In Eclipse Theia versions 1.66.0 and up until including 1.73.1, the "@theia/plugin-ext" backend exposes the "/hostedPlugin/:pluginId/:path(*)" HTTP endpoint, which resolves the requested file path with "path.resolve(localPath, filePath)" without verifying that the resolved path stays within the plugin's directory. An unauthenticated network attacker can send percent-encoded "../" sequences ("%2e%2e%2f") that decode into the path parameter and escape the plugin directory, allowing arbitrary files readable by the Theia backend process to be retrieved. Plugin IDs are derived deterministically from a plugin's publisher and name, so built-in plugins serve as reliable anchors that require no prior knowledge of the target system.