A second‑order expression injection flaw exists in the Form nodes of n8n. By submitting form data that begins with an equals sign, an unauthenticated attacker can cause the system to evaluate the content twice, allowing injection of arbitrary n8n expressions. When this injection is chained with an expression sandbox escape vulnerability, the attacker can execute code on the host machine. The vulnerability is limited to the data available in the n8n expression context unless this escalation path is taken.

This flaw affects n8n releases older than 2.10.1, 2.9.3, and 1.123.22. The affected vendor is n8n‑io and the product is n8n, usable on Node.js environments.

The CVSS base score of 9.5 indicates critical severity. The EPSS score of less than 1 % shows that exploitation is unlikely at present, and the vulnerability is not listed in the CISA KEV catalog. Attackers would need to provide specially crafted form input on a configured workflow that evaluates expressions without sanitization, and then chain a sandbox escape. If those conditions are met, the result is remote code execution. The primary attack vector is unauthenticated submission of a malicious form.