August 25, 2026

Advisory Details (11)

Auto-updated Aug 25, 2026
Patch available. Sources: redhat.
redhat Patch Available
https://access.redhat.com/errata/RHSA-2026:30650
redhat Patch Available
https://access.redhat.com/errata/RHSA-2026:26547
redhat Patch Available
https://access.redhat.com/errata/RHSA-2026:26546
redhat Patch Available
https://access.redhat.com/errata/RHSA-2026:23264
redhat Patch Available
https://access.redhat.com/errata/RHSA-2026:23262
generic

GO-2026-5026 - Go Packages

https://pkg.go.dev/vuln/GO-2026-5026
generic
https://groups.google.com/g/golang-announce/c/iI-mYSI0lu8
generic
https://groups.google.com/g/golang-announce/c/94pEornpRlI
generic

net/http, x/net/idna: ToUnicode accepts Punycode labels encoding pure ASCII labels · Issue #78760 · golang/go · GitHub

https://go.dev/issue/78760
generic
https://go.dev/cl/767220
redhat Patch Available
https://access.redhat.com/errata/RHSA-2026:30651

Vendor Advisories for CVE-2026-39821(20)

These vendors published their own advisory mentioning this CVE — often with vendor-specific remediation steps + affected product lists not in NVD.

Patch Availability(30)

Vendor / EcosystemFixed in / PatchReleasedSource
redhatosbuild-composer-0:132.2-10.el9_62026-08-20redhat
redhatmulticluster-engine/hypershift-addon-rhel9-operator:17869120062026-08-19redhat
redhatgrafana-pcp-0:5.1.1-15.el9_62026-08-18redhat
redhatopenshift-logging/eventrouter-rhel9:17866451072026-08-18redhat
redhatopenshift-clients-0:4.20.0-202608101541.p2.g1367d50.assembly.stream.el82026-08-18redhat
redhatgrafana-pcp-0:5.3.0-1.el10_02026-08-18redhat
redhatodf4/rook-ceph-rhel9-operator:17867068802026-08-18redhat
redhatcert-manager/jetstack-cert-manager-rhel9:17866138052026-08-13redhat
redhatetcd-0:3.3.23-22.el8ost2026-08-13redhat
redhatworkload-availability/machine-deletion-remediation-rhel9-operator:17852035082026-08-12redhat
redhatquay/quay-rhel9:17861819812026-08-12redhat
redhatworkload-availability/self-node-remediation-rhel9-operator:17852030362026-08-12redhat
redhatworkload-availability/storage-based-remediation-rhel9-operator:17852034742026-08-12redhat
redhatworkload-availability/fence-agents-remediation-rhel9-operator:17852030792026-08-12redhat
redhatrh-lightspeed-runtimes/runtimes-inventory-rhel9-operator:17864837822026-08-12redhat
redhatworkload-availability/node-healthcheck-rhel9-operator:17852027232026-08-12redhat
redhatgolang-github-danielqsj-kafka_exporter2026-08-12redhat
redhatworkload-availability/node-maintenance-rhel9-operator:17852028472026-08-12redhat
redhatrhc-1:0.2.7-1.el9_6.52026-08-12redhat
redhatrhc-worker-script-0:0.11-1.el7_92026-08-12redhat
redhatopentelemetry-collector-0:0.152.1-1.el10_02026-08-11redhat
redhatopentelemetry-collector-0:0.152.1-1.el9_42026-08-11redhat
redhatopentelemetry-collector-0:0.152.1-1.el9_62026-08-11redhat
redhatrhc-1:0.3.2-5.el10_02026-08-11redhat
redhatmulticluster-globalhub/multicluster-globalhub-agent-rhel9:17857759482026-08-11redhat
redhatignition-0:2.21.0-7.el9_62026-08-10redhat
redhatsatellite/iop-vmaas-rhel9:17859233082026-08-06redhat
redhatyggdrasil-0:0.4.7-5.el10_02026-08-06redhat
redhatgrafana-0:10.2.6-22.el9_62026-08-06redhat
redhatsatellite/iop-vmaas-rhel9:17859299942026-08-06redhat

Patches are aggregated from vendor advisories (Red Hat, Microsoft, Cisco, GitHub) and package ecosystems (OSV, GHSA). Multiple rows for the same upstream release have been deduplicated.

