Vulnerability in mermaid (CVE-2026-71437)
Summary
Vulnerability in mermaid (CVE-2026-71437). Risk of unauthorized operations or information disclosure. Exploitable via horizontal. Mitigation: upgrade to 11.16.1 or later.
Original description
The description as published by the source.
Rendering an untrusted architecture-beta diagram lets the diagram author write an arbitrary property with the value horizontal or vertical onto Object.prototype. A group id of __proto__ is accepted as a valid parent.
Impact
Any code in the same realm that reads a property of that name from an arbitrary object, or enumerates an object with bare for...in, observes the injected value (which can only be the string horizontal or vertical.
This may mean corrupted option/config defaults, bypassed truthiness checks, causing denial of service or logic corruption in the embedding application.
Because the injected value cannot be an object or function, this is not directly exploitable for remote code execution.
PoC
architecture-beta
group mermaidPrototypePollutionMarker(cloud)[Marker]
service a(server)[A] in __proto__
service b(server)[B] in mermaidPrototypePollutionMarker
a:R -- L:b
The vulnerable write was introduced in commit cb0a4703bdf01d47508bde1c08aa9a980d70bc20 and first shipped in mermaid@11.5.0. The lines are unchanged in every release since.
Patches
This has been patched by https://github.com/mermaid-js/mermaid/commit/99af3fc35ef0a9a9c8c6314521344d67523ddccf, released in Mermaid v11.16.1
Workarounds
There are no known workarounds. Please upda…
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-2026-71437 has been found in mermaid.
Attackers can target a specific entry point like horizontal over the network to misuse the product.
Risk of unauthorized operations or information disclosure.
What to do: upgrade mermaid to 11.16.1 or later.
If unsure, ask your IT team or search "mermaid CVE-2026-71437" on the vendor's site.
CVE-2026-71437 (mermaid) — CWE-1321
Attack surface: horizontal / vertical / Object.prototype / __proto__
Patched: 11.16.1 — apply immediately
Workaround: There are no known workarounds.
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-1321) exists in mermaid. Attackers reach the vulnerable code path via horizontal.
Check the vendor advisory for the affected versions of mermaid. Attack surface: horizontal / vertical / Object.prototype / __proto__.
Severity: ?. Risk of unauthorized operations or information disclosure.
Update to 11.16.1.
Workaround: There are no known workarounds.
Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'mermaid' . 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 'mermaid' . | 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
mermaid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
There are no known workarounds.パッチが適用されるまでの応急処置として、There are no known workarounds. 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 mermaid to 11.16.1ステージング環境で 11.16.1 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。
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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。