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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.
AI summary snake-internal / snake-material-v2
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
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 (7 steps)
Concrete steps and command examples for SOC/SRE teams to execute in order
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1Identify 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。
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6Apply 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と、Step 3 で見つけた異常検知の続報チェックを含めること。
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7Post-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 Step 3 と同じログクエリ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"}]}]
References
- advisory https://nvd.nist.gov/vuln/detail/CVE-2026-27820
- package https://github.com/ruby/zlib
- web https://github.com/ruby/zlib/security/advisories/GHSA-g857-hhfv-j68w
- web https://hackerone.com/reports/3467067
- web https://github.com/rubysec/ruby-advisory-db/blob/master/gems/zlib/CVE-2026-27820.yml
- web https://www.ruby-lang.org/en/news/2026/03/05/buffer-overflow-zlib-cve-2026-27820
- web https://github.com/advisories/GHSA-g857-hhfv-j68w