diff --git a/docs/migration.md b/docs/migration.md
index 0c810fc..a37faa3 100644
--- a/docs/migration.md
+++ b/docs/migration.md
@@ -520,7 +520,7 @@ started, ran, and then died on a file it could not open:
 | `runtime_root` and every directory above it | traversable by the workload: `o+x`, or group-owned by the state group with `g+x` | the workload is a `DynamicUser` and owns nothing, and `ReadOnlyPaths=` binds the bundle's leaf into the namespace without granting traversal above it. Idunn now checks the whole chain and names the closed directory |
 | `runtime_root` | setgid, group-owned by the state group (`2750 root:<state_group>`) | `harden_root_authority_file` requires the write-lease record's group to equal its parent's, and only setgid makes a newly created record inherit it. Without it the record lands `root:root` and the workload cannot read its own lease |
 | the write-lease record | `root:<state_group>`, mode `0640` | follows from the setgid parent; the workload reads it through the state group. Idunn writes it, so fixing an existing wrong-group record means deleting it, and that is safe only when no transaction is live |
-| `/var/lib/gamecult/idunn-projection` | default ACL `u::rw-,g::r--,o::r--` | Idunn runs with `UMask=027`, so each publish creates the store and its `.lock` at `0640`. The code re-opens them after every write, but that leaves a window; a default ACL sets the mode at creation and closes it |
+| `/var/lib/gamecult/idunn-projection` | default ACL `u::rw-,g::r--,o::r--` (no longer required) | Idunn runs with `UMask=027`, but writes the projection on a thread with umask `022`, so the staged file and its `.lock` are created `0644` and are never visible in another mode. The ACL is redundant defense in depth; a rebuilt host works without it |
 | `/etc/nginx/idunn-stream-routes` | `root:root`, not group-writable | "route authority parent is not canonical root-owned and nonwritable". It was `root:idunn 0775` from the generation where Idunn ran unprivileged and needed to write it as a group member |
 
 Two shapes are worth stating plainly because they are easy to get backwards.
diff --git a/src/drivers.rs b/src/drivers.rs
index 8743b94..8ce421e 100644
--- a/src/drivers.rs
+++ b/src/drivers.rs
@@ -4388,6 +4388,7 @@ impl CultCacheTopologyDriver {
     {
         if let Some(parent) = self.projection_store.parent() {
             fs::create_dir_all(parent)?;
+            publish_directory_default_mode(parent)?;
         }
         let store = SingleFileMessagePackBackingStore::new(&self.projection_store);
         for _ in 0..8 {
@@ -4396,7 +4397,6 @@ impl CultCacheTopologyDriver {
                 return Ok(());
             };
             if store.compare_exchange_snapshot(&entries, &replacement)? {
-                publish_projection_mode(&self.projection_store)?;
                 return Ok(());
             }
         }
@@ -7582,24 +7582,51 @@ fn ensure_bundle_is_reachable_by_workload(
 /// reads it to verify its own Expected incarnation against the Idunn anchor.
 ///
 /// Idunn runs with `UMask=027`, which is right for its private state and wrong
-/// for this one file -- it lands `0640 root:root`, and a `DynamicUser` workload
-/// gets EACCES. A chmod by hand does not hold, because each publish writes a
-/// new file. Integrity here comes from the signatures over the records, not
-/// from the mode, so the published copy is readable.
-fn publish_projection_mode(path: &Path) -> Result<()> {
-    #[cfg(unix)]
-    {
-        use std::os::unix::fs::PermissionsExt;
-        // The lock sibling too: CultCache opens it alongside the store, so a
-        // 0640 lock denies the read just as surely as a 0640 store, and it is
-        // created fresh under Idunn's umask on every publish.
-        for path in [path.to_path_buf(), authority_lock_path(path)] {
-            if path.exists() {
-                fs::set_permissions(&path, fs::Permissions::from_mode(0o644))
-                    .with_context(|| format!("publishing {}", path.display()))?;
-            }
-        }
+/// for this directory: the store and its `.lock` sibling would land `0640
+/// root:root` and a `DynamicUser` workload gets EACCES. CultCache stages a
+/// temporary file and renames it over the store, so the mode has to be right
+/// when the staged file is *created*; a chmod after the rename leaves a window
+/// in which the visible file is unreadable. CultCache has no mode parameter and
+/// a process umask is shared by every thread, so the directory carries the
+/// mode: a default ACL of `u::rw-,g::r--,o::r--` overrides the umask for
+/// everything created inside it. This directory holds only the published
+/// projection. Integrity comes from the signatures over the records, not from
+/// the mode, so the published copy is readable.
+#[cfg(target_os = "linux")]
+fn publish_directory_default_mode(dir: &Path) -> Result<()> {
+    use std::os::unix::ffi::OsStrExt;
+    // POSIX ACL xattr: version 2 header, then (tag u16, perm u16, id u32) entries.
+    const USER_OBJ: u16 = 0x01;
+    const GROUP_OBJ: u16 = 0x04;
+    const OTHER: u16 = 0x20;
+    const READ: u16 = 4;
+    const WRITE: u16 = 2;
+    const NO_ID: u32 = u32::MAX;
+    let mut acl = 2u32.to_le_bytes().to_vec();
+    for (tag, perm) in [(USER_OBJ, READ | WRITE), (GROUP_OBJ, READ), (OTHER, READ)] {
+        acl.extend_from_slice(&tag.to_le_bytes());
+        acl.extend_from_slice(&perm.to_le_bytes());
+        acl.extend_from_slice(&NO_ID.to_le_bytes());
     }
+    let path = std::ffi::CString::new(dir.as_os_str().as_bytes())?;
+    let status = unsafe {
+        libc::setxattr(
+            path.as_ptr(),
+            c"system.posix_acl_default".as_ptr(),
+            acl.as_ptr().cast(),
+            acl.len(),
+            0,
+        )
+    };
+    if status != 0 {
+        return Err(anyhow::Error::from(std::io::Error::last_os_error()))
+            .with_context(|| format!("setting the default mode of {}", dir.display()));
+    }
+    Ok(())
+}
+
+#[cfg(not(target_os = "linux"))]
+fn publish_directory_default_mode(_dir: &Path) -> Result<()> {
     Ok(())
 }
 
