diff --git a/codex-rs/linux-sandbox/src/daemon_mounts.rs b/codex-rs/linux-sandbox/src/daemon_mounts.rs
index 713688b59c..fbe724c7f0 100644
--- a/codex-rs/linux-sandbox/src/daemon_mounts.rs
+++ b/codex-rs/linux-sandbox/src/daemon_mounts.rs
@@ -39,14 +39,19 @@ fn check_mounts(
         let [_, _, mount_device, root, destination] = fields.as_slice() else {
             return Err(invalid());
         };
-        let root = mount_path(root)?;
         let destination = mount_path(destination)?;
-        if *mount_device == device.as_bytes()
+        // Only roots on the socket filesystem can identify aliases. Other
+        // filesystems can use non-path roots such as nsfs `mnt:[inode]`, but
+        // their destinations still matter for nested-mount checks.
+        let root = (*mount_device == device.as_bytes())
+            .then(|| mount_path(root))
+            .transpose()?;
+        if let Some(root) = &root
             && let Ok(relative) = directory.strip_prefix(&destination)
         {
             locations.insert(root.join(relative));
         }
-        mounts.push((*mount_device, root, destination));
+        mounts.push((root, destination));
     }
     // Overmounts can leave hidden entries in mountinfo. Require every possible
     // containing mount to agree instead of guessing which root is visible.
@@ -54,10 +59,10 @@ fn check_mounts(
         return Err(invalid());
     }
     let location = locations.into_iter().next().ok_or_else(invalid)?;
-    for (mount_device, root, destination) in &mounts {
+    for (root, destination) in &mounts {
         // Nested mounts can introduce another filesystem (or an individual socket) under the mask.
         let nested = destination != directory && destination.starts_with(directory);
-        let alias = if *mount_device == device.as_bytes() {
+        let alias = if let Some(root) = root {
             if let Ok(relative) = location.strip_prefix(root) {
                 Some(destination.join(relative))
             } else if root.starts_with(&location) {
diff --git a/codex-rs/linux-sandbox/src/daemon_mounts_tests.rs b/codex-rs/linux-sandbox/src/daemon_mounts_tests.rs
index 04e0fe5d25..de0f3c208b 100644
--- a/codex-rs/linux-sandbox/src/daemon_mounts_tests.rs
+++ b/codex-rs/linux-sandbox/src/daemon_mounts_tests.rs
@@ -29,6 +29,44 @@ fn rejects_only_mounts_that_compromise_the_directory(root: &str, destination: &s
     );
 }
 
+#[test_case("0:2", "mnt:[4026532835]", "/run/snapd/ns/example.mnt", Ok(()); "unrelated mount namespace")]
+#[test_case("0:2", "net:[4026531840]", "/run/netns/example", Ok(()); "unrelated network namespace")]
+#[test_case("0:2", "mnt:[4026532835]", "/tmp/codex-daemon-1000/ns", Err(io::ErrorKind::PermissionDenied); "nested namespace mount")]
+#[test_case("0:1", "mnt:[4026532835]", "/run/snapd/ns/example.mnt", Err(io::ErrorKind::Other); "non-path root on socket filesystem")]
+#[test_case("0:2", "mnt:[4026532835]", "relative/ns", Err(io::ErrorKind::Other); "relative destination")]
+#[test_case("0:2", "mnt:[4026532835]", "/run/snapd/ns/\\invalid", Err(io::ErrorKind::Other); "invalid destination escape")]
+fn validates_namespace_mounts_by_device_and_destination(
+    device: &str,
+    root: &str,
+    destination: &str,
+    expected: Result<(), io::ErrorKind>,
+) {
+    let mounts = format!(
+        "1 0 0:1 / / rw - ext4 disk rw\n2 1 {device} {root} {destination} rw - nsfs nsfs rw\n"
+    );
+    assert_eq!(
+        check_mounts(
+            Path::new("/tmp/codex-daemon-1000"),
+            "0:1",
+            mounts.as_bytes()
+        )
+        .map_err(|error| error.kind()),
+        expected
+    );
+}
+
+#[test]
+fn unrelated_namespace_mount_does_not_hide_a_socket_alias() {
+    let mounts = b"1 0 0:1 / / rw - ext4 disk rw\n\
+                   2 1 0:2 net:[4026531840] /run/netns/example rw - nsfs nsfs rw\n\
+                   3 1 0:1 /tmp/codex-daemon-1000 /alias rw - ext4 disk rw\n";
+    assert_eq!(
+        check_mounts(Path::new("/tmp/codex-daemon-1000"), "0:1", mounts)
+            .map_err(|error| error.kind()),
+        Err(io::ErrorKind::PermissionDenied)
+    );
+}
+
 #[test]
 fn rejects_alias_when_tmp_is_itself_a_bind_mount() {
     let mounts = b"1 0 0:1 / / rw - ext4 disk rw\n2 1 0:1 /backing/tmp /tmp rw - ext4 disk rw\n";