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
10.0EG
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS PROB: 0.7%CVSS: 9.6Exploit: None knownExposed services: Not assessed

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
10.0CRITICALmedium confidence
EG Risk
45
EG Risk 45/100

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%
EPSS PROB
0.7%
EPSS %ILE
51st
KEV
Not listed

Published

May 22, 2026

Last Modified

September 17, 2026

Advisory Details (11)

Auto-updated Oct 7, 2026
Patch available. Sources: redhat.
redhatMentions a patch (unverified)
https://access.redhat.com/errata/RHSA-2026:30650
redhatMentions a patch (unverified)
https://access.redhat.com/errata/RHSA-2026:26547
redhatMentions a patch (unverified)
https://access.redhat.com/errata/RHSA-2026:26546
redhatMentions a patch (unverified)
https://access.redhat.com/errata/RHSA-2026:23264
redhatMentions a patch (unverified)
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
redhatMentions a patch (unverified)
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 / EcosystemFix (by version range) / 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 rangeFix by version rangeDependents
golang.org/x/net—
  • every version up to 0.55.0: fixed in 0.55.0
—
stdlib—
  • every version up to 1.25.13: fixed in 1.25.13
  • 1.26.0-0 up to 1.26.6: fixed in 1.26.6
  • 1.27.0-0 up to 1.27.0-rc.3: fixed in 1.27.0-rc.3
—
Debian:11(1)
PackageVulnerable rangeFix by version rangeDependents
golang-golang-x-net1:0.0+git20210119.5f4716e+dfsg-4 ... 1:0.7.0+dfsg-1 (40 versions)
  • every version on: no fix on record
—
Debian:12(1)
PackageVulnerable rangeFix by version rangeDependents
golang-golang-x-net1:0.10.0-1 ... 1:0.7.0+dfsg-1 (27 versions)
  • every version on: no fix on record
—
Debian:13(1)
PackageVulnerable rangeFix by version rangeDependents
golang-golang-x-net1:0.27.0-2 ... 1:0.58.0-1 (10 versions)
  • every version on: no fix on record
—
Debian:14(1)
PackageVulnerable rangeFix by version rangeDependents
golang-golang-x-net1:0.27.0-2 ... 1:0.53.0-2~bpo13+1 (6 versions)
  • every version up to 1:0.55.0-1: fixed in 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 8× in last 7d / 51× 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 704 total refreshes for this CVE.

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

Related CVEs(same product + 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 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 0.7% probability of exploitation in the next 30 days (51st percentile of EPSS-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 (CRITICAL).
How do I remediate CVE-2026-39821?
A fix for CVE-2026-39821 is available: update to the fixed version the vendor names in its advisory. The vendor advisories EchelonGraph has for CVE-2026-39821 are linked in the Vendor Advisories panel on this page.

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