CVE-2026-39821 (GCVE-0-2026-39821)

Vulnerability from cvelistv5 – Published: 2026-05-22 15:01 – Updated: 2026-05-27 13:13
VLAI
Title
Invoking failure to reject ASCII-only Punycode-encoded labels in golang.org/x/net/idna
Summary
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".
Severity
9.6 (Critical)
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N
SSVC
Exploitation: none Automatable: no Technical Impact: total
CISA Coordinator (v2.0.3)
CWE
  • CWE-1289 - Improper Validation of Unsafe Equivalence in Input
Assigner
Go
References
Impacted products
Vendor Product Version
golang.org/x/net golang.org/x/net/idna Affected: 0 , < 0.55.0 (semver)
Create a notification for this product.
Credits
KC1zs4 (https://github.com/KC1zs4)
Show details on NVD website
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Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

Sightings

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Nomenclature

  • Seen: The vulnerability was mentioned, discussed, or observed by the user.
  • Confirmed: The vulnerability has been validated from an analyst's perspective.
  • Published Proof of Concept: A public proof of concept is available for this vulnerability.
  • Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
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  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.


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