Affected Packages

(5 across 5 ecosystems)
Debian:11(1)
PackageVulnerable rangeFixed inDependents
golang-golang-x-net1:0.0+git20210119.5f4716e+dfsg-4 ... 1:0.7.0+dfsg-1 (40 versions)——
Debian:12(1)
PackageVulnerable rangeFixed inDependents
golang-golang-x-net1:0.10.0-1 ... 1:0.7.0+dfsg-1 (26 versions)——
Debian:13(1)
PackageVulnerable rangeFixed inDependents
golang-golang-x-net1:0.27.0-2 ... 1:0.57.0-1 (9 versions)——
Debian:14(1)
PackageVulnerable rangeFixed inDependents
golang-golang-x-net1:0.27.0-2 ... 1:0.53.0-2~bpo13+1 (6 versions)1:0.55.0-1—
Go(1)
PackageVulnerable rangeFixed inDependents
golang.org/x/net—0.55.0—

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

All Vendor Advisories

(25)

Every vendor that published an advisory referencing this CVE — pulled from our cve_vendor_advisories aggregation. Click any row for the vendor's original advisory page.

Data Freshness Timeline

(refreshed 44× in last 7d / 284× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

Showing the most recent 100 of 611 total refreshes for this CVE.

  1. 2026-08-25 13:49 UTCEPSS rescore
  2. 2026-08-25 13:23 UTCVendor advisory
  3. 2026-08-25 13:23 UTCGHSA enrichment
  4. 2026-08-25 12:19 UTCEG score recompute
  5. 2026-08-25 12:19 UTCVendor advisory
  6. 2026-08-25 12:19 UTCGHSA enrichment
  7. 2026-08-24 13:29 UTCVendor advisory
  8. 2026-08-24 13:29 UTCGHSA enrichment
  9. 2026-08-24 12:24 UTCEG score recompute
  10. 2026-08-24 12:24 UTCVendor advisory
  11. 2026-08-24 12:24 UTCGHSA enrichment
  12. 2026-08-24 00:18 UTCVendor advisory
  13. 2026-08-24 00:18 UTCGHSA enrichment
  14. 2026-08-23 20:30 UTCVendor advisory
  15. 2026-08-23 20:30 UTCGHSA enrichment
  16. 2026-08-23 16:42 UTCEG score recompute
  17. 2026-08-23 16:42 UTCVendor advisory
  18. 2026-08-23 16:41 UTCGHSA enrichment
  19. 2026-08-23 00:19 UTCEPSS rescore
  20. 2026-08-22 00:53 UTCEG score recompute
  21. 2026-08-22 00:53 UTCVendor advisory
  22. 2026-08-22 00:53 UTCGHSA enrichment
  23. 2026-08-21 23:49 UTCEPSS rescore
  24. 2026-08-21 21:05 UTCVendor advisory
  25. 2026-08-21 21:05 UTCGHSA enrichment
Show 75 more
  1. 2026-08-21 13:19 UTCVendor advisory
  2. 2026-08-21 13:19 UTCGHSA enrichment
  3. 2026-08-21 12:31 UTCEG score recompute
  4. 2026-08-21 12:31 UTCVendor advisory
  5. 2026-08-21 12:31 UTCGHSA enrichment
  6. 2026-08-20 22:55 UTCEPSS rescore
  7. 2026-08-20 18:39 UTCVendor advisory
  8. 2026-08-20 18:39 UTCGHSA enrichment
  9. 2026-08-20 13:57 UTCVendor advisory
  10. 2026-08-20 13:57 UTCGHSA enrichment
  11. 2026-08-20 12:43 UTCEG score recompute
  12. 2026-08-20 12:43 UTCVendor advisory
  13. 2026-08-20 12:43 UTCGHSA enrichment
  14. 2026-08-19 17:03 UTCEPSS rescore
  15. 2026-08-19 12:26 UTCVendor advisory
  16. 2026-08-19 12:26 UTCGHSA enrichment
  17. 2026-08-19 12:10 UTCEG score recompute
  18. 2026-08-19 12:10 UTCVendor advisory
  19. 2026-08-19 12:10 UTCGHSA enrichment
  20. 2026-08-18 13:48 UTCEPSS rescore
  21. 2026-08-18 12:39 UTCVendor advisory
  22. 2026-08-18 12:39 UTCGHSA enrichment
  23. 2026-08-18 12:17 UTCEG score recompute
  24. 2026-08-18 12:17 UTCVendor advisory
  25. 2026-08-18 12:17 UTCGHSA enrichment
  26. 2026-08-17 13:47 UTCEPSS rescore
