diff --git a/deploy/deploy-test-server.sh b/deploy/deploy-test-server.sh
index d97b160..1d4c359 100755
--- a/deploy/deploy-test-server.sh
+++ b/deploy/deploy-test-server.sh
@@ -130,7 +130,8 @@ $sudo_cmd cp /opt/left4me/deploy/files/usr/local/lib/systemd/system/left4me-refr
 $sudo_cmd cp /opt/left4me/deploy/files/usr/local/lib/systemd/system/left4me-refresh-global-overlays.timer /usr/local/lib/systemd/system/left4me-refresh-global-overlays.timer
 $sudo_cmd cp /opt/left4me/deploy/files/usr/local/libexec/left4me/left4me-systemctl /usr/local/libexec/left4me/left4me-systemctl
 $sudo_cmd cp /opt/left4me/deploy/files/usr/local/libexec/left4me/left4me-journalctl /usr/local/libexec/left4me/left4me-journalctl
-$sudo_cmd chmod 0755 /usr/local/libexec/left4me/left4me-systemctl /usr/local/libexec/left4me/left4me-journalctl
+$sudo_cmd cp /opt/left4me/deploy/files/usr/local/libexec/left4me/left4me-overlay /usr/local/libexec/left4me/left4me-overlay
+$sudo_cmd chmod 0755 /usr/local/libexec/left4me/left4me-systemctl /usr/local/libexec/left4me/left4me-journalctl /usr/local/libexec/left4me/left4me-overlay
 $sudo_cmd cp /opt/left4me/deploy/files/etc/sudoers.d/left4me /etc/sudoers.d/left4me
 $sudo_cmd chmod 0440 /etc/sudoers.d/left4me
 $sudo_cmd visudo -cf /etc/sudoers.d/left4me
diff --git a/deploy/files/etc/sudoers.d/left4me b/deploy/files/etc/sudoers.d/left4me
index 3f0f864..02cccc6 100644
--- a/deploy/files/etc/sudoers.d/left4me
+++ b/deploy/files/etc/sudoers.d/left4me
@@ -1,3 +1,4 @@
 Defaults:left4me !requiretty
 left4me ALL=(root) NOPASSWD: /usr/local/libexec/left4me/left4me-systemctl *
 left4me ALL=(root) NOPASSWD: /usr/local/libexec/left4me/left4me-journalctl *
+left4me ALL=(root) NOPASSWD: /usr/local/libexec/left4me/left4me-overlay mount *, /usr/local/libexec/left4me/left4me-overlay umount *
diff --git a/deploy/files/usr/local/libexec/left4me/left4me-overlay b/deploy/files/usr/local/libexec/left4me/left4me-overlay
new file mode 100644
index 0000000..e53eeb5
--- /dev/null
+++ b/deploy/files/usr/local/libexec/left4me/left4me-overlay
@@ -0,0 +1,188 @@
+#!/usr/bin/python3
+"""Privileged overlay mount helper for left4me.
+
+Invoked via sudo by the left4me runtime user. Validates inputs strictly,
+then enters PID 1's mount namespace via nsenter to perform the actual
+mount/umount syscall, so the resulting mount lives in the host namespace
+and is visible to the systemd-managed gameserver units.
+
+Verbs:
+    mount <name>    Reads ${LEFT4ME_ROOT}/instances/<name>/instance.env
+                    for L4D2_LOWERDIRS, validates every lowerdir is
+                    under one of installation/overlays/workshop_cache/
+                    global_overlay_cache, then mounts the kernel
+                    overlay at runtime/<name>/merged.
+    umount <name>   Unmounts runtime/<name>/merged.
+
+Set LEFT4ME_OVERLAY_PRINT_ONLY=1 to print the would-be argv (one line,
+shell-quoted) and exit 0 instead of execv. Used by tests.
