Pulpcore: pulpcore: relative_path_validator bypass via directory traversal in filesystemexport

A path traversal vulnerability was found in pulpcore. The relative_path_validator function only verifies that content paths do not begin with "/" but fails to block directory traversal sequences such as "../" anywhere in the path. An authenticated administrator can craft a relative_path containing embedded traversal sequences (e.g., "looking/normal/../../../../etc/shadow") that escapes the intended export directory during FilesystemExport operations. Because the file content is also user-controlled (uploaded artifact), this allows arbitrary file write to any location writable by the Pulp service user, potentially leading to service compromise or further system exploitation.