Answer in brief
CVE-2026-39821 records a Critical severity (CVSS 9.6) vulnerability in Invoking failure to reject ASCII-only Punycode-encoded labels in golang.org/x/net/idna. The current sources do not mark it as known exploited. The current feed maps golang.org/x/net/golang.org/x/net/idna (generic). Check affected ranges and fixed versions before updating.
Analysis pending evidence review
HOL Guard separates source facts from reviewed analysis. See the methodology.
Answer in brief
CVE-2026-39821 records a Critical severity (CVSS 9.6) vulnerability in Invoking failure to reject ASCII-only Punycode-encoded labels in golang.org/x/net/idna. The current sources do not mark it as known exploited. The current feed maps golang.org/x/net/golang.org/x/net/idna (generic). Check affected ranges and fixed versions before updating.
Analysis pending evidence review
HOL Guard separates source facts from reviewed analysis. See the methodology.
CVSS is 9.6. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.
Analysis status
Analysis pending evidence review
Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.
The current feed maps golang.org/x/net/golang.org/x/net/idna (generic). Check affected ranges and fixed versions before updating.
| Product | Affected versions | Fixed versions |
|---|---|---|
| cpe:2.3:a:golang:net:*:*:*:*:*:go:*:* | Not reported | Not reported |
| Package | Affected range | Fixed version |
|---|---|---|
| golang.org/x/net/golang.org/x/net/idnageneric | >=0 <0.55.0 | 0.55.0 |
Published upstream
May 22, 2026
Evidence: source:cvelist:source_dates:source-dates:recordSource modified
Aug 5, 2026
Evidence: source:cvelist:source_dates:source-dates:recordFirst seen by HOL
Jul 1, 2026
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".
Quoted source text, attributed separately from HOL analysis.
CVSS is 9.6. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.
Analysis status
Analysis pending evidence review
Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.
The current feed maps golang.org/x/net/golang.org/x/net/idna (generic). Check affected ranges and fixed versions before updating.
| Product | Affected versions | Fixed versions |
|---|---|---|
| cpe:2.3:a:golang:net:*:*:*:*:*:go:*:* | Not reported | Not reported |
| Package | Affected range | Fixed version |
|---|---|---|
| golang.org/x/net/golang.org/x/net/idnageneric | >=0 <0.55.0 | 0.55.0 |
Published upstream
May 22, 2026
Evidence: source:cvelist:source_dates:source-dates:recordSource modified
Aug 5, 2026
Evidence: source:cvelist:source_dates:source-dates:recordFirst seen by HOL
Jul 1, 2026
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".
Quoted source text, attributed separately from HOL analysis.