diff --git a/codex-rs/core/src/config_loader/macos.rs b/codex-rs/core/src/config_loader/macos.rs
index 5570e97c54..85773201c2 100644
--- a/codex-rs/core/src/config_loader/macos.rs
+++ b/codex-rs/core/src/config_loader/macos.rs
@@ -154,6 +154,16 @@ fn parse_managed_config_base64(encoded: &str) -> io::Result<Option<ManagedAdminC
                     return Ok(None);
                 }
             };
+            if let Some(dropped_entry) = sanitized
+                .dropped_entries
+                .iter()
+                .find(|entry| is_invalid_security_managed_config_entry(entry))
+            {
+                return Err(io::Error::new(
+                    io::ErrorKind::InvalidData,
+                    format!("Error parsing managed config from MDM: {dropped_entry}"),
+                ));
+            }
             for dropped_entry in &sanitized.dropped_entries {
                 tracing::warn!(
                     dropped_entry = %dropped_entry,
@@ -182,6 +192,15 @@ fn parse_managed_config_base64(encoded: &str) -> io::Result<Option<ManagedAdminC
     }
 }
 
+fn is_invalid_security_managed_config_entry(dropped_entry: &str) -> bool {
+    let path = dropped_entry
+        .split_once(':')
+        .map_or(dropped_entry, |(path, _)| path)
+        .trim();
+    let top_level = path.split(['.', '[']).next().unwrap_or(path);
+    matches!(top_level, "approval_policy" | "sandbox_mode")
+}
+
 fn parse_managed_requirements_base64(encoded: &str) -> io::Result<ConfigRequirementsToml> {
     let source = managed_preferences_requirements_source();
     let raw_toml = decode_managed_preferences_base64(encoded).map_err(|err| {
diff --git a/codex-rs/core/src/config_loader/tests.rs b/codex-rs/core/src/config_loader/tests.rs
index c05e4543cd..6dc15ac02b 100644
--- a/codex-rs/core/src/config_loader/tests.rs
+++ b/codex-rs/core/src/config_loader/tests.rs
@@ -468,6 +468,48 @@ writable_roots = ["relative/path"]
     Ok(())
 }
 
+#[cfg(target_os = "macos")]
+#[tokio::test]
+async fn managed_preferences_invalid_security_config_entry_fails_closed() -> anyhow::Result<()> {
+    use base64::Engine;
+
+    for (payload, expected_fragment) in [
+        (
+            "approval_policy = \"bogus\"\nmodel = \"managed\"\n",
+            "approval_policy",
+        ),
+        (
+            "sandbox_mode = \"bogus\"\nmodel = \"managed\"\n",
+            "sandbox_mode",
+        ),
+    ] {
+        let tmp = tempdir()?;
+        let err = load_config_layers_state(
+            tmp.path(),
+            Some(AbsolutePathBuf::try_from(tmp.path())?),
+            &[] as &[(String, TomlValue)],
+            LoaderOverrides {
+                managed_config_path: Some(tmp.path().join("managed_config.toml")),
+                managed_preferences_base64: Some(
+                    base64::prelude::BASE64_STANDARD.encode(payload.as_bytes()),
+                ),
+                macos_managed_config_requirements_base64: None,
+            },
+            CloudRequirementsLoader::default(),
+        )
+        .await
+        .expect_err("invalid managed security config should fail closed");
+
+        assert_eq!(err.kind(), std::io::ErrorKind::InvalidData);
+        let message = err.to_string();
+        assert!(message.contains("Error parsing managed config from MDM"));
+        assert!(message.contains(expected_fragment), "{message}");
+        assert!(message.contains("bogus"), "{message}");
+    }
+
+    Ok(())
+}
+
 #[cfg(target_os = "macos")]
 #[tokio::test]
 async fn managed_preferences_ignore_invalid_payload() -> anyhow::Result<()> {