diff --git a/packages/markitdown/tests/test_file_paths.py b/packages/markitdown/tests/test_file_paths.py
new file mode 100644
index 0000000000..2930c2c522
--- /dev/null
+++ b/packages/markitdown/tests/test_file_paths.py
@@ -0,0 +1,138 @@
+"""File URI validation must reject network paths before opening a file."""
+
+import os
+from pathlib import Path
+
+import pytest
+
+import markitdown._uri_utils as uri_utils
+from markitdown import MarkItDown
+from markitdown._uri_utils import _is_unc_or_device_path, file_uri_to_path
+
+
+UNC_URI_PATHS = [
+    "//server.example/share/x.txt",
+    "///server.example/share/x.txt",
+    "////server.example/share/x.txt",
+    r"/\server.example/share/x.txt",
+    r"\/server.example/share/x.txt",
+    r"\\server.example\share\x.txt",
+    "/%2Fserver.example/share/x.txt",
+    "/%2fserver.example/share/x.txt",
+    "/%5Cserver.example/share/x.txt",
+    "/%5cserver.example/share/x.txt",
+    "%2F%2Fserver.example/share/x.txt",
+    "%5C%5Cserver.example%5Cshare%5Cx.txt",
+    "/%5C%3F%5CUNC%5Cserver.example%5Cshare%5Cx.txt",
+    "/%5C.%5CUNC%5Cserver.example%5Cshare%5Cx.txt",
+    "/%5C%3F%5CC:%5CTemp%5Cx.txt",
+    "/%5C.%5CPhysicalDrive0",
+]
+
+BLOCKED_URIS = (
+    [
+        f"file://{authority}{path}"
+        for authority in ("", "localhost")
+        for path in UNC_URI_PATHS
+        # An authority must be followed by a slash to introduce its path.
+        if path.startswith("/")
+    ]
+    + [f"file:{path}" for path in UNC_URI_PATHS if not path.startswith("/")]
+    + [
+        "FILE:////server.example/share/x.txt",
+        "file:////C:/Temp/x.txt",
+    ]
+)
+
+
+@pytest.mark.parametrize("uri", BLOCKED_URIS)
+def test_file_uri_rejects_unc_or_device_path(uri: str) -> None:
+    with pytest.raises(ValueError, match="UNC"):
+        file_uri_to_path(uri)
+
+
+class _NoFileAccessMarkItDown(MarkItDown):
+    def convert_local(self, *args, **kwargs):
+        pytest.fail("Rejected file URIs must not reach convert_local()")
+
+
+@pytest.fixture(scope="module")
+def no_file_access_markitdown() -> MarkItDown:
+    return _NoFileAccessMarkItDown(enable_builtins=False)
+
+
+@pytest.mark.parametrize("method", ["convert", "convert_uri"])
+@pytest.mark.parametrize("uri", BLOCKED_URIS + ["file://server.example/share/x.txt"])
+def test_convert_rejects_remote_file_uri_before_file_access(
+    no_file_access_markitdown: MarkItDown, method: str, uri: str
+) -> None:
+    with pytest.raises(ValueError, match="Unsupported file URI"):
+        getattr(no_file_access_markitdown, method)(uri)
+
+
+@pytest.mark.parametrize(
+    "path, blocked",
+    [
+        (r"\\server\share\x.txt", True),
+        ("//server/share/x.txt", True),
+        (r"/\server/share/x.txt", True),
+        (r"\/server/share/x.txt", True),
+        (r"\\?\UNC\server\share\x.txt", True),
+        (r"\\?\C:\Temp\x.txt", True),
+        (r"\\.\PhysicalDrive0", True),
+        (r"C:\Temp\x.txt", False),
+        ("C:/Temp/x.txt", False),
+        ("/home/user/x.txt", False),
+        ("relative.txt", False),
+    ],
+)
+def test_resolved_path_validation(path: str, blocked: bool) -> None:
+    assert _is_unc_or_device_path(path) is blocked
+
+
+def test_file_uri_rejects_unc_after_conversion(monkeypatch: pytest.MonkeyPatch) -> None:
+    # Check the final guard independently of the URI prefix check. Replace only
+    # the helper's converter binding, leaving the stdlib functions unchanged.
+    monkeypatch.setattr(
+        uri_utils, "url2pathname", lambda _: "//server.example/share/x.txt"
+    )
+
+    with pytest.raises(ValueError, match="UNC"):
+        file_uri_to_path("file:///local.txt")
+
+
+@pytest.mark.parametrize("filename", ["notes.txt", "notes café.txt", "%5C%5Cx.txt"])
+@pytest.mark.parametrize("authority", ["", "localhost"])
+def test_local_file_uri_conversion(
+    tmp_path: Path, filename: str, authority: str
+) -> None:
+    local_file = tmp_path / filename
+    local_file.write_text("Local file contents", encoding="utf-8")
+    uri = local_file.as_uri().replace("file://", f"file://{authority}", 1)
+
+    netloc, path = file_uri_to_path(uri)
+
+    assert netloc == (authority or None)
+    assert path == str(local_file)
+    markitdown = MarkItDown()
+    assert markitdown.convert(uri).markdown == "Local file contents"
+    assert markitdown.convert(str(local_file)).markdown == "Local file contents"
+
+
+@pytest.mark.skipif(os.name != "nt", reason="Requires native Windows path conversion")
+@pytest.mark.parametrize("authority", ["", "localhost"])
+@pytest.mark.parametrize(
+    "uri_path, expected",
+    [
+        ("/C:/Temp/notes.txt", r"C:\Temp\notes.txt"),
+        ("/C%3A/Temp/notes.txt", r"C:\Temp\notes.txt"),
+        ("/C%3a/Temp/notes.txt", r"C:\Temp\notes.txt"),
+        ("/C:/Temp/notes%20caf%C3%A9.txt", "C:\\Temp\\notes café.txt"),
+        ("/C:/Temp/%255C%255Cx.txt", r"C:\Temp\%5C%5Cx.txt"),
+    ],
+)
+def test_windows_local_drive_uri(authority: str, uri_path: str, expected: str) -> None:
+    netloc, path = file_uri_to_path(f"file://{authority}{uri_path}")
+
+    assert netloc == (authority or None)
+    assert path == expected