CVE-2026-398219.6CriticalCWE-1289

CVE-2026-39821: Invoking failure to reject ASCII-only Punycode-encoded labels in golang.org/x/net/idna

Go standard library · net/http

Vendor fix available · Affected: Go standard library net/http · Exploitation: possible (1 to 10%)

Not in CyberSIXT coverage yet: no article names this CVE. This page shows the public record.

1.8%probability of exploitation in the next 30 days

79th percentile

050100

Higher than 79% of scored CVEs

9.6CriticalCVSS 3.1

LowMediumHighCrit.

DescriptionCVE.org

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".

Affected products3

From the CVE record. A product or version that is not listed is unknown, not unaffected.

Go standard library · net/httpbefore 1.25.13; from 1.26.0-0 before 1.26.6; from 1.27.0-0 before 1.27.0-rc.3
Go standard library · net/http/internal/http2before 1.25.13; from 1.26.0-0 before 1.26.6; from 1.27.0-0 before 1.27.0-rc.3
golang.org/x/net · golang.org/x/net/idnabefore 0.55.0

On the Atlas

Every published CVE is on the CyberSIXT Atlas, grouped by weakness, vendor and month.

Open the Atlas

Exploitation: Possible (1 to 10%). EPSS between 1% and 10%.
CISA KEV: Not listed
Public check: No public nuclei template
Weakness: Improper Validation of Unsafe Equivalence in Input · Input Validation and Logic
Vendor: Go standard library
Published: May 2026
Band: Possible (1 to 10%), May 2026

CoverageNo articles yet

No CyberSIXT article names CVE-2026-39821 yet, so there is no coverage timeline, action brief or change log for it. Everything on this page comes from the public CVE record, CISA KEV, EPSS and the nuclei template index. Search the archive or see the CVEs in the news.

References

28 more references are in the CVE record.