+"""
+
+import os
+import re
+import shlex
+import sys
+from pathlib import Path
+
+NAME_RE = re.compile(r"^[a-z0-9][a-z0-9_-]{0,63}$")
+DEFAULT_ROOT = "/var/lib/left4me"
+LOWERDIR_ALLOWLIST = (
+    "installation",
+    "overlays",
+    "global_overlay_cache",
+    "workshop_cache",
+)
+MAX_LOWERDIRS = 500
+NSENTER = "/usr/bin/nsenter"
+MOUNT_BIN = "/bin/mount"
+UMOUNT_BIN = "/bin/umount"
+
+
+def die(msg: str) -> None:
+    sys.stderr.write(f"left4me-overlay: {msg}\n")
+    sys.exit(1)
+
+
+def root() -> Path:
+    return Path(os.environ.get("LEFT4ME_ROOT") or DEFAULT_ROOT)
+
+
+def validate_name(name: str) -> str:
+    if not NAME_RE.fullmatch(name):
+        die(f"invalid instance name: {name!r}")
+    return name
+
+
+def parse_lowerdirs(env_path: Path) -> list[str]:
+    if not env_path.is_file():
+        die(f"instance.env not found: {env_path}")
+    raw = None
+    for line in env_path.read_text().splitlines():
+        if "=" not in line:
+            continue
+        key, value = line.split("=", 1)
+        if key.strip() == "L4D2_LOWERDIRS":
+            raw = value
+            break
+    if raw is None:
+        die(f"L4D2_LOWERDIRS not set in {env_path}")
+    if raw == "":
+        die(f"L4D2_LOWERDIRS is empty in {env_path}")
+    parts = raw.split(":")
+    if any(p == "" for p in parts):
+        die(f"L4D2_LOWERDIRS contains an empty entry: {raw!r}")
+    if len(parts) > MAX_LOWERDIRS:
+        die(f"L4D2_LOWERDIRS has {len(parts)} entries (cap {MAX_LOWERDIRS})")
+    return parts
+
+
+def canonical_under(allowed_roots: list[Path], path: Path) -> Path:
+    try:
+        canonical = path.resolve(strict=True)
+    except (FileNotFoundError, RuntimeError):
+        die(f"path does not exist or has a symlink loop: {path}")
+    for r in allowed_roots:
+        if canonical == r or r in canonical.parents:
+            return canonical
+    die(f"path is outside the permitted roots: {path} (resolved: {canonical})")
+
+
+_LISTXATTR = getattr(os, "listxattr", None)
+
+
+def _entry_has_fuse_xattr(path: str) -> str | None:
+    if _LISTXATTR is None:
+        return None
+    try:
+        attrs = _LISTXATTR(path, follow_symlinks=False)
+    except OSError:
+        return None
+    for a in attrs:
+        if a.startswith("user.fuseoverlayfs."):
+            return a
+    return None
+
+
+def assert_no_fuse_xattrs(upper: Path) -> None:
+    if not upper.exists() or _LISTXATTR is None:
+        return
+    for dirpath, dirnames, filenames in os.walk(upper):
+        for entry in (dirpath, *(os.path.join(dirpath, n) for n in dirnames),
+                      *(os.path.join(dirpath, n) for n in filenames)):
+            tainted = _entry_has_fuse_xattr(entry)
+            if tainted:
+                die(
+                    f"upperdir contains fuse-overlayfs xattr {tainted!r} on {entry}; "
+                    "wipe upper/ and work/ before mounting"
+                )
+
+
+def exec_or_print(argv: list[str]) -> None:
+    if os.environ.get("LEFT4ME_OVERLAY_PRINT_ONLY") == "1":
+        print(" ".join(shlex.quote(a) for a in argv))
+        sys.exit(0)
+    os.execv(argv[0], argv)
+
+
+def cmd_mount(name: str) -> None:
+    name = validate_name(name)
+    r = root()
+    instance_env = r / "instances" / name / "instance.env"
+    raw_lowerdirs = parse_lowerdirs(instance_env)
+
+    allowed_roots = [(r / sub).resolve() for sub in LOWERDIR_ALLOWLIST]
+    canonical_lowerdirs = [str(canonical_under(allowed_roots, Path(p))) for p in raw_lowerdirs]
+
+    runtime_name_dir = (r / "runtime" / name).resolve(strict=True)
+    upper = (runtime_name_dir / "upper").resolve(strict=True)
+    work = (runtime_name_dir / "work").resolve(strict=True)
+    merged = (runtime_name_dir / "merged").resolve(strict=True)
+    for label, path in (("upper", upper), ("work", work), ("merged", merged)):
+        if path.parent != runtime_name_dir:
+            die(f"{label} resolved outside runtime/{name}: {path}")
+
+    assert_no_fuse_xattrs(upper)
+
+    options = f"lowerdir={':'.join(canonical_lowerdirs)},upperdir={upper},workdir={work}"
+    argv = [
+        NSENTER,
+        "--mount=/proc/1/ns/mnt",
