Prototype pollution in mermaid - CVE-2026-71438
Published: August 4, 2026 / Updated: August 7, 2026
Vulnerability identifier: #VU140936
CSH Severity: Medium
CVSS v4: 4.6 [CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:A/VC:N/VI:L/VA:L/SC:N/SI:N/SA:N]
CVE-ID: CVE-2026-71438
CWE-ID: CWE-1321
Exploitation vector: Local access
Exploit availability:
No public exploit available
Vulnerability details
The vulnerability allows a local privileged user to modify object prototypes.
The vulnerability exists due to improperly controlled modification of object prototype attributes in the mermaid configuration setters when merging caller-supplied configuration objects into the internal config. A local privileged user can supply a crafted configuration object to modify object prototypes.
The issue is only reachable if an application forwards attacker-controlled data directly into these configuration entry points; user-controlled configuration in diagram code and YAML frontmatter is already protected.
Affected software
mermaid
Fedora
nextcloud
Fedora
nextcloud
How to mitigate CVE-2026-71438
Install security update from vendor's website.
mermaid - addressed in versions 10.9.8, 11.16.1
nextcloud - addressed in versions 34.0.3-1.el10_2, 34.0.3-1.el10_3, 34.0.3-1.fc44
nextcloud - addressed in versions 34.0.3-1.el10_2, 34.0.3-1.el10_3, 34.0.3-1.fc44