Zoho ManageEngine ADSelfService Plus - Authentication Bypass (CVE-2021-40539)
391Exploiting IPs reported
Zoho ManageEngine ADSelfService Plus version 6113 and prior is vulnerable to REST API authentication bypass with resultant remote code execution.
CrowdSec analysis
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.
Insights from the CrowdSec network reveal that the attackers trying to exploit CVE-2021-40539 are composed of a fairly even mix of opportunistic and targeted actors. Some attackers employ preliminary reconnaissance, while others use indiscriminate scanning. CrowdSec network telemetry also shows that exploitation of CVE-2021-40539 has significantly declined over the past week. Attack volumes are well below the long-term average, suggesting attackers are rapidly losing interest. The vulnerability appears to be falling out of active use across most threat landscapes.
Exploitation efforts typically focus on endpoints containing /restapi/logoncustomization, /restapi/connection, or /help/admin-guide/test.jsp.
Exploitation
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Protection
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Blocklist
With our advanced worldwide network detection, CrowdSec can provide a list of IPs known for exploiting the vulnerability.
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