+        "--",
+        MOUNT_BIN,
+        "-t", "overlay",
+        "overlay",
+        "-o", options,
+        str(merged),
+    ]
+    exec_or_print(argv)
+
+
+def cmd_umount(name: str) -> None:
+    name = validate_name(name)
+    r = root()
+    runtime_name_dir = (r / "runtime" / name).resolve(strict=True)
+    merged = (runtime_name_dir / "merged").resolve(strict=True)
+    if merged.parent != runtime_name_dir:
+        die(f"merged resolved outside runtime/{name}: {merged}")
+    argv = [
+        NSENTER,
+        "--mount=/proc/1/ns/mnt",
+        "--",
+        UMOUNT_BIN,
+        str(merged),
+    ]
+    exec_or_print(argv)
+
+
+def main(argv: list[str]) -> None:
+    if len(argv) != 3 or argv[1] not in ("mount", "umount"):
+        sys.stderr.write("usage: left4me-overlay mount|umount <name>\n")
+        sys.exit(2)
+    if argv[1] == "mount":
+        cmd_mount(argv[2])
+    else:
+        cmd_umount(argv[2])
+
+
+if __name__ == "__main__":
+    main(sys.argv)
diff --git a/deploy/tests/test_deploy_artifacts.py b/deploy/tests/test_deploy_artifacts.py
index 792bb24..626bedd 100644
--- a/deploy/tests/test_deploy_artifacts.py
+++ b/deploy/tests/test_deploy_artifacts.py
@@ -13,6 +13,7 @@ GLOBAL_REFRESH_SERVICE = DEPLOY / "files/usr/local/lib/systemd/system/left4me-re
 GLOBAL_REFRESH_TIMER = DEPLOY / "files/usr/local/lib/systemd/system/left4me-refresh-global-overlays.timer"
 SYSTEMCTL_HELPER = DEPLOY / "files/usr/local/libexec/left4me/left4me-systemctl"
 JOURNALCTL_HELPER = DEPLOY / "files/usr/local/libexec/left4me/left4me-journalctl"
+OVERLAY_HELPER = DEPLOY / "files/usr/local/libexec/left4me/left4me-overlay"
 SUDOERS = DEPLOY / "files/etc/sudoers.d/left4me"
 HOST_ENV = DEPLOY / "templates/etc/left4me/host.env"
 WEB_ENV_TEMPLATE = DEPLOY / "templates/etc/left4me/web.env.template"
@@ -146,10 +147,36 @@ def test_sudoers_allows_only_left4me_helpers_not_raw_system_tools():
         "left4me ALL=(root) NOPASSWD: "
         "/usr/local/libexec/left4me/left4me-journalctl *"
     ) in sudoers
+    assert "/usr/local/libexec/left4me/left4me-overlay mount *" in sudoers
+    assert "/usr/local/libexec/left4me/left4me-overlay umount *" in sudoers
     assert "/bin/systemctl" not in sudoers
     assert "/usr/bin/systemctl" not in sudoers
     assert "/bin/journalctl" not in sudoers
     assert "/usr/bin/journalctl" not in sudoers
+    assert "/bin/mount" not in sudoers
+    assert "/bin/umount" not in sudoers
+
+
+def test_overlay_helper_is_python_with_strict_validation():
+    text = OVERLAY_HELPER.read_text()
+    assert text.startswith("#!/usr/bin/python3")
+    # Validation surface
+    assert "NAME_RE = re.compile" in text
+    assert "LOWERDIR_ALLOWLIST" in text
+    assert "user.fuseoverlayfs." in text
+    assert "MAX_LOWERDIRS = 500" in text
+    # Mounts via PID 1's mount namespace
+    assert "/proc/1/ns/mnt" in text
+    assert "nsenter" in text
+    # Verbs are mount and umount (not unmount)
+    assert '"mount"' in text and '"umount"' in text
+    assert '"unmount"' not in text
+
+
+def test_deploy_script_installs_overlay_helper_with_executable_mode():
+    script = DEPLOY_SCRIPT.read_text()
+    assert "/usr/local/libexec/left4me/left4me-overlay" in script
+    assert "chmod 0755" in script and "left4me-overlay" in script
 
 
 def test_env_templates_contain_required_defaults():
diff --git a/l4d2host/fs/kernel_overlayfs.py b/l4d2host/fs/kernel_overlayfs.py
new file mode 100644
index 0000000..164eec0
--- /dev/null
+++ b/l4d2host/fs/kernel_overlayfs.py
@@ -0,0 +1,53 @@
+from pathlib import Path
+from typing import Callable
+
+from l4d2host.fs.base import OverlayMounter
+from l4d2host.process import run_command
+
+
+HELPER_PATH = "/usr/local/libexec/left4me/left4me-overlay"
+
+
+class KernelOverlayFSMounter(OverlayMounter):
+    # Delegates the actual mount/umount syscalls to the privileged
+    # left4me-overlay helper. The helper takes only the instance name and
+    # rederives lowerdirs/upper/work/merged from disk; the OverlayMounter
+    # ABC accepts those args for compatibility, so we extract the name
+    # from the merged path's parent directory.
