diff --git a/backend/tests/preserve_on_behalf_of.rs b/backend/tests/preserve_on_behalf_of.rs
index 36bb5173ee..7c738d3eba 100644
--- a/backend/tests/preserve_on_behalf_of.rs
+++ b/backend/tests/preserve_on_behalf_of.rs
@@ -1495,6 +1495,57 @@ async fn test_app_draft_policy_derives_on_behalf_of_email(db: Pool<Postgres>) ->
     Ok(())
 }
 
+/// A draft is stored unvalidated, so its principal is whatever its author typed. Resolving it
+/// through the instance-wide fallback would let any member turn a guessed username into a
+/// non-member superadmin's address.
+#[sqlx::test(fixtures("preserve_on_behalf_of"))]
+async fn test_app_draft_derivation_does_not_disclose_non_member_email(
+    db: Pool<Postgres>,
+) -> anyhow::Result<()> {
+    initialize_tracing().await;
+
+    let server = ApiServer::start(db.clone()).await?;
+    let port = server.addr.port();
+    let base = format!("http://localhost:{port}/api/w/test-workspace");
+    let path = "u/test-user-2/leak_probe";
+
+    // test-user-2 is an ordinary member; superadmin-external has no `usr` row here.
+    sqlx::query!(
+        "INSERT INTO draft (workspace_id, email, path, typ, value)
+         VALUES ($1, $2, $3, 'raw_app', $4)",
+        "test-workspace",
+        "test2@windmill.dev",
+        path,
+        json!({
+            "path": path,
+            "summary": "Probe",
+            "value": { "type": "rawapp", "inline_script": null },
+            "policy": {
+                "execution_mode": "anonymous",
+                "triggerables": {},
+                "on_behalf_of": "u/superadmin-external"
+            }
+        })
+    )
+    .execute(&db)
+    .await?;
+
+    for url in [
+        format!("{base}/drafts/get_own/raw_app/{path}"),
+        format!("{base}/apps/get/p/{path}?get_draft=true&raw_app=true"),
+    ] {
+        let resp = authed(client().get(&url), "SECRET_TOKEN_2").send().await?;
+        assert_eq!(resp.status(), 200, "Should read own draft at {url}");
+        let body = resp.text().await?;
+        assert!(
+            !body.contains("superadmin-external@windmill.dev"),
+            "Draft derivation must not disclose a non-member superadmin's address via {url}: {body}"
+        );
+    }
+
+    Ok(())
+}
+
 /// Test schedule update preserves email/edited_by correctly
 #[sqlx::test(fixtures("preserve_on_behalf_of"))]
 async fn test_schedule_update_preserves_email(db: Pool<Postgres>) -> anyhow::Result<()> {
diff --git a/backend/windmill-api/src/apps.rs b/backend/windmill-api/src/apps.rs
index 03420a5c73..99fdf48e9e 100644
--- a/backend/windmill-api/src/apps.rs
+++ b/backend/windmill-api/src/apps.rs
@@ -288,6 +288,55 @@ async fn derive_policy_on_behalf_of_email(
     Ok(())
 }
 
+/// The draft variant of the derivation. A draft is stored exactly as its editor sent it, with
+/// no `resolve_on_behalf_of` pass, so its principal is caller-controlled: anyone who can save a
+/// draft under their own `u/` path chooses it. `u/` is therefore resolved against workspace
+/// membership alone, never `resolve_username_to_email`'s instance `password` fallback, which
+/// would turn a guessed username into a non-member superadmin's address. A principal that names
+/// no member is left alone rather than answered.
+///
+/// `g/` and the bare-address form skip the lookup: the first is synthetic and the second is
+/// already the address, so neither tells the caller anything their draft did not.
+async fn derive_draft_on_behalf_of_email_in_place(
+    db: &DB,
+    w_id: &str,
+    obj: &mut serde_json::Map<String, serde_json::Value>,
+) -> Result<bool> {
+    let Some(permissioned_as) = obj
+        .get("on_behalf_of")
+        .and_then(|v| v.as_str())
+        .map(|s| s.to_string())
+    else {
+        return Ok(false);
+    };
+    let email = match permissioned_as.strip_prefix(windmill_common::users::PERMISSIONED_AS_USER_PREFIX)
+    {
+        Some(username) => {
+            let member = sqlx::query_scalar!(
+                "SELECT email FROM usr WHERE workspace_id = $1 AND username = $2",
+                w_id,
+                username
+            )
+            .fetch_optional(db)
+            .await?;
+            match member {
+                Some(email) => email,
+                None => return Ok(false),
+            }
+        }
+        None => {
+            windmill_common::users::get_email_from_permissioned_as_uncached(
+                &permissioned_as,
+                w_id,
+                db,
+            )
+            .await?
+        }
+    };
+    obj.insert("on_behalf_of_email".to_string(), json!(email));
+    Ok(true)
+}
+
 /// Same derivation on an already-parsed policy. Reports whether it rewrote the address, so
 /// callers holding the raw form only pay to re-serialize when there was a principal to derive.
 async fn derive_on_behalf_of_email_in_place(
@@ -328,7 +377,7 @@ async fn derive_draft_policy_on_behalf_of_email(
     else {
         return Ok(());
     };
-    derive_on_behalf_of_email_in_place(db, w_id, policy, true).await?;
+    derive_draft_on_behalf_of_email_in_place(db, w_id, policy).await?;
     Ok(())
 }
 
@@ -354,7 +403,7 @@ pub(crate) async fn derive_stored_draft_policy_on_behalf_of_email(
     let Some(policy) = obj.get_mut("policy").and_then(|p| p.as_object_mut()) else {
         return Ok(());
     };
-    if derive_on_behalf_of_email_in_place(db, w_id, policy, true).await? {
+    if derive_draft_on_behalf_of_email_in_place(db, w_id, policy).await? {
         value.0 = to_raw_value(&obj);
     }
     Ok(())