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, 4:16 p.m.
Last Modified :
May 29, 2026, 3:26 p.m.
Remotely Exploit :
Yes !
Source :
[email protected]
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS 3.1 | CRITICAL | 134c704f-9b21-4f2e-91b3-4a467353bcc0 | ||||
| CVSS 3.1 | CRITICAL | 134c704f-9b21-4f2e-91b3-4a467353bcc0 |
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.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-39821.
| URL | Resource |
|---|---|
| https://go.dev/cl/767220 | Issue Tracking |
| https://go.dev/issue/78760 | Issue Tracking |
| https://groups.google.com/g/golang-announce/c/iI-mYSI0lu8 | Mailing List |
| https://pkg.go.dev/vuln/GO-2026-5026 | Vendor Advisory |
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-39821 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-39821
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Results are limited to the first 15 repositories due to potential performance issues.
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.
-
Initial Analysis by [email protected]
May. 29, 2026
Action Type Old Value New Value Added CPE Configuration OR *cpe:2.3:a:golang:net:*:*:*:*:*:go:*:* versions up to (excluding) 0.55.0 Added Reference Type Go Project: https://go.dev/cl/767220 Types: Issue Tracking Added Reference Type Go Project: https://go.dev/issue/78760 Types: Issue Tracking Added Reference Type Go Project: https://groups.google.com/g/golang-announce/c/iI-mYSI0lu8 Types: Mailing List Added Reference Type Go Project: https://pkg.go.dev/vuln/GO-2026-5026 Types: Vendor Advisory -
CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0
May. 27, 2026
Action Type Old Value New Value Added CVSS V3.1 AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N Removed CVSS V3.1 AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N -
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