MongoDb Reachable Assertion Vulnerability - CVE-2026-9749

This issue can occur when running an aggregation pipeline that uses the internal exchange stage configured with key-range partitioning and order-preserving delivery. If a single key range produces enough documents to fill its exchange buffer (that is many results are routed to the same consumer) the server reaches the code path where a full per-consumer buffer is detected but the internal quothigh watermarkquot for that key range is not updated as intended.