+    def mount(
+        self,
+        *,
+        lowerdirs: str,
+        upperdir: Path,
+        workdir: Path,
+        merged: Path,
+        on_stdout: Callable[[str], None] | None = None,
+        on_stderr: Callable[[str], None] | None = None,
+        passthrough: bool = False,
+        should_cancel: Callable[[], bool] | None = None,
+    ) -> None:
+        del lowerdirs, upperdir, workdir
+        run_command(
+            ["sudo", "-n", HELPER_PATH, "mount", merged.parent.name],
+            on_stdout=on_stdout,
+            on_stderr=on_stderr,
+            passthrough=passthrough,
+            should_cancel=should_cancel,
+        )
+
+    def unmount(
+        self,
+        *,
+        merged: Path,
+        on_stdout: Callable[[str], None] | None = None,
+        on_stderr: Callable[[str], None] | None = None,
+        passthrough: bool = False,
+        should_cancel: Callable[[], bool] | None = None,
+    ) -> None:
+        run_command(
+            ["sudo", "-n", HELPER_PATH, "umount", merged.parent.name],
+            on_stdout=on_stdout,
+            on_stderr=on_stderr,
+            passthrough=passthrough,
+            should_cancel=should_cancel,
+        )
diff --git a/l4d2host/tests/test_kernel_overlayfs.py b/l4d2host/tests/test_kernel_overlayfs.py
new file mode 100644
index 0000000..296e880
--- /dev/null
+++ b/l4d2host/tests/test_kernel_overlayfs.py
@@ -0,0 +1,76 @@
+from pathlib import Path
+
+import pytest
+
+
+HELPER_PATH = "/usr/local/libexec/left4me/left4me-overlay"
+
+
+def test_mount_invokes_helper_with_name_only(monkeypatch: pytest.MonkeyPatch) -> None:
+    from l4d2host.fs.kernel_overlayfs import KernelOverlayFSMounter
+
+    calls: list[list[str]] = []
+
+    def fake_run_command(cmd, **kwargs):
+        del kwargs
+        calls.append(list(cmd))
+
+    monkeypatch.setattr("l4d2host.fs.kernel_overlayfs.run_command", fake_run_command)
+
+    KernelOverlayFSMounter().mount(
+        lowerdirs="/var/lib/left4me/installation",
+        upperdir=Path("/var/lib/left4me/runtime/alpha/upper"),
+        workdir=Path("/var/lib/left4me/runtime/alpha/work"),
+        merged=Path("/var/lib/left4me/runtime/alpha/merged"),
+    )
+
+    assert calls == [["sudo", "-n", HELPER_PATH, "mount", "alpha"]]
+
+
+def test_unmount_invokes_helper_with_umount_verb(monkeypatch: pytest.MonkeyPatch) -> None:
+    from l4d2host.fs.kernel_overlayfs import KernelOverlayFSMounter
+
+    calls: list[list[str]] = []
+
+    def fake_run_command(cmd, **kwargs):
+        del kwargs
+        calls.append(list(cmd))
+
+    monkeypatch.setattr("l4d2host.fs.kernel_overlayfs.run_command", fake_run_command)
+
+    KernelOverlayFSMounter().unmount(merged=Path("/var/lib/left4me/runtime/alpha/merged"))
+
+    assert calls == [["sudo", "-n", HELPER_PATH, "umount", "alpha"]]
+
+
+def test_mount_propagates_run_command_kwargs(monkeypatch: pytest.MonkeyPatch) -> None:
+    from l4d2host.fs.kernel_overlayfs import KernelOverlayFSMounter
+
+    captured: dict = {}
+
+    def fake_run_command(cmd, **kwargs):
+        captured["cmd"] = list(cmd)
+        captured["kwargs"] = kwargs
+
+    monkeypatch.setattr("l4d2host.fs.kernel_overlayfs.run_command", fake_run_command)
+
+    out: list[str] = []
+    err: list[str] = []
+    KernelOverlayFSMounter().mount(
+        lowerdirs="/var/lib/left4me/installation",
+        upperdir=Path("/var/lib/left4me/runtime/alpha/upper"),
