CVE-2021-39341: OptinMonster <= 2.6.4 Unprotected REST-API Endpoints

Is it exploited?

Reported, not confirmed by CISA. VulnCheck KEV lists it as exploited in the wild. CISA has not added it to its Known Exploited Vulnerabilities catalog as of 29 Oct 2026.

VulnCheck KEV lists it since 27 Oct 2021. It cites 1 public report, the first from 27 Oct 2021.

• patchstack.com

How likely is exploitation in the next 30 days?

21.9%

EPSS score, percentile 97.7 of all scored records. A probability estimated by FIRST, not a report of exploitation.

How severe?

8.2 High

CVSS v3.1 base score, given by the CNA

What should I do?

Update to version 2.6.5, or newer.Solution from the CVE record.

CISA has set no required action, because the vulnerability is not in KEV.

The CVE record links no vendor advisory, patch or mitigation.

Which versions are affected?

Affected

• 2.6.4 through 2.6.4

Versions by product

Source: CISA KEV 2026.10.29, as of 29 Oct 2026; VulnCheck KEV, as of 30 Oct 2026; FIRST EPSS, as of 30 Oct 2026; CVE List, as of 30 Oct 2026. About the data.

Description

OptinMonster <= 2.6.4 Unprotected REST-API Endpoints

The OptinMonster WordPress plugin is vulnerable to sensitive information disclosure and unauthorized setting updates due to insufficient authorization validation via the logged_in_or_has_api_key function in the ~/OMAPI/RestApi.php file that can used to exploit inject malicious web scripts on sites with the plugin installed. This affects versions up to, and including, 2.6.4.

Source: CVE List, as of 30 Oct 2026. About the data.

What to do

Advisories and patches

The CVE record links no vendor advisory, patch or mitigation.

This summary is based on the CVE record as of 30 Oct 2026. Check the vendor's advisory before you act: affected versions and fixes can change.

Affected products and versions

Source: CVE List, as of 30 Oct 2026. About the data.

Timeline