diff --git a/docker/Start-Codeman.sh b/docker/Start-Codeman.sh
index 0109b69a..41fa2678 100644
--- a/docker/Start-Codeman.sh
+++ b/docker/Start-Codeman.sh
@@ -230,6 +230,9 @@ accept_appdata_path() {
     '~') p=$HOME ;;
     '~/'*) p="$HOME/${p#\~/}" ;;
   esac
+  while [[ "$p" == *//* ]]; do
+    p=${p//\/\//\/}
+  done
   while [[ "$p" == */ && "$p" != / ]]; do
     p=${p%/}
   done
@@ -247,6 +250,14 @@ accept_appdata_path() {
     printf '  Use an absolute path, one that starts with /.\n' >&2
     return 1
   fi
+  # The refusals below compare strings, so a dot segment could spell any folder
+  # they exist to keep out.
+  case "$p/" in
+    */./* | */../*)
+      printf '  Spell the path out, without . or .. in it.\n' >&2
+      return 1
+      ;;
+  esac
   # The folder becomes the container's home directory, so its `.codeman` is
   # the server's state directory: $HOME itself would share state.json with a
   # Codeman installed directly on this machine.
diff --git a/test/docker-start-setup.test.ts b/test/docker-start-setup.test.ts
index 25d517eb..f207b286 100644
--- a/test/docker-start-setup.test.ts
+++ b/test/docker-start-setup.test.ts
@@ -343,6 +343,13 @@ describe('Start-Codeman.sh first run (no docker/.env yet)', () => {
     ['a relative path', () => 'codeman-data', /absolute path/],
     ['the checkout, which is the image build context', ({ repo }: { repo: string }) => repo, /copied into the image/],
     ['a folder inside the checkout', ({ repo }: { repo: string }) => join(repo, 'data'), /copied into the image/],
+    // join() would resolve these, which is the whole point: the refusals above compare strings.
+    ['HOME spelled through ..', ({ home }: { home: string }) => `${home}/../home`, /without \. or \.\./],
+    ['the checkout spelled through ..', ({ home }: { home: string }) => `${home}/../repo/data`, /without \. or \.\./],
+    ['a path ending in ..', ({ home }: { home: string }) => `${home}/data/..`, /without \. or \.\./],
+    ['a native install spelled through .', ({ home }: { home: string }) => `${home}/./.codeman`, /without \. or \.\./],
+    ['a native install behind a doubled slash', ({ home }: { home: string }) => `${home}//.codeman`, /installed directly/],
+    ['the checkout behind a doubled slash', ({ repo }: { repo: string }) => `${repo}//data`, /copied into the image/],
   ])('refuses %s as the data folder and writes nothing', (_name, appdata, reason) => {
     const r = runStart(['--setup-only'], { appdata });
     expect(r.status).toBe(1);
@@ -402,6 +409,12 @@ describe('Start-Codeman.sh first run (no docker/.env yet)', () => {
     expect(r.envKind).toBe('symlink');
   });
 
+  it('keeps a folder name that merely starts with a dot or dots', () => {
+    const r = runStart(['--setup-only'], { appdata: ({ home }) => `${home}/..data/.state` });
+    expect(r.status, r.stderr).toBe(0);
+    expect(envValue(r.env as string, 'CODEMAN_APPDATA_PATH')).toBe(`${r.home}/..data/.state`);
+  });
+
   it('never rewrites an existing docker/.env', () => {
     const existing = '# hand-written\nCODEMAN_PASSWORD=mine-and-only-mine\nCODEMAN_APPDATA_PATH=/x\n';
     const r = runStart(['--setup-only'], { existingEnv: existing });