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CVE-2026-39821 critical CVSS 9.6

Vulnerability in golang.org/x/net (CVE-2026-39821)

Summary

Vulnerability in golang.org/x/net (CVE-2026-39821). Confidential information can be exposed externally. Mitigation: upgrade to 0.55.0 or later.

Original description

The description as published by the source.

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".

Source: OSV.dev (Open Source Vulnerabilities) NVD GitHub Advisory OSV

Auto-generated summary

Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.

A vulnerability tracked as CVE-2026-39821 has been found in golang.org/x/net.
Confidential information can be exposed externally. CVSS score: 9.6/10.
What to do: upgrade golang.org/x/net to 0.55.0 or later.
If unsure, ask your IT team or search "golang.org/x/net CVE-2026-39821" on the vendor's site.

CVE-2026-39821 (golang.org/x/net) — CWE-1289 / CVSS v3 9.6
Attack vector: remote (network-reachable) / no user interaction
Patched: 0.55.0 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.

❓ What is the problem

A vulnerability (CWE-1289) exists in golang.org/x/net.

📍 Affected scope

Check the vendor advisory for the affected versions of golang.org/x/net.

🔥 Severity

Severity: Critical (CVSS 9.6/10). Confidential information can be exposed externally.

🔧 How to fix

Update to 0.55.0.

🛡️ Workaround

Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.

🔍 Detection

Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'golang.org/x/net' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.

Response actions

Concrete steps and command examples for SOC/SRE teams to execute in order

  1. 1
    Identify exposure identify 
    grep -r 'golang.org/x/net' . | grep -v node_modules

    リポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で golang.org/x/net を grep し、稼働しているサービス・バージョンを把握する。

  2. 2
    Apply patch patch 
    Upgrade golang.org/x/net to 0.55.0

    ステージング環境で 0.55.0 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。

  3. 3
    Post-deployment verification verify 
    Confirm patched version is live in production

    パッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。

Affected packages

Go golang.org/x/net
[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"0.55.0"}]}]
Go stdlib
[{"type":"SEMVER","events":[{"introduced":"0"},{"fixed":"1.25.13"},{"introduced":"1.26.0-0"},{"fixed":"1.26.6"},{"introduced":"1.27.0-0"},{"fixed":"1.27.0-rc.3"}]}]

Fix status by distribution

Fixed packages published by each distribution for this CVE

References

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