Mermaid configuration APIs allow prototype pollution

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

2.4 · CVSS:4.0/AV:L/AC:L/AT:P/PR:H/UI:A/VC:N/VI:L/VA:L/SC:H/SI:H/SA:H · score source

EPSS

• CVE-2026-71438: 0.35% estimated exploitation activity in the next 30 days; percentile 29.06 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary Mermaid's configuration setters (mermaid.initialize, mermaidAPI.setConfig, and mermaidAPI.updateSiteConfig) merge the caller-supplied configuration object into Mermaid's internal config using the assignWithDepth deep-merge helper that is vulnerable to prototype pollution. Because these APIs are intended to receive trusted configuration supplied by the application integrating Mermaid, Mermaid assesses the practical risk as low. The vulnerability is only reachable if an application forwards attacker-controlled data directly into one of these configuration entry points, which is outside their documented usage. User-controlled configuration (e.g. configuration in diagram code using %%{init: {}}%% or YAML frontmatter) are already protected from prototype pollution. Patches This has been patched in https://github.com/mermaid-js/mermaid/commit/2cd6dcf735533b323507e3e889ffdea870540b43 and released in Mermaid v11.16.1. A backport has been made for the v10 branch in…

Published by the source

6 Aug 2026, 19:59 UTC

Last modified by the source

6 Aug 2026, 19:59 UTC

Source last checked

26 Oct 2026, 13:00 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-c4c3-pg64-4m4v

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# Patch Next advisory brief: CVE-2026-71438

Advisory: Mermaid configuration APIs allow prototype pollution
IDs: CVE-2026-71438, GHSA-c4c3-pg64-4m4v
Record: https://patchnext.com/advisories/cve-2026-71438 (revision 1, source checked 26 Oct 2026, 13:00 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Low (CVSS 2.4)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: mermaid 11.16.1, 10.9.8. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm mermaid: affected >= 11.0.0-alpha.1, < 11.16.1; < 10.9.8; fixed in 10.9.8, 11.16.1

## Suggested inspection (read-only)
- `npm ls mermaid --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on mermaid.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-c4c3-pg64-4m4v (GitHub-reviewed advisory GHSA-c4c3-pg64-4m4v lists affected ranges: npm mermaid >=11.0.0-alpha.1 =0 <10.9.8)

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