CVE-2026-71437 mermaid-js CVE debrief

Who should care

Defenders responsible for systems using Mermaid, especially those processing user-supplied diagram text, should assess exposure and prioritize verification and updates. This includes operators, platform administrators, vulnerability management teams, and security teams who need to verify Mermaid versions, assess exposure, and prioritize updates to mitigate potential risks.

Why it matters

CVE-2026-71437 is a prototype pollution vulnerability in Mermaid that allows attackers to affect the behavior of embedding applications. Defenders should verify Mermaid versions, assess exposure, and prioritize updates to mitigate potential risks.

• Potential modification of application behavior through prototype pollution
• Need for verification of Mermaid version and exposure in systems
• Priority for updating to version 11.16.1 or later
• Monitoring for potential exploitation attempts

Technical summary

Mermaid, a JavaScript tool for creating diagrams and charts, is vulnerable to prototype pollution in versions 11.5.0 to 11.16.1. An attacker can pollute Object.prototype by defining a group with an id of __proto__ in a diagram, potentially affecting the behavior of the embedding application. The issue is fixed in version 11.16.1. This vulnerability allows defenders to assess exposure and prioritize verification and updates. Mermaid's Architecture Diagrams are susceptible to prototype pollution when a diagram defines a group with an id of __proto__.

Defensive priority

Defenders should prioritize verifying Mermaid versions and updating to 11.16.1 or later if vulnerable. They should also assess exposure in their systems, especially where user-supplied diagram text is processed.

Recommended defensive actions

• Verify Mermaid version and update to 11.16.1 or later if vulnerable
• Assess exposure in systems where user-supplied diagram text is processed
• Monitor for potential exploitation attempts
• Review compensating controls for exposed systems while remediation is scheduled and verified
• Check relevant monitoring, detection, and logs for exposed assets that need extra review
• Track exceptions, retest remediated assets, and close the item only after evidence is documented
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up

Evidence notes

The CVE record and NVD entry provide details on the vulnerability, its impact, and the fix. However, additional information on exploitation or affected systems is limited. Defenders should verify Mermaid versions, assess exposure, and prioritize updates to mitigate potential risks. The vulnerability allows an attacker to pollute Object.prototype by defining a group with an id of __proto__ in a diagram, potentially affecting the behavior of the embedding application. Limited information is available on the number of affected systems or

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.