Local File Inclusion (LFI) via Improper URL Handling in uptime-kuma's `Real-Browser` monitor

Uptime Kuma is an open source, self-hosted monitoring tool. An Improper URL Handling Vulnerability allows an attacker to access sensitive local files on the server by exploiting the file:/// protocol. This vulnerability is triggered via the "real-browser" request type, which takes a screenshot of the URL provided by the attacker. By supplying local file paths, such as file:///etc/passwd, an attacker can read sensitive data from the server. This vulnerability arises because the system does not properly validate or sanitize the user input for the URL field. Specifically: 1. The URL input (``) allows users to input arbitrary file paths, including those using the file:/// protocol, without server-side validation. 2. The server then uses the user-provided URL to make a request, passing it to a browser instance that performs the "real-browser" request, which takes a screenshot of the content at the given URL. If a local file path is entered (e.g., file:///etc/passwd), the browser fetches and captures the file’s content. Since the user input is not validated, an attacker can manipulate the URL to request local files (e.g., file:///etc/passwd), and the system will capture a screenshot of the file's content, potentially exposing sensitive data. Any authenticated user who can submit a URL in "real-browser" mode is at risk of exposing sensitive data through screenshots of these files. This issue has been addressed in version 1.23.16 and all users are advised to upg…