CVE-2026-39821

CRITICALPre-NVD 9.610.0▲
EchelonGraph scoreMEDIUM confidence

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

Triggered by: GitHub Security Advisory CVSS
Sources: epss, ghsa, nvd
9.6
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS: 0%CVSS: 9.6Exploit: NoneExposed: 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)
EPSS
38.5%
KEV
Not listed

Published

May 22, 2026

Last Modified

July 22, 2026

Advisory Details (4)

Auto-updated May 23, 2026
No patch confirmed yet.
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

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

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(9)

Vendor / EcosystemFixed in / PatchReleasedSource
redhatgit-lfs-0:3.4.1-11.el8_102026-06-29redhat
redhatgit-lfs-0:3.7.1-4.el9_8.12026-06-29redhat
ubuntugolang-golang-x-net-dev (1:0.0+git20190811.74dc4d7+dfsg-1ubuntu0.1~esm3) @ focal2026-06-29ubuntu
redhatmulticluster-engine/multicloud-manager-rhel9:17823085182026-06-28redhat
redhatrhacm2/thanos-rhel9:17824654122026-06-28redhat
redhatadvanced-cluster-security/rhacs-main-rhel8:17816864582026-06-17redhat
redhatadvanced-cluster-security/rhacs-main-rhel8:17816864462026-06-17redhat
redhatgolang1-25-main-1.25.11-2.hum12026-06-04redhat
redhatgolang1-26-main-1.26.4-2.hum12026-06-04redhat

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

(1 across 1 ecosystem)
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

(10)

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 8× in last 7d / 65× 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 315 total refreshes for this CVE.

