An attacker with basic CRUD permissions on a replicated collection can run the applyOps command with specially malformed oplog entries, resulting in a potential denial of service on secondaries. This issue affects MongoDB Server v4.0 versions prior to 4.0.25; MongoDB Server v4.2 versions prior to 4.2.14; MongoDB Server v4.4 versions prior to 4.4.6.

References

• https://nvd.nist.gov/vuln/detail/CVE-2021-20330
• https://jira.mongodb.org/browse/SERVER-36263