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

CVE ID
CVE-2021-20330

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.

CVSS score
This issue's CVSS:3.1 severity is scored at 6.5 using the following scoring metrics:
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Affected versions

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.

CWE
CWE-20 Improper Input Validation

Underlying operating systems affected
ALL

As of SERVER-25994, a user can run applyOps if they have the privileges to perform each individual operation specified in the the applyOps command. However, applyOps is more powerful than other commands in that it avoids certain input validation (see SERVER-27096, SERVER-32941, SERVER-32952, and SERVER-32305). This is done intentionally, since applyOps is supposed to behave similarly to oplog application, where the primary does all validation and the secondary applies the changes exactly as the primary specified without validation. This feature is important to products that mimic oplog application, such as mongomirror and mongorestore. However, users should not be able to bypass validation simply because they have permission to write to a collection. Instead, applyOps should require a special privilege for bypassing validation.

We will create a new privilege bypassing system-level invariants in applyOps. Today, this privilege will be required in order to run applyOps at all, since we have not implemented a version of applyOps that performs validation. The privilege will be included in dbAdminAnyDatabase, which is included in the custom role atlasAdmin and the temporary user that we create for Live Imports (mongomirror).