+        workdir=Path("/var/lib/left4me/runtime/alpha/work"),
+        merged=Path("/var/lib/left4me/runtime/alpha/merged"),
+        on_stdout=out.append,
+        on_stderr=err.append,
+        passthrough=False,
+        should_cancel=lambda: False,
+    )
+
+    assert captured["cmd"][0:3] == ["sudo", "-n", HELPER_PATH]
+    captured["kwargs"]["on_stdout"]("hi")
+    captured["kwargs"]["on_stderr"]("oops")
+    assert out == ["hi"]
+    assert err == ["oops"]
+    assert captured["kwargs"]["passthrough"] is False
+    assert callable(captured["kwargs"]["should_cancel"])
diff --git a/l4d2host/tests/test_overlay_helper.py b/l4d2host/tests/test_overlay_helper.py
new file mode 100644
index 0000000..0e3977e
--- /dev/null
+++ b/l4d2host/tests/test_overlay_helper.py
@@ -0,0 +1,168 @@
+import os
+import shlex
+import subprocess
+import sys
+from pathlib import Path
+
+import pytest
+
+
+HELPER_SOURCE = (
+    Path(__file__).resolve().parents[2]
+    / "deploy"
+    / "files"
+    / "usr"
+    / "local"
+    / "libexec"
+    / "left4me"
+    / "left4me-overlay"
+)
+
+
+def _setup_instance(root: Path, name: str = "alpha", lowerdirs: list[str] | None = None) -> None:
+    """Create the on-disk shape the helper expects."""
+    (root / "installation").mkdir(parents=True, exist_ok=True)
+    (root / "overlays" / "workshop").mkdir(parents=True, exist_ok=True)
+    if lowerdirs is None:
+        lowerdirs = [str(root / "overlays" / "workshop"), str(root / "installation")]
+    inst_dir = root / "instances" / name
+    inst_dir.mkdir(parents=True)
+    (inst_dir / "instance.env").write_text(
+        f"L4D2_PORT=27015\nL4D2_ARGS=-tickrate 100\nL4D2_LOWERDIRS={':'.join(lowerdirs)}\n"
+    )
+    runtime = root / "runtime" / name
+    (runtime / "upper").mkdir(parents=True)
+    (runtime / "work").mkdir(parents=True)
+    (runtime / "merged").mkdir(parents=True)
+
+
+def _run(args: list[str], root: Path, extra_env: dict[str, str] | None = None) -> subprocess.CompletedProcess:
+    env = {
+        **os.environ,
+        "LEFT4ME_ROOT": str(root),
+        "LEFT4ME_OVERLAY_PRINT_ONLY": "1",
+    }
+    if extra_env:
+        env.update(extra_env)
+    return subprocess.run(
+        [sys.executable, str(HELPER_SOURCE), *args],
+        env=env,
+        capture_output=True,
+        text=True,
+    )
+
+
+def test_mount_prints_expected_nsenter_command(tmp_path: Path) -> None:
+    _setup_instance(tmp_path)
+
+    result = _run(["mount", "alpha"], tmp_path)
+
+    assert result.returncode == 0, result.stderr
+    parts = shlex.split(result.stdout.strip())
+    assert parts[0] == "/usr/bin/nsenter"
+    assert parts[1] == "--mount=/proc/1/ns/mnt"
+    assert parts[2] == "--"
+    assert parts[3] == "/bin/mount"
+    assert parts[4:6] == ["-t", "overlay"]
+    assert parts[6] == "overlay"
+    assert parts[7] == "-o"
+    options = parts[8]
+    assert f"upperdir={tmp_path}/runtime/alpha/upper" in options
+    assert f"workdir={tmp_path}/runtime/alpha/work" in options
+    assert f"lowerdir={tmp_path}/overlays/workshop:{tmp_path}/installation" in options
+    assert parts[9] == str(tmp_path / "runtime" / "alpha" / "merged")
+
+
+def test_umount_prints_expected_nsenter_command(tmp_path: Path) -> None:
+    _setup_instance(tmp_path)
+
+    result = _run(["umount", "alpha"], tmp_path)
+
+    assert result.returncode == 0, result.stderr
+    parts = shlex.split(result.stdout.strip())
+    assert parts == [
