diff --git a/src/scitacean/client.py b/src/scitacean/client.py
index 898bd4d7..0045b3ba 100644
--- a/src/scitacean/client.py
+++ b/src/scitacean/client.py
@@ -277,7 +277,7 @@ def upload_new_dataset_now(self, dataset: Dataset) -> Dataset:
             Note the error message if that happens.
         """
         dataset = dataset.replace(source_folder=self._source_folder_for(dataset))
-        files_to_upload = _files_to_upload(dataset.files)
+        files_to_upload = _files_to_upload(dataset, self.file_transfer)
         self.scicat.validate_dataset_model(dataset.make_upload_model())
         with self._connect_for_file_upload(dataset, files_to_upload) as con:
             # TODO check if any remote file is out of date.
@@ -1327,15 +1327,39 @@ def is_up_to_date(file: File) -> bool:
     ]
 
 
-def _files_to_upload(files: Iterable[File]) -> list[File]:
-    for file in files:
+def _files_to_upload(
+    dataset: Dataset,
+    file_transfer: FileTransfer | None,
+) -> list[File]:
+    for file in dataset.files:
         if file.is_on_local and file.is_on_remote:
             raise ValueError(
                 f"Refusing to upload file at remote_path={file.remote_path} "
                 "because it is both on local and remote and it is unclear what "
                 "to do. If you want to perform the upload, set the local path to None."
             )
-    return [file for file in files if file.local_path is not None]
+
+    to_upload = [file for file in dataset.files if file.is_on_local]
+    if not to_upload:
+        return []
+
+    if file_transfer is not None:
+        source_folder = file_transfer.source_folder_for(dataset)
+        outside = [
+            (file.remote_path, file.local_path)
+            for file in to_upload
+            if not (source_folder / file.remote_path)
+            .resolve()
+            .is_relative_to(source_folder)
+        ]
+        if outside:
+            raise ValueError(
+                "Refusing to upload files that would be placed outside of the "
+                f"source folder '{source_folder.posix}': {[str(l) for _, l in outside]}"
+                f" with remote paths {[r.posix for r, _ in outside]}."
+            )
+
+    return to_upload
 
 
 class _NullUploadConnection:
diff --git a/src/scitacean/filesystem.py b/src/scitacean/filesystem.py
index 1df15cff..e63569ae 100644
--- a/src/scitacean/filesystem.py
+++ b/src/scitacean/filesystem.py
@@ -127,6 +127,36 @@ def parent(self) -> RemotePath:
             return RemotePath(base)
         return RemotePath(parts[0] or base)
 
+    def is_absolute(self) -> bool:
+        """Return True if the path is absolute."""
+        return self._path.startswith("/")
+
+    def resolve(self) -> RemotePath:
+        """Resolve any parent segments in a path.
+
+        This does *not* resolve symlinks or the current working directory like
+        :meth:`pathlib.Path.resolve`.
+
+        Returns
+        -------
+        :
+            A new remote path with resolved parent segments.
+        """
+        parts: list[str] = []
+        for part in self._path.split("/"):
+            if part == ".." and parts:
+                parts.pop()
+            else:
+                parts.append(part)
+        res = RemotePath(*parts)
+        if self.is_absolute():
+            res._path = "/" + res._path
+        return res
+
+    def is_relative_to(self, other: RemotePath) -> bool:
+        """Check whether this path is relative to another."""
+        return self.posix.startswith(other.posix)
+
     def truncated(self, max_length: int = 255) -> RemotePath:
         """Return a new remote path with all path segments truncated.
 
diff --git a/tests/filesystem_test.py b/tests/filesystem_test.py
index e3e10baa..dc1db64c 100644
--- a/tests/filesystem_test.py
+++ b/tests/filesystem_test.py
@@ -151,6 +151,29 @@ def test_remote_path_parent() -> None:
     assert RemotePath("relative/sub").parent == RemotePath("relative")
 
 
+def test_remote_path_resolve() -> None:
+    assert RemotePath("").resolve() == RemotePath("")
+    assert RemotePath("/").resolve() == RemotePath("/")
+    assert RemotePath("/a").resolve() == RemotePath("/a")
+    assert RemotePath("file").resolve() == RemotePath("file")
+    assert RemotePath("../up").resolve() == RemotePath("../up")
+    assert RemotePath("base/../up").resolve() == RemotePath("up")
+    assert RemotePath("/base/../up").resolve() == RemotePath("/up")
+    assert RemotePath("/base/../mid/../up").resolve() == RemotePath("/up")
+    assert RemotePath("/base/mid/../up").resolve() == RemotePath("/base/up")
+    assert RemotePath("base/mid/../up").resolve() == RemotePath("base/up")
+    assert RemotePath("base/mid/inner/../../file").resolve() == RemotePath("base/file")
+
+
+def test_remote_path_is_relative_to() -> None:
+    path = RemotePath("/source/nested/file.txt")
+    assert path.is_relative_to(RemotePath("/"))
+    assert path.is_relative_to(RemotePath("/source"))
+    assert path.is_relative_to(RemotePath("/source/nested"))
+    assert not path.is_relative_to(RemotePath("/other-top"))
+    assert not path.is_relative_to(RemotePath("/source/other-nested"))
+
+
 def test_remote_path_truncated() -> None:
     assert RemotePath("something-long.txt").truncated(10) == "someth.txt"
     assert RemotePath("longlonglong/short").truncated(5) == "longl/short"
diff --git a/tests/upload_test.py b/tests/upload_test.py
index b640b04e..9ec1b181 100644
--- a/tests/upload_test.py
+++ b/tests/upload_test.py
@@ -276,6 +276,21 @@ def test_upload_uploads_files_to_source_folder(
     )
 
 
+@pytest.mark.parametrize("remote_prefix", ["../relative/", "/absolute/"])
+def test_upload_rejects_files_outside_of_source_folder(
+    client: FakeClient,
+    dataset_with_files: Dataset,
+    fs: FakeFilesystem,
+    remote_prefix: str,
+) -> None:
+    make_file(fs, path="bad", contents=b"This wants to be outside the source folder")
+    dataset_with_files.add_files(
+        File.from_local("bad", remote_path=remote_prefix + "bad")
+    )
+    with pytest.raises(ValueError, match="outside of the source folder"):
+        client.upload_new_dataset_now(dataset_with_files)
+
+
 def test_upload_does_not_create_dataset_if_file_upload_fails(
     dataset_with_files: Dataset, fs: FakeFilesystem
 ) -> None: