diff --git a/src/handler/sshkeys.py b/src/handler/sshkeys.py
index 8a6cf8d..5528892 100644
--- a/src/handler/sshkeys.py
+++ b/src/handler/sshkeys.py
@@ -53,6 +53,10 @@ def materialize_private_key(hostname: str, private_key: str) -> str:
         private_key += "\n"
     fd = os.open(path, os.O_WRONLY | os.O_CREAT | os.O_TRUNC, 0o600)
     with os.fdopen(fd, "w") as fh:
+        # O_CREAT's mode only applies to brand-new files; an existing file keeps
+        # whatever mode something else gave it (e.g. a k8s fsGroup remount adding
+        # group rw), and ssh refuses keys readable beyond the owner.
+        os.fchmod(fd, 0o600)
         fh.write(private_key)
     return path
 
diff --git a/tests/test_git_servers.py b/tests/test_git_servers.py
index c5f5c76..a42e1b4 100644
--- a/tests/test_git_servers.py
+++ b/tests/test_git_servers.py
@@ -80,6 +80,18 @@ def test_materialize_private_key_is_0600_under_projects_root(env):
         assert fh.read() == "KEYDATA\n"
 
 
+def test_materialize_resets_mode_on_existing_file(env):
+    # A k8s fsGroup remount (fsGroupChangePolicy: Always) chmods volume files to
+    # group-rw between pod starts; rematerializing must restore 0600 or ssh
+    # refuses the key.
+    from handler import sshkeys
+
+    path = sshkeys.materialize_private_key("github.com", "KEYDATA")
+    os.chmod(path, 0o660)
+    path = sshkeys.materialize_private_key("github.com", "KEYDATA")
+    assert stat.S_IMODE(os.stat(path).st_mode) == 0o600
+
+
 def test_materialize_sanitizes_hostname(env):
     from handler import sshkeys