The vulnerability exists due to a combination of two factors. First, the SysFileApi controller inherits a generic create method from VLifeApi. According to the GitHub issue, a misconfiguration allows unauthenticated access to the /sysFile/create endpoint. This allows an attacker to create a SysFile database entry and control the value of the fileName field, injecting path traversal sequences (e.g., ../../../etc/passwd).
Second, the SysFileApi controller has several methods (image, pdf, download) that retrieve a SysFile record by its ID and use the fileName field to construct a file path for reading from the filesystem. These methods directly concatenate a base path with the user-controlled fileName without any validation or sanitization. When an attacker calls one of these endpoints with the ID of the maliciously created record, the application reads and returns the content of an arbitrary file on the server, leading to a classic path traversal vulnerability.