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CVE-2026-27459 critical CVSS 9.8

Vulnerability in pyopenssl (CVE-2026-27459)

Summary

Vulnerability in pyopenssl (CVE-2026-27459). Successful exploitation can lead to full system takeover. Exploitable via set_cookie_generate_callback. Mitigation: upgrade to 26.0.0 or later.

Original description

The description as published by the source.

pyOpenSSL is a Python wrapper around the OpenSSL library. Starting in version 22.0.0 and prior to version 26.0.0, if a user provided callback to set_cookie_generate_callback returned a cookie value greater than 256 bytes, pyOpenSSL would overflow an OpenSSL provided buffer. Starting in version 26.0.0, cookie values that are too long are now rejected.

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-2026-27459 has been found in pyopenssl.
Attackers can target a specific entry point like set_cookie_generate_callback over the network to misuse the product.
Successful exploitation can lead to full system takeover. CVSS score: 9.8/10.
What to do: upgrade pyopenssl to 26.0.0 or later.
If unsure, ask your IT team or search "pyopenssl CVE-2026-27459" on the vendor's site.

CVE-2026-27459 (pyopenssl) — CWE-120 / CVSS v3 9.8
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: set_cookie_generate_callback
Patched: 26.0.0 — 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-120) exists in pyopenssl. Attackers reach the vulnerable code path via set_cookie_generate_callback without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of pyopenssl. Attack surface: set_cookie_generate_callback.

🔥 Severity

Severity: Critical (CVSS 9.8/10). Successful exploitation can lead to full system takeover.

🔧 How to fix

Update to 26.0.0.

🛡️ 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 'pyopenssl' . 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 'pyopenssl' . | 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 pyopenssl 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 pyopenssl to 26.0.0

    ステージング環境で 26.0.0 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

PyPI pyopenssl
[{"type":"ECOSYSTEM","events":[{"introduced":"22.0.0"},{"fixed":"26.0.0"}]}]

Fix status by distribution

Fixed packages published by each distribution for this CVE

References

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