  27. 2026-08-17 12:19 UTCVendor advisory
  28. 2026-08-17 12:19 UTCGHSA enrichment
  29. 2026-08-17 12:12 UTCVendor advisory
  30. 2026-08-17 12:12 UTCGHSA enrichment
  31. 2026-08-17 09:43 UTCVendor advisory
  32. 2026-08-17 09:43 UTCGHSA enrichment
  33. 2026-08-16 16:48 UTCEG score recompute
  34. 2026-08-16 16:48 UTCVendor advisory
  35. 2026-08-16 16:48 UTCGHSA enrichment
  36. 2026-08-16 14:56 UTCEPSS rescore
  37. 2026-08-16 10:55 UTCVendor advisory
  38. 2026-08-16 10:55 UTCGHSA enrichment
  39. 2026-08-16 07:07 UTCVendor advisory
  40. 2026-08-16 07:07 UTCGHSA enrichment
  41. 2026-08-16 03:20 UTCEG score recompute
  42. 2026-08-16 03:20 UTCVendor advisory
  43. 2026-08-16 03:20 UTCGHSA enrichment
  44. 2026-08-16 02:14 UTCEPSS rescore
  45. 2026-08-15 01:35 UTCEG score recompute
  46. 2026-08-15 01:35 UTCVendor advisory
  47. 2026-08-15 01:35 UTCGHSA enrichment
  48. 2026-08-15 01:30 UTCEPSS rescore
  49. 2026-08-14 21:47 UTCVendor advisory
  50. 2026-08-14 21:47 UTCGHSA enrichment
  51. 2026-08-14 17:59 UTCVendor advisory
  52. 2026-08-14 17:59 UTCGHSA enrichment
  53. 2026-08-14 13:27 UTCVendor advisory
  54. 2026-08-14 13:27 UTCGHSA enrichment
  55. 2026-08-14 12:29 UTCVendor advisory
  56. 2026-08-14 12:29 UTCGHSA enrichment
  57. 2026-08-14 09:50 UTCVendor advisory
  58. 2026-08-14 09:50 UTCGHSA enrichment
  59. 2026-08-14 06:02 UTCVendor advisory
  60. 2026-08-14 06:02 UTCGHSA enrichment
  61. 2026-08-14 02:14 UTCVendor advisory
  62. 2026-08-14 02:13 UTCGHSA enrichment
  63. 2026-08-13 22:25 UTCVendor advisory
  64. 2026-08-13 22:25 UTCGHSA enrichment
  65. 2026-08-13 22:10 UTCEG score recompute
  66. 2026-08-13 22:10 UTCVendor advisory
  67. 2026-08-13 22:10 UTCGHSA enrichment
  68. 2026-08-13 22:00 UTCEPSS rescore
  69. 2026-08-13 13:37 UTCVendor advisory
  70. 2026-08-13 13:37 UTCGHSA enrichment
  71. 2026-08-13 12:27 UTCVendor advisory
  72. 2026-08-13 12:27 UTCGHSA enrichment
  73. 2026-08-13 08:04 UTCVendor advisory
  74. 2026-08-13 08:04 UTCGHSA enrichment
  75. 2026-08-13 03:50 UTCVendor advisory

Frequently asked(5)

What is CVE-2026-39821?
CVE-2026-39821 is a critical vulnerability published on May 22, 2026. 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…
When was CVE-2026-39821 disclosed?
CVE-2026-39821 was first published in the National Vulnerability Database on May 22, 2026, with the most recent update on August 25, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2026-39821 actively exploited?
CVE-2026-39821 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 1% probability of exploitation in the next 30 days, which ranks it in the top 49.9% of all scored CVEs.
What is the CVSS score of CVE-2026-39821?
CVE-2026-39821 has a CVSS v3.1 base score of 9.6 (CISA-ADP / Vulnrichment enrichment; NVD's own analysis pending). EchelonGraph synthesises NVD + CISA KEV + FIRST EPSS + GHSA into a combined EG score of 10.0.
How do I remediate CVE-2026-39821?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2026-39821, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

Dependency Blast Radius

See which npm, PyPI, Go, and Maven packages are affected by CVE-2026-39821

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