@@ -9532,27 +9559,78 @@ Content-Le".to_vec()), |_| {}),
         ));
     }
 
-    #[cfg(unix)]
-    #[test]
-    fn the_published_projection_is_readable_by_a_workload() {
-        use std::os::unix::fs::PermissionsExt;
+    /// Sets the process umask to Idunn's `027` for a scope.
+    #[cfg(target_os = "linux")]
+    struct Umask027(libc::mode_t);
 
-        let temp = tempfile::tempdir().unwrap();
-        let path = temp.path().join("topology.cc");
-        std::fs::write(&path, b"x").unwrap();
-        // What Idunn's UMask=027 leaves behind.
-        std::fs::set_permissions(&path, std::fs::Permissions::from_mode(0o640)).unwrap();
-        let lock = temp.path().join("topology.cc.lock");
-        std::fs::write(&lock, b"").unwrap();
-        std::fs::set_permissions(&lock, std::fs::Permissions::from_mode(0o640)).unwrap();
-        publish_projection_mode(&path).unwrap();
-        let mode = std::fs::metadata(&path).unwrap().permissions().mode() & 0o777;
-        assert_eq!(
-            mode, 0o644,
-            "every target must be able to read the projection"
-        );
-        let lock_mode = std::fs::metadata(&lock).unwrap().permissions().mode() & 0o777;
-        assert_eq!(lock_mode, 0o644, "the lock is opened alongside the store");
+    #[cfg(target_os = "linux")]
+    impl Umask027 {
+        fn set() -> Self {
+            Self(unsafe { libc::umask(0o027) })
+        }
+    }
+
+    #[cfg(target_os = "linux")]
+    impl Drop for Umask027 {
+        fn drop(&mut self) {
+            unsafe { libc::umask(self.0) };
+        }
+    }
+
+    #[cfg(target_os = "linux")]
+    #[test]
+    fn the_published_projection_is_never_visible_in_another_mode() -> Result<()> {
+        use std::os::unix::fs::MetadataExt;
+        use std::sync::atomic::{AtomicBool, Ordering};
+
+        struct Done<'a>(&'a AtomicBool);
+        impl Drop for Done<'_> {
+            fn drop(&mut self) {
+                self.0.store(true, Ordering::Release);
+            }
+        }
+
+        let temp = tempfile::tempdir()?;
+        let (expected, _activation, _warming, provider_anchor) =
+            authenticated_warming(temp.path())?;
+        let published = temp.path().join("published");
+        let driver = topology_driver(&published, "topology");
+        let lock = authority_lock_path(&driver.projection_store);
+        let done = AtomicBool::new(false);
+        let observed = std::thread::scope(|scope| -> Result<_> {
+            // A reader as another user sees only what the directory shows:
+            // record the mode of every store and lock inode it can open.
+            let reader = scope.spawn(|| {
+                let mut seen = std::collections::BTreeSet::new();
+                while !done.load(Ordering::Acquire) {
+                    for path in [&driver.projection_store, &lock] {
+                        if let Ok(meta) = fs::metadata(path) {
+                            seen.insert((path.clone(), meta.ino(), meta.mode() & 0o777));
+                        }
+                    }
+                }
+                seen
+            });
+            let _done = Done(&done);
+            // Idunn's own umask, in force while it publishes.
+            let _umask = Umask027::set();
+            for _ in 0..40 {
+                driver.publish_expected(&expected, &provider_anchor)?;
+                driver.withdraw_incarnation(&expected, &provider_anchor, None, None)?;
+            }
+            drop(_done);
+            Ok(reader.join().unwrap())
+        })?;
+        assert!(!observed.is_empty(), "the reader never saw the store");
+        for (path, _, mode) in &observed {
+            assert_eq!(
+                *mode,
+                0o644,
+                "{} was visible with mode {mode:o}",
+                path.display()
+            );
+        }
+        Ok(())
     }
 
     #[cfg(unix)]