diff --git a/SECURITY.md b/SECURITY.md
index 71fa342..2b2ad77 100644
--- a/SECURITY.md
+++ b/SECURITY.md
@@ -45,7 +45,8 @@ sites.
 On Desktop Only the Hub is not published at all. It is reachable only from the
 machine itself, through the desktop application window on localhost. Desktop
 Only is the role most likely to be used away from home, and a root-capable admin
-UI has no business listening on a coffee-shop network.
+UI has no business listening on a coffee-shop network. The firewall there opens
+no TCP port at all; the only port open is UDP 5353, for mDNS.
 `sovran_systemsOS.hub.directPort = true` in `custom.nix` opens port 8937 if you
 do want to reach a Desktop Only Hub from another device.
 
diff --git a/modules/core/sshd-localhost.nix b/modules/core/sshd-localhost.nix
index 76be80f..9d4841a 100644
--- a/modules/core/sshd-localhost.nix
+++ b/modules/core/sshd-localhost.nix
@@ -9,6 +9,13 @@
 
   services.openssh = {
     enable = true;
+    # sshd listens on 127.0.0.1 only here, so there is nothing for the firewall
+    # to let in. NixOS opens sshd's ports by default (openFirewall = true)
+    # whether or not sshd listens on them, which left port 22 open on every
+    # role, Desktop Only included. The roles that do publish SSH open it
+    # themselves: the sshd feature (sshd.nix) and remote deploy
+    # (remote-deploy.nix) both add 22 explicitly.
+    openFirewall = lib.mkDefault false;
     listenAddresses = lib.mkDefault [
       { addr = "127.0.0.1"; port = 22; }
     ];
diff --git a/tests/test_ssh_exposure.py b/tests/test_ssh_exposure.py
new file mode 100644
index 0000000..fc915c1
--- /dev/null
+++ b/tests/test_ssh_exposure.py
@@ -0,0 +1,64 @@
+"""Guards for when port 22 is open in the firewall.
+
+sshd-localhost.nix gives every role "ssh root@localhost" by listening on
+127.0.0.1 only. NixOS opens sshd's ports in the firewall by default whether or
+not sshd listens on them, which left port 22 open on every role, Desktop Only
+included, with nothing behind it. The roles that really publish SSH open it
+explicitly, so the default has to stay off.
+
+Like the other nix-file checks these read the modules as text: nothing is run
+and nothing touches the network.
+"""
+
+import os
+import re
+import unittest
+
+_ROOT = os.path.normpath(os.path.join(os.path.dirname(__file__), ".."))
+
+
+def _read(*parts):
+    with open(os.path.join(_ROOT, *parts), encoding="utf-8") as f:
+        return f.read()
+
+
+def _without_comments(src):
+    return "\n".join(l for l in src.splitlines() if not l.lstrip().startswith("#"))
+
+
+class LocalhostSshdDoesNotOpenThePort(unittest.TestCase):
+
+    def test_the_firewall_is_not_opened_by_default(self):
+        code = _without_comments(_read("modules", "core", "sshd-localhost.nix"))
+        self.assertRegex(code, r"openFirewall\s*=\s*lib\.mkDefault\s+false\s*;")
+
+    def test_it_still_listens_on_loopback_only(self):
+        code = _without_comments(_read("modules", "core", "sshd-localhost.nix"))
+        self.assertRegex(code, r'addr\s*=\s*"127\.0\.0\.1"')
+        self.assertNotIn("0.0.0.0", code)
+
+
+class PublishedSshOpensItsOwnPort(unittest.TestCase):
+    """Turning the default off must not close the roles that want SSH open."""
+
+    def test_the_sshd_feature_opens_22_and_only_when_enabled(self):
+        src = _without_comments(_read("modules", "sshd.nix"))
+        self.assertRegex(src, r"lib\.mkIf\s+config\.sovran_systemsOS\.features\.sshd")
+        self.assertRegex(src, r"networking\.firewall\.allowedTCPPorts\s*=\s*\[\s*22\s*\]")
+
+    def test_remote_deploy_opens_22_and_only_when_enabled(self):
+        src = _without_comments(_read("modules", "core", "remote-deploy.nix"))
+        self.assertRegex(src, r"lib\.mkIf\s+cfg\.enable")
+        self.assertRegex(src, r"networking\.firewall\.allowedTCPPorts\s*=\s*\[\s*22\s*\]")
+
+
+class DesktopOnlyDocumentsWhatItOpens(unittest.TestCase):
+
+    def test_security_policy_says_desktop_opens_no_tcp_port(self):
+        text = " ".join(_read("SECURITY.md").split())  # the file is line-wrapped
+        self.assertIn("opens no TCP port", text)
+        self.assertIn("UDP 5353", text)
+
+
+if __name__ == "__main__":
+    unittest.main()