Vulnerability in h2 (CVE-2026-71554)
Summary
Vulnerability in h2 (CVE-2026-71554). Risk of unauthorized operations or information disclosure. Exploitable via Host header. Mitigation: upgrade to 4.4.1 or later.
Original description
The description as published by the source.
Impact
h2 <=4.4.0 accepts request header blocks containing more than one Host header, and forwards every Host header to the consuming application. Where the consumer downgrades HTTP/2 to HTTP/1.1, the resulting request carries two Host header lines, which is a request smuggling primitive (CWE-444).
Patches
Patched and fixed in v4.4.1
Workarounds
Users of the h2 library are advised to check and follow HTTP semantics best practices in their application code. h2 provides best effort sanity checks, but ultimately the calling code is responsible to ensure proper and safe usage of HTTP/2 as provided by h2, hyperframe, and hpack.
References
Similar to the previously disclosed and fixed duplicate content-length issue.
Source: GitHub Security Advisories (Global) 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-2026-71554 has been found in h2.
Attackers can target a specific entry point like Host header over the network to misuse the product.
Risk of unauthorized operations or information disclosure. CVSS score: 5.3/10.
What to do: upgrade h2 to 4.4.1 or later.
If unsure, ask your IT team or search "h2 CVE-2026-71554" on the vendor's site.
CVE-2026-71554 (h2) — CWE-444 / CVSS v3 5.3
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: Host header
Patched: 4.4.1 — apply immediately
Workaround: Users of the h2 library are advised to check and follow HTTP semantics best practices in their application code.
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.
A vulnerability (CWE-444) exists in h2. Attackers reach the vulnerable code path via Host header without authentication.
Check the vendor advisory for the affected versions of h2. Attack surface: Host header.
Severity: Medium (CVSS 5.3/10). Risk of unauthorized operations or information disclosure.
Update to 4.4.1.
Workaround: Users of the h2 library are advised to check and follow HTTP semantics best practices in their application code.
Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'h2' . 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
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1Identify exposure identify
grep -r 'h2' . | 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
h2を grep し、稼働しているサービス・バージョンを把握する。 -
2Apply temporary workaround mitigate
Users of the h2 library are advised to check and follow HTTP semantics best practices in their application code.パッチが適用されるまでの応急処置として、Users of the h2 library are advised to check and follow HTTP semantics best practices in their application code. を実施。回避策の副作用 (機能低下) を確認した上で。
-
3Apply patch patch
Upgrade h2 to 4.4.1ステージング環境で 4.4.1 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。
-
4Post-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
Fix status by distribution
Fixed packages published by each distribution for this CVE
- BellSoft Alpaquita BELL-CVE-2026-71554 py3-h2 Fixed in: 4.1.0-r2
- Debian DEBIAN-CVE-2026-71554 python-h2
- Ubuntu UBUNTU-CVE-2026-71554 python-h2
References
- advisory github.com
- advisory nvd.nist.gov
- package github.com
- package pypi.org
- web github.com
- web github.com