CVE-2026-120803: MongoDB SBE $locf Window Accumulator Use-After-Free Enables Heap OOB Read and Server Crash

A use-after-free issue exists in MongoDB’s SBE (Slot-Based Execution) implementation of a window function accumulator used by certain $setWindowFields aggregation pipelines. Under specific conditions, the accumulator can retain a dangling pointer to memory that has since been freed. When this stale pointer is later dereferenced during serialization of the carried-forward value, it can result in a heap out-of-bounds read that may expose adjacent heap contents and crash the mongod server. An authenticated user with readWrite privileges on a database can trigger this issue by crafting a specific aggregation pipeline.