CVE-2026-21858 is a maximum severity vulnerability dubbed 'Ni8mare' affecting n8n workflow automation platform. The flaw stems from improper input validation weakness that allows remote, unauthenticated attackers to take control over locally deployed n8n instances after gaining access to files on the underlying server. The vulnerability is specifically a content-type confusion in how n8n parses data. An n8n instance is potentially vulnerable if it has an active workflow with a Form Submission trigger accepting a file element, and a Form Ending node returning a binary file. The vulnerability can be exploited to expose secrets stored on the instance, forge session cookies to bypass authentication, inject sensitive files into workflows, or even execute arbitrary commands.