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
37.9%
KEV
Not listed

Published

May 22, 2026

Last Modified

July 8, 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 / 227× 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 413 total refreshes for this CVE.

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

Related CVEs(same vendor + same CWE)

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 8, 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 37.9% 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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