+        "/usr/bin/nsenter",
+        "--mount=/proc/1/ns/mnt",
+        "--",
+        "/bin/umount",
+        str(tmp_path / "runtime" / "alpha" / "merged"),
+    ]
+
+
+@pytest.mark.parametrize("bad_name", ["..", "../escape", "FOO", "foo bar", "foo/bar", ""])
+def test_rejects_bad_instance_name(tmp_path: Path, bad_name: str) -> None:
+    result = _run(["mount", bad_name], tmp_path)
+    assert result.returncode != 0
+    assert "invalid instance name" in result.stderr or "usage:" in result.stderr
+
+
+def test_rejects_lowerdir_outside_allowlist(tmp_path: Path) -> None:
+    _setup_instance(tmp_path, lowerdirs=["/etc"])
+    result = _run(["mount", "alpha"], tmp_path)
+    assert result.returncode != 0
+    assert "outside the permitted roots" in result.stderr
+
+
+def test_rejects_lowerdir_traversal(tmp_path: Path) -> None:
+    # An overlay subdirectory whose path uses .. to escape the overlays root.
+    _setup_instance(tmp_path, lowerdirs=[str(tmp_path / "overlays" / "..") + "/etc"])
+    result = _run(["mount", "alpha"], tmp_path)
+    assert result.returncode != 0
+    assert "outside the permitted roots" in result.stderr or "path does not exist" in result.stderr
+
+
+def test_rejects_lowerdir_symlink_escape(tmp_path: Path) -> None:
+    _setup_instance(tmp_path)
+    sneaky = tmp_path / "overlays" / "sneaky"
+    os.symlink("/etc", sneaky)
+    # rewrite instance.env to point at the symlink
+    inst_env = tmp_path / "instances" / "alpha" / "instance.env"
+    inst_env.write_text(f"L4D2_LOWERDIRS={sneaky}\n")
+    result = _run(["mount", "alpha"], tmp_path)
+    assert result.returncode != 0
+    assert "outside the permitted roots" in result.stderr
+
+
+def test_rejects_missing_instance_env(tmp_path: Path) -> None:
+    (tmp_path / "instances" / "alpha").mkdir(parents=True)
+    runtime = tmp_path / "runtime" / "alpha"
+    (runtime / "upper").mkdir(parents=True)
+    (runtime / "work").mkdir(parents=True)
+    (runtime / "merged").mkdir(parents=True)
+    result = _run(["mount", "alpha"], tmp_path)
+    assert result.returncode != 0
+    assert "instance.env not found" in result.stderr
+
+
+def test_rejects_lowerdir_count_over_cap(tmp_path: Path) -> None:
+    (tmp_path / "installation").mkdir()
+    many = [str(tmp_path / "installation")] * 501
+    _setup_instance(tmp_path, lowerdirs=many)
+    result = _run(["mount", "alpha"], tmp_path)
+    assert result.returncode != 0
+    assert "501 entries" in result.stderr
+
+
+def test_rejects_empty_lowerdir_entry(tmp_path: Path) -> None:
+    (tmp_path / "installation").mkdir()
+    _setup_instance(
+        tmp_path,
+        lowerdirs=[str(tmp_path / "installation"), "", str(tmp_path / "installation")],
+    )
+    result = _run(["mount", "alpha"], tmp_path)
+    assert result.returncode != 0
+    assert "empty entry" in result.stderr
+
+
+@pytest.mark.skipif(sys.platform != "linux", reason="user.* xattrs are Linux-only")
+def test_rejects_upperdir_with_fuseoverlayfs_xattr(tmp_path: Path) -> None:
+    _setup_instance(tmp_path)
+    tainted = tmp_path / "runtime" / "alpha" / "upper" / "deleted-thing"
+    tainted.write_bytes(b"")
+    try:
+        os.setxattr(tainted, "user.fuseoverlayfs.opaque", b"y")
+    except OSError:
+        pytest.skip("filesystem doesn't support user.* xattrs")
+    result = _run(["mount", "alpha"], tmp_path)
+    assert result.returncode != 0
+    assert "fuse-overlayfs xattr" in result.stderr