CVE 2019-2393

A user authorized to perform database queries may trigger denial of service by issuing specially crafted queries, which use $lookup and collations. This issue affects MongoDB Server v4.2 versions prior to 4.2.1; MongoDB Server v4.0 versions prior to 4.0.13 and MongoDB Server v3.6 versions prior to 3.6.15.

See the CVE page on Mitre.org for more details.