What’s Vulnerable › Vulnerabilities

CVE-2021-20330: Specific replication command with malformed oplog entries can crash secondaries

• Severity: medium (CVSS 6.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.1% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-12-15 · updated 2026-06-17
• Weakness: CWE-20

Description

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 4.0.27; MongoDB Server v4.2 versions prior to 4.2.16; MongoDB Server v4.4 versions prior to 4.4.9.

Affected products

• MongoDB: from 4.0.0 before 4.0.25; from 4.2.0 before 4.2.14; from 4.4.0 before 4.4.6 — fixed in 4.0.25, 4.2.14, 4.4.6

How to fix

Upgrade to a fixed version: MongoDB 4.0.25, 4.2.14, 4.4.6. Check the vendor advisory for mitigations.

References

• https://jira.mongodb.org/browse/SERVER-36263
• https://jira.mongodb.org/browse/SERVER-36263

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