Decap CMS before 3.11.0 contains a path traversal vulnerability in the local proxy containment guard that uses plain string prefix comparison without path separator validation. Attackers can access sibling directories whose names begin with the repository directory name to read, write, or delete files outside the intended repository root. The vulnerability is classified as CWE‑22 and can compromise file system integrity, confidentiality, and potentially the availability of critical system files if the attacker can write or delete them.

The affected component is Decap CMS (decaporg/decap-cms), specifically all releases prior to version 3.11.0. The decap-server library is part of this product. The vulnerability stems from a local proxy containment guard that uses plain string prefix comparison without path separator validation. Administrators should ensure that the local proxy endpoint is not exposed to untrusted users and verify the version of decap-cms in use.

The CVSS score of 9.3 indicates high severity, but the EPSS score of less than 1% suggests a low probability of exploitation at present, and the flaw is not listed in the CISA KEV catalog. The path traversal requires that the attacker interacts with the local proxy endpoint, implying that either local code execution or remote access to the decap‑server instance is necessary. Based on the description, it is inferred that an attacker could exploit the flaw by crafting a specially crafted URL that bypasses the guard, after gaining access to the endpoint, and then traverse to arbitrary directories relative to the repository root.