CVE-2026-39821
Invoking failure to reject ASCII-only Punycode-encoded labels in golang.org/x/net/idna
Description
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".
INFO
Published Date :
May 22, 2026, 3:01 p.m.
Last Modified :
May 22, 2026, 3:01 p.m.
Remotely Exploit :
No
Source :
Go
Affected Products
The following products are affected by CVE-2026-39821
vulnerability.
Even if cvefeed.io is aware of the exact versions of the
products
that
are
affected, the information is not represented in the table below.
No affected product recoded yet
Solution
- Update the idna package to the latest version.
- Validate hostnames after Punycode decoding.
- Implement strict hostname validation checks.
- Review privilege checks for hostname handling.
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The following list is the news that have been mention
CVE-2026-39821 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-39821 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
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CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0
May. 22, 2026
Action Type Old Value New Value Added CVSS V3.1 AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N Added CWE CWE-1289 -
New CVE Received by [email protected]
May. 22, 2026
Action Type Old Value New Value Added Description 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". Added Reference https://go.dev/cl/767220 Added Reference https://go.dev/issue/78760 Added Reference https://groups.google.com/g/golang-announce/c/iI-mYSI0lu8 Added Reference https://pkg.go.dev/vuln/GO-2026-5026