diff --git a/backend/app/db.py b/backend/app/db.py
index ede28db..98838fd 100644
--- a/backend/app/db.py
+++ b/backend/app/db.py
@@ -581,12 +581,80 @@ def set_auto_search_enabled_for_non_admin_users(enabled: bool) -> int:
 
 
 def verify_user_password(username: str, password: str) -> Optional[Dict[str, Any]]:
-    user = get_user_by_username(username)
-    if not user:
+    # Resolve case-insensitive duplicates safely by only considering local-provider rows.
+    with _connect() as conn:
+        rows = conn.execute(
+            """
+            SELECT id, username, password_hash, role, auth_provider, jellyseerr_user_id,
+                   created_at, last_login_at, is_blocked, auto_search_enabled,
+                   jellyfin_password_hash, last_jellyfin_auth_at
+            FROM users
+            WHERE username = ? COLLATE NOCASE
+            ORDER BY
+                CASE WHEN username = ? THEN 0 ELSE 1 END,
+                id ASC
+            """,
+            (username, username),
+        ).fetchall()
+    if not rows:
         return None
-    if not verify_password(password, user["password_hash"]):
-        return None
-    return user
+    for row in rows:
+        provider = str(row[4] or "local").lower()
+        if provider != "local":
+            continue
+        if not verify_password(password, row[2]):
+            continue
+        return {
+            "id": row[0],
+            "username": row[1],
+            "password_hash": row[2],
+            "role": row[3],
+            "auth_provider": row[4],
+            "jellyseerr_user_id": row[5],
+            "created_at": row[6],
+            "last_login_at": row[7],
+            "is_blocked": bool(row[8]),
+            "auto_search_enabled": bool(row[9]),
+            "jellyfin_password_hash": row[10],
+            "last_jellyfin_auth_at": row[11],
+        }
+    return None
+
+
+def get_users_by_username_ci(username: str) -> list[Dict[str, Any]]:
+    with _connect() as conn:
+        rows = conn.execute(
+            """
+            SELECT id, username, password_hash, role, auth_provider, jellyseerr_user_id,
+                   created_at, last_login_at, is_blocked, auto_search_enabled,
+                   jellyfin_password_hash, last_jellyfin_auth_at
+            FROM users
+            WHERE username = ? COLLATE NOCASE
+            ORDER BY
+                CASE WHEN username = ? THEN 0 ELSE 1 END,
+                id ASC
+            """,
+            (username, username),
+        ).fetchall()
+    results: list[Dict[str, Any]] = []
+    for row in rows:
+        results.append(
+            {
+                "id": row[0],
+                "username": row[1],
+                "password_hash": row[2],
+                "role": row[3],
+                "auth_provider": row[4],
+                "jellyseerr_user_id": row[5],
+                "created_at": row[6],
+                "last_login_at": row[7],
+                "is_blocked": bool(row[8]),
+                "auto_search_enabled": bool(row[9]),
+                "jellyfin_password_hash": row[10],
+                "last_jellyfin_auth_at": row[11],
+            }
+        )
+    return results
 
 
 def set_user_password(username: str, password: str) -> None:
diff --git a/backend/app/routers/auth.py b/backend/app/routers/auth.py
index f6fe65e..0a546ad 100644
--- a/backend/app/routers/auth.py
+++ b/backend/app/routers/auth.py
@@ -8,6 +8,7 @@ from ..db import (
     create_user_if_missing,
     set_last_login,
     get_user_by_username,
+    get_users_by_username_ci,
     set_user_password,
     set_jellyfin_auth_cache,
     set_user_jellyseerr_id,
@@ -82,9 +83,26 @@ def _extract_jellyseerr_user_id(response: dict) -> int | None:
 
 @router.post("/login")
 async def login(form_data: OAuth2PasswordRequestForm = Depends()) -> dict:
+    # Provider placeholder passwords must never be accepted by the local-login endpoint.
+    if form_data.password in {"jellyfin-user", "jellyseerr-user"}:
+        raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="Invalid credentials")
+    matching_users = get_users_by_username_ci(form_data.username)
+    has_external_match = any(
+        str(user.get("auth_provider") or "local").lower() != "local" for user in matching_users
+    )
+    if has_external_match:
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail="This account uses external sign-in. Use the external sign-in option.",
+        )
     user = verify_user_password(form_data.username, form_data.password)
     if not user:
         raise HTTPException(status_code=status.HTTP_401_UNAUTHORIZED, detail="Invalid credentials")
+    if str(user.get("auth_provider") or "local").lower() != "local":
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail="This account uses external sign-in. Use the external sign-in option.",
+        )
     if user.get("is_blocked"):
         raise HTTPException(status_code=status.HTTP_403_FORBIDDEN, detail="User is blocked")
     token = create_access_token(user["username"], user["role"])