Zoho ManageEngine ADSelfService Plus - Authentication Bypass (CVE-2021-40539)

CVE-2021-40539 is a critical vulnerability in Zoho ManageEngine ADSelfService Plus, enabling remote code execution by bypassing REST API authentication mechanisms.

CrowdSec has been tracking this vulnerability and its exploits since 7th of April 2025.

According to CrowdSec data, while opportunistic exploitation dominates, a portion of threat actors trying to exploit CVE-2021-40539 apply basic targeting methods such as port or service detection. This indicates emerging patterns of selective targeting. Telemetry from the CrowdSec network also shows that exploitation activity for CVE-2021-40539 remains steady week-over-week. Attack volumes are consistent with long-term trends, indicating sustained interest from threat actors. CVE-2021-40539 continues to be an active part of the threat landscape and will likely remain this way for the forseeable future.

Exploitation efforts typically focus on endpoints containing /restapi/logoncustomization, /restapi/connection, or /help/admin-guide/test.jsp.