diff --git a/scripts/governance_snapshot.sh b/scripts/governance_snapshot.sh
index 27915efed..7baccf1d6 100755
--- a/scripts/governance_snapshot.sh
+++ b/scripts/governance_snapshot.sh
@@ -60,8 +60,26 @@ g() {
 }
 
 # Owned by the invoker and writable by no one else (-perm -g+w / -o+w: BSD find has no `/`).
+# GROUP-WRITABLE IN A PRIVATE GROUP IS STILL OWNER-ONLY (pub-claude, 2026-10-23). Ubuntu gives each user a
+# group of their own and a umask of 002, so every clone there is g+w: on pub the hourly snapshot refused its
+# own checkout from the hour #520 landed. A group bit reaches someone else only if the group has someone
+# else in it, so g+w passes when the file's group lists no member but the invoker and is no other account's
+# primary group. Without getent (macOS) that cannot be asked, and g+w is refused as before; a gid with no
+# group entry is refused too. The check is absence-of-evidence: it trusts getent to enumerate every account,
+# which holds on files-backed hosts (pub, cbp, HUB) but not under sssd/LDAP with enumeration off.
+private_group() {
+  command -v getent >/dev/null 2>&1 || return 1
+  pg_gid="$(stat -c %g "$1" 2>/dev/null || stat -f %g "$1" 2>/dev/null)" || return 1
+  pg_me="$(id -un)"
+  pg_entry="$(getent group "$pg_gid")" || return 1
+  pg_members="$(printf '%s\n' "$pg_entry" | cut -d: -f4)"
+  [ -z "$pg_members" ] || [ "$pg_members" = "$pg_me" ] || return 1
+  [ -z "$(getent passwd | awk -F: -v g="$pg_gid" -v me="$pg_me" '$4 == g && $1 != me')" ]
+}
 owner_only() {
-  [ -e "$1" ] && [ -O "$1" ] && [ -z "$(find "$1" -maxdepth 0 \( -perm -g+w -o -perm -o+w \) 2>/dev/null || true)" ]
+  [ -e "$1" ] && [ -O "$1" ] || return 1
+  [ -z "$(find "$1" -maxdepth 0 -perm -o+w 2>/dev/null || true)" ] || return 1
+  [ -z "$(find "$1" -maxdepth 0 -perm -g+w 2>/dev/null || true)" ] || private_group "$1"
 }
 if [ "$PRIVATE" -eq 0 ]; then
   GITDIR="$(g rev-parse --absolute-git-dir 2>/dev/null || true)"
diff --git a/scripts/tests/test_governance_snapshot.py b/scripts/tests/test_governance_snapshot.py
index 46432800e..486610e1d 100644
--- a/scripts/tests/test_governance_snapshot.py
+++ b/scripts/tests/test_governance_snapshot.py
@@ -137,8 +137,31 @@ def test_a_deploy_step_reads_the_deploys_own_clone_at_the_deployed_build_without
     assert _git("-C", private, "rev-parse", "origin/main") == before      # no fetch into it
 
 
-def test_a_source_someone_else_can_write_is_refused(world):
-    os.chmod(world["src"], os.stat(world["src"]).st_mode | stat.S_IWGRP)
+def _group_is_private(path):
+    """The script's rule, asked of Python: nobody but the invoker is in the file's group.
+    It restates the rule, so it proves which branch the script takes, not that the rule is right."""
+    import grp, pwd
+    gid, me = os.stat(path).st_gid, pwd.getpwuid(os.getuid()).pw_name
+    try:
+        members = grp.getgrgid(gid).gr_mem
+    except KeyError:
+        return False
+    return set(members) <= {me} and not [p for p in pwd.getpwall() if p.pw_gid == gid and p.pw_name != me]
+
+
+def test_a_source_anyone_can_write_is_refused(world):
+    os.chmod(world["src"], os.stat(world["src"]).st_mode | stat.S_IWOTH)
     r = _run(world)
     assert r.returncode == 3 and "not owned by" in r.stderr, (r.returncode, r.stderr)
     assert _snapshot(world) is None
+
+
+def test_a_group_writable_source_is_refused_unless_the_group_is_the_invokers_alone(world):
+    # Ubuntu's private group + umask 002 makes every clone g+w; that bit reaches no one else.
+    os.chmod(world["src"], os.stat(world["src"]).st_mode | stat.S_IWGRP)
+    r = _run(world)
+    if shutil.which("getent") and _group_is_private(world["src"]):
+        assert r.returncode == 0 and _snapshot(world) is not None, (r.returncode, r.stderr)
+    else:
+        assert r.returncode == 3 and "not owned by" in r.stderr, (r.returncode, r.stderr)
+        assert _snapshot(world) is None