An issue in MongoDB server’s aggregation framework allows an authenticated user with read privileges to cause a denial of service by sending specially crafted aggregate commands using an internal expression. Despite its internal naming, this expression is registered as accessible to any authenticated client. By supplying crafted input parameters across multiple concurrent connections, each evaluation triggers unbounded memory allocations with no per-expression memory limits enforced. This can exhaust available server memory and cause the mongod process to be terminated by the operating system’s out-of-memory killer.