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CVE-2026-27820 critical CVSS 9.8

Vulnerability in zlib (CVE-2026-27820)

Summary

Vulnerability in zlib (CVE-2026-27820). Successful exploitation can lead to full system takeover. Exploitable via zstream_buffer_ungets. Mitigation: upgrade to 3.0.1 or later.

Original description

The description as published by the source.

Details

A buffer overflow vulnerability exists in Zlib::GzipReader.

The zstream_buffer_ungets function prepends caller-provided bytes ahead of previously produced output but fails to guarantee the backing Ruby string has enough capacity before the memmove shifts the existing data. This can lead to memory corruption when the buffer length exceeds capacity.

Recommended action

We recommend to update the zlib gem to version 3.2.3 or later. In order to ensure compatibility with bundled version in older Ruby series, you may update as follows instead:

  • For Ruby 3.2 users: Update to zlib 3.0.1
  • For Ruby 3.3 users: Update to zlib 3.1.2

You can use gem update zlib to update it. If you are using bundler, please add gem "zlib", ">= 3.2.3" to your Gemfile.

Affected versions

zlib gem 3.2.2 or lower

Credits

calysteon

References

Source: NIST National Vulnerability Database NVD GitHub Advisory

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-2026-27820 has been found in zlib.
Attackers can target a specific entry point like zstream_buffer_ungets over the network to misuse the product.
Successful exploitation can lead to full system takeover. CVSS score: 9.8/10.
What to do: upgrade zlib to 3.0.1 or later.
If unsure, ask your IT team or search "zlib CVE-2026-27820" on the vendor's site.

CVE-2026-27820 (zlib) — CWE-120 / CVSS v3 9.8
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: zstream_buffer_ungets / zlib
Patched: 3.0.1 — 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 zlib. Attackers reach the vulnerable code path via zstream_buffer_ungets without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of zlib. Attack surface: zstream_buffer_ungets / zlib.

🔥 Severity

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

🔧 How to fix

Update to 3.0.1.

🛡️ 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 'zlib' . 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 'zlib' . | 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 zlib 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 zlib to 3.0.1

    ステージング環境で 3.0.1 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

RubyGems zlib
[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"3.0.1"}]}]

Fix status by distribution

Fixed packages published by each distribution for this CVE

References

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