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
51%
KEV
Not listed

Published

May 22, 2026

Last Modified

September 17, 2026

Advisory Details (11)

Auto-updated Sep 15, 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-0:75-10.el8_82026-09-17redhat
redhatrhcos-418.94.202609031320-02026-09-17redhat
redhatose-crio-credential-provider-0:4.21.0-202609021357.p2.g83d4961.assembly.stream.el102026-09-15redhat
redhatgrafana-0:10.2.6-26.el10_02026-09-15redhat
redhatgrafana-pcp-0:5.1.1-9.el9_42026-09-14redhat
redhatgrafana-pcp-0:5.1.1-5.el9_2.22026-09-14redhat
redhatgit-lfs-0:3.6.1-2.el9_6.72026-09-14redhat
redhatgit-lfs-0:3.2.0-2.el9_2.102026-09-14redhat
redhatgit-lfs-0:3.6.1-2.el10_0.72026-09-14redhat
redhatosbuild-composer-0:165.1-5.el10_22026-09-11redhat
redhatrhcos-416.94.202609011112-02026-09-10redhat
redhatoadp/oadp-velero-rhel9:17888604612026-09-09redhat
redhatimage-builder-0:52.1-1.el9_8.22026-09-09redhat
redhatosbuild-composer-0:101.5-2.el8_102026-09-09redhat
redhatosbuild-composer-0:165.1-4.el9_8.12026-09-08redhat
redhatrhoai/odh-trustyai-service-operator-rhel9:17873300732026-09-08redhat
redhatimage-builder-0:52.1-1.el10_2.22026-09-08redhat
redhatignition-0:2.15.0-2.el9_2.12026-09-08redhat
redhatosbuild-composer-0:134.1-10.el10_02026-09-03redhat
redhatosbuild-composer-0:76.1-7.el9_22026-08-31redhat
redhatgolang-0:1.26.5-1.el9_22026-08-31redhat
redhatrhoai/odh-training-operator-rhel9:17873616772026-08-27redhat
redhatmicroshift-0:4.16.69-202608202336.p0.g763478c.assembly.4.16.69.el92026-08-26redhat
redhatrhc-1:0.2.2-1.el9_2.42026-08-26redhat
redhatrhacm2/thanos-rhel9:17876852392026-08-26redhat
redhatrhc-1:0.2.4-9.el9_42026-08-26redhat
redhatosbuild-composer-0:101.3-4.el9_4.52026-08-25redhat
redhatignition-0:2.21.0-2.el10_0.32026-08-25redhat
redhatignition-0:2.17.0-2.el9_4.12026-08-25redhat
redhatosbuild-composer-0:132.2-10.el9_62026-08-20redhat

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

(6 across 5 ecosystems)
Go(2)
PackageVulnerable rangeFixed inDependents
golang.org/x/net—0.55.0—
stdlib—1.27.0-rc.3—
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 (27 versions)——
Debian:13(1)
PackageVulnerable rangeFixed inDependents
golang-golang-x-net1:0.27.0-2 ... 1:0.58.0-1 (10 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—

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 23× in last 7d / 116× 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 689 total refreshes for this CVE.

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

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 September 17, 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 48.6% 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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