# CVE-2021-40539: Zoho ManageEngine ADSelfService Plus - Authentication Bypass

> Live exploitation tracking for CVE-2021-40539 by the CrowdSec Live Exploit Tracker, built from real-world attack data crowdsourced from CrowdSec's global network.

Page: https://tracker.crowdsec.net/cves/CVE-2021-40539

## Key facts

- **Exploitation phase:** Background Noise. Continuous low-level scanning or exploitation attempts are observed, mostly opportunistic and automated.
- **CVSS score:** 9.8
- **Public exploit available:** Yes
- **Affected products:** ManageEngine ADSelfService Plus
- **Weaknesses:** CWE-706 (Use of Incorrectly-Resolved Name or Reference)
- **Tags:** web_application

## Description

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](https://euvd.enisa.europa.eu/vulnerability/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.
Data from the CrowdSec community also indicates a gradual decrease in attacks targeting CVE-2021-40539. While still present in the wild, exploitation levels have dropped noticeably week-over-week. This may signal that the vulnerability is becoming less relevant or that defenses are improving fast enough for attackers to lose interest.

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

## Timeline

- 2021-09-07: CVE Published. CVE-2021-40539 is published to NVD.
- 2021-11-03: CISA KEV. CVE-2021-40539 is added to the Known Exploited Vulnerabilities catalog by CISA.
- 2021-11-17: CISA Remediation Deadline. Expiration of the CISA remediation deadline as described in [BOD 22-01](https://www.cisa.gov/news-events/directives/bod-22-01-reducing-significant-risk-known-exploited-vulnerabilities).
- 2025-04-07: Rule Released. CrowdSec releases a rule to detect CVE-2021-40539 exploitation attempts against the CrowdSec Network.
- 2025-04-07: CrowdSec First Seen. CrowdSec observes in-the-wild exploitation of CVE-2021-40539 for the first time.

## References

- https://www.manageengine.com
- https://github.com/projectdiscovery/nuclei-templates/blob/main/http/cves/2021/CVE-2021-40539.yaml

## Live exploitation data

The [Live Exploit Tracker page](https://tracker.crowdsec.net/cves/CVE-2021-40539) follows observed exploitation activity for CVE-2021-40539. [Request an API key](https://tracker.crowdsec.net/request-api-key) for full access to its exploitation timeline, attacking IP addresses and blocklists.