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CVE-2026-39821

Golang Net privilege escalation

9.6 CRITICALEPSS 0.69%
  1. Published22 May 2026

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".

CVSS 3.1 breakdown

Attack vectorNetwork
Attack complexityLow
Privileges requiredLow
User interactionNone
ScopeChanged
ConfidentialityHigh
IntegrityHigh
AvailabilityNone
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N

Affected products

Golang Net< 0.55.0

Remediation

Fixed in
Golang Net≥ 0.55.0
Patch & vendor advisories

References

and 149 more references

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Frequently asked

Is CVE-2026-39821 being actively exploited?

CVE-2026-39821 is not in the CISA KEV catalog. Its EPSS exploit probability is 0.69%.

How severe is CVE-2026-39821?

CVE-2026-39821 has a CVSS 3.1 base score of 9.6, rated critical. EPSS estimates a 0.69% probability of exploitation.

Which products are affected by CVE-2026-39821?

Affected products include Golang Net.

How do I fix CVE-2026-39821?

Upgrade affected software to a fixed version, for example Golang Net 0.55.0 or later, then confirm no affected versions remain in your inventory.

How Secuno handles this

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Data from the NVD, CISA KEV catalog and FIRST EPSS. CVSS 3.1 base score shown where available.