Vulnerability Intelligence Report
Invoking failure to reject ASCII-only Punycode-encoded labels in golang.org/x/net/idna
CVE-2026-39821
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".
No Active Exploit Signals
CVSS Base Score
8.2
HIGH
Exploitability:1.8
Impact Score:5.8
EPSS Probability:0.69%
Executive Threat Verdict
Evaluating...
Evaluating Threat Landscape...
Assessing known weaponization, exploitation telemetry, and federal advisories.
Attack Surface
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Authentication
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Weaponization
—
SSVC Action
—
Weaknesses (CWE)
CWE-1289 ↗CWE-1289: Improper Validation of Unsafe Equivalence in Input
Affected Products & Versions
| Vendor | Product | Affected Versions |
|---|---|---|
| Go standard library | net/http | 0 < 1.25.13 (affected), 1.26.0-0 < 1.26.6 (affected), 1.27.0-0 < 1.27.0-rc.3 (affected) |
| Go standard library | net/http/internal/http2 | 0 < 1.25.13 (affected), 1.26.0-0 < 1.26.6 (affected), 1.27.0-0 < 1.27.0-rc.3 (affected) |
| golang.org/x/net | golang.org/x/net/idna | 0 < 0.55.0 (affected) |
References & Technical Advisories
No reference links found.
Threat Intelligence Signals
EPSS Score
0.692%
GitHub Advisory
Identity & Timeline
| Status | PUBLISHED |
| Assigning Authority | Go Project · Vendor · USA |
| Reserved | 2026-04-07T18:13:03 |
| Published | 2026-05-22T15:01:21 |
| Last Updated | 2026-09-17T12:04:30 |
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