  1. 2026-07-22 14:08 UTCEPSS rescore
  2. 2026-07-21 15:24 UTCEPSS rescore
  3. 2026-07-20 17:08 UTCEPSS rescore
  4. 2026-07-19 14:31 UTCEPSS rescore
  5. 2026-07-19 02:29 UTCEPSS rescore
  6. 2026-07-18 10:04 UTCEPSS rescore
  7. 2026-07-17 22:59 UTCOSV refresh
  8. 2026-07-16 17:03 UTCEPSS rescore
  9. 2026-07-15 16:57 UTCEPSS rescore
  10. 2026-07-15 16:57 UTCEPSS rescore
  11. 2026-07-15 02:00 UTCEPSS rescore
  12. 2026-07-13 22:30 UTCEPSS rescore
  13. 2026-07-13 06:13 UTCEPSS rescore
  14. 2026-07-13 06:13 UTCEPSS rescore
  15. 2026-07-12 05:46 UTCEPSS rescore
  16. 2026-07-11 08:27 UTCEPSS rescore
  17. 2026-07-09 19:10 UTCEPSS rescore
  18. 2026-07-08 15:15 UTCEPSS rescore
  19. 2026-07-07 13:46 UTCEPSS rescore
  20. 2026-07-06 16:27 UTCEPSS rescore
  21. 2026-07-06 16:27 UTCEPSS rescore
  22. 2026-07-06 02:23 UTCEPSS rescore
  23. 2026-07-06 02:23 UTCEPSS rescore
  24. 2026-07-05 02:30 UTCEPSS rescore
  25. 2026-07-04 06:31 UTCEPSS rescore
Show 75 more
  1. 2026-07-01 15:06 UTCEPSS rescore
  2. 2026-06-30 23:22 UTCEPSS rescore
  3. 2026-06-29 20:26 UTCVendor advisory
  4. 2026-06-29 20:26 UTCGHSA enrichment
  5. 2026-06-29 16:38 UTCVendor advisory
  6. 2026-06-29 16:38 UTCGHSA enrichment
  7. 2026-06-29 14:06 UTCEPSS rescore
  8. 2026-06-28 14:07 UTCEPSS rescore
  9. 2026-06-28 11:37 UTCVendor advisory
  10. 2026-06-28 11:37 UTCGHSA enrichment
  11. 2026-06-28 07:49 UTCVendor advisory
  12. 2026-06-28 07:49 UTCGHSA enrichment
  13. 2026-06-28 04:56 UTCEPSS rescore
  14. 2026-06-28 03:05 UTCVendor advisory
  15. 2026-06-28 03:05 UTCGHSA enrichment
  16. 2026-06-27 23:14 UTCVendor advisory
  17. 2026-06-27 23:14 UTCGHSA enrichment
  18. 2026-06-27 19:23 UTCVendor advisory
  19. 2026-06-27 19:23 UTCGHSA enrichment
  20. 2026-06-27 11:50 UTCVendor advisory
  21. 2026-06-27 11:50 UTCGHSA enrichment
  22. 2026-06-27 07:19 UTCVendor advisory
  23. 2026-06-27 07:19 UTCGHSA enrichment
  24. 2026-06-27 03:08 UTCEPSS rescore
  25. 2026-06-26 23:03 UTCVendor advisory
  26. 2026-06-26 23:03 UTCGHSA enrichment
  27. 2026-06-26 19:12 UTCVendor advisory
  28. 2026-06-26 19:12 UTCGHSA enrichment
  29. 2026-06-26 03:18 UTCVendor advisory
  30. 2026-06-26 03:18 UTCGHSA enrichment
  31. 2026-06-25 21:43 UTCVendor advisory
  32. 2026-06-25 21:43 UTCGHSA enrichment
  33. 2026-06-25 17:45 UTCVendor advisory
  34. 2026-06-25 17:45 UTCGHSA enrichment
  35. 2026-06-25 13:56 UTCVendor advisory
  36. 2026-06-25 13:56 UTCGHSA enrichment
  37. 2026-06-25 13:49 UTCEPSS rescore
  38. 2026-06-25 13:49 UTCEPSS rescore
  39. 2026-06-24 14:05 UTCEPSS rescore
  40. 2026-06-23 21:33 UTCEPSS rescore
  41. 2026-06-22 14:25 UTCEPSS rescore
  42. 2026-06-22 14:25 UTCEPSS rescore
  43. 2026-06-21 14:56 UTCEPSS rescore
  44. 2026-06-21 14:56 UTCEPSS rescore
  45. 2026-06-21 01:59 UTCEPSS rescore
  46. 2026-06-19 19:25 UTCEPSS rescore
  47. 2026-06-18 17:52 UTCEPSS rescore
  48. 2026-06-18 17:52 UTCEPSS rescore
  49. 2026-06-18 03:46 UTCVendor advisory
  50. 2026-06-18 03:46 UTCGHSA enrichment
  51. 2026-06-17 23:45 UTCVendor advisory
  52. 2026-06-17 23:45 UTCGHSA enrichment
  53. 2026-06-17 19:56 UTCVendor advisory
  54. 2026-06-17 19:56 UTCGHSA enrichment
  55. 2026-06-17 17:53 UTCEPSS rescore
  56. 2026-06-17 14:48 UTCVendor advisory
  57. 2026-06-17 14:48 UTCGHSA enrichment
  58. 2026-06-17 10:55 UTCGHSA enrichment
  59. 2026-06-17 03:11 UTCVendor advisory
  60. 2026-06-17 03:11 UTCGHSA enrichment
  61. 2026-06-16 23:21 UTCVendor advisory
  62. 2026-06-16 23:21 UTCGHSA enrichment
  63. 2026-06-16 19:22 UTCVendor advisory
  64. 2026-06-16 19:22 UTCGHSA enrichment
  65. 2026-06-16 17:52 UTCEPSS rescore
  66. 2026-06-16 10:31 UTCVendor advisory
  67. 2026-06-16 10:31 UTCGHSA enrichment
  68. 2026-06-16 06:29 UTCVendor advisory
  69. 2026-06-16 06:29 UTCGHSA enrichment
  70. 2026-06-16 02:32 UTCVendor advisory
  71. 2026-06-16 02:32 UTCGHSA enrichment
  72. 2026-06-15 22:45 UTCVendor advisory
  73. 2026-06-15 22:45 UTCGHSA enrichment
  74. 2026-06-15 17:49 UTCEPSS rescore
  75. 2026-06-15 14:54 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 July 22, 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 38.5% percentile likelihood of exploitation in the next 30 days — higher percentiles indicate greater predicted risk.
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

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