WebDyne versions from 3.007 before 3.022 for Perl execute .psp files outside the document root via path traversal in the PSGI API route fallback.
When a request matches no file, the PSGI handler retries it as an API route through api_filename. That keeps .. segments in the request path and only checks that the joined path starts with the document root, which a request for /../private passes.
A remote attacker can run the embedded Perl in any .psp file the server can read, such as one uploaded to a directory outside the document root.
Upgrade to WebDyne 3.022 or later.