How severe is CVE-2021-40539?

CVE-2021-40539 has a CVSS score of 9.8/10, rated CRITICAL. Immediate patching is strongly recommended.

Which products are affected by CVE-2021-40539?

CVE-2021-40539 affects the following products: zohocorp manageengine adselfservice plus.

How do I patch CVE-2021-40539?

Apply the zohocorp manageengine adselfservice plus security patch or hotfix for CVE-2021-40539 on the affected version, following the vendor advisory for your exact build.

How do I detect CVE-2021-40539 across my infrastructure?

Inventory every zohocorp manageengine adselfservice plus deployment and check each version against the affected-products list above. Because the exploit targets the running system rather than your application code, also watch for remote code execution at the network and runtime layer — O3 flags the exploit behaviour from runtime telemetry and egress traffic even before a vulnerable build is confirmed.

How do I mitigate CVE-2021-40539 if there is no patch (or I can't update yet)?

Cut exposure now: restrict the management/admin interface to trusted networks, segment the device, and apply the vendor's recommended configuration mitigations and any WAF/IPS signature. O3's runtime protection blocks the exploit chain at execution, holding the line on unpatched or end-of-life systems until you can patch.

How does O3 Security protect against CVE-2021-40539?

O3 detects and blocks CVE-2021-40539 exploitation at runtime: eBPF exploit-chain detection, plus L7 egress monitoring that catches the post-exploitation callback and severs the attacker's outbound channel.

Is CVE-2021-40539 actively exploited in the wild?

Yes. There are 7 known exploit references for CVE-2021-40539, including 6 in-the-wild exploitations observed on GitHub and other sources. Treat this as actively exploitable and prioritize patching immediately. All exploit code should only be run in an isolated sandbox environment for research or authorized testing — never against production systems without explicit written authorization.

What is the EPSS score for CVE-2021-40539?

CVE-2021-40539 has an EPSS (Exploit Prediction Scoring System) score of 99.0%, placing it in the 100th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. A score above 50% indicates significantly elevated exploitation risk — immediate patching is strongly recommended.

Is CVE-2021-40539 in the CISA Known Exploited Vulnerabilities catalog?

Yes. CVE-2021-40539 (Zoho ManageEngine ADSelfService Plus Authentication Bypass Vulnerability) was added to CISA's Known Exploited Vulnerabilities (KEV) catalog on 2021-11-03. CISA required all US federal civilian agencies to remediate this vulnerability by 2021-11-17. The required action was: Apply updates per vendor instructions. This CVE has been used in known ransomware campaigns. Being listed in the KEV catalog is the strongest possible confirmation of active exploitation — treat this as a critical priority.

What type of vulnerability is CVE-2021-40539?

CVE-2021-40539 is classified as CWE-706 (CWE-706). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

Which products and versions are affected by CVE-2021-40539?

CVE-2021-40539 affects zohocorp manageengine adselfservice plus (1 version, e.g. 6.1). Check the affected-products list on this page against your deployed version to confirm exposure.

Is there exploit tooling available for CVE-2021-40539?

A Nuclei detection template exists for CVE-2021-40539, meaning automated scanners can fingerprint it. Public exploit tooling significantly increases real-world risk — prioritize patching and ensure detection coverage. Exploit tooling should only ever be used in isolated, authorized testing environments.

When was CVE-2021-40539 published, and has it been updated?

CVE-2021-40539 was published on September 7, 2021 and was last updated on June 17, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🛡️

CISA KEV · ACTIVELY EXPLOITED

CRITICAL severity

CVE-2021-40539

CRITICAL

CVE-2021-40539 is a critical-severity (CVSS 9.8) CWE-706 vulnerability in zohocorp manageengine adselfservice plus. It is in CISA's Known Exploited Vulnerabilities catalog (added 2021-11-03) and has been used in ransomware campaigns — treat it as actively exploited and patch now. O3 Security detects and blocks CVE-2021-40539 exploitation at runtime via eBPF exploit-chain detection and L7 egress monitoring, even on unpatched systems.

Zoho ManageEngine ADSelfService Plus version 6113 and prior is vulnerable to REST API authentication bypass with resultant remote code execution.

Published

Sep 7, 2021

Updated

Jun 17, 2026

Affected

1 product

Patched

See advisory

Exploits

7 known

Exploitation data as of Aug 27, 2026 · OSV.dev, NVD, CISA KEV, FIRST.org (EPSS)

Exploitation Status

Actively exploited in the wild

Confirmed by CISA's Known Exploited Vulnerabilities catalog on 2021-11-03 — including use in ransomware campaigns. Federal agencies were required to remediate by 2021-11-17.
CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
A successful exploit gives an attacker total control of the affected component, not partial access.

Exploitation and automatability from CISA (KEV catalog and SSVC triage) for CVE-2021-40539.

EPSS Exploitation Probability

via FIRST.org ↗

99.0%probability of exploitation in next 30 days

Very High Risk0.00%

Lower risk than most CVEs100th percentile — riskier than 100% of all scored CVEsHighest risk

EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.

How urgent is this, really

CVE-2021-40539 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE sits: patch-first territory.

Where this sits among everything scored

Of 368,770 CVEs with a current EPSS score, this one falls in the ≥ 90% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.

Description

Zoho ManageEngine ADSelfService Plus version 6113 and prior is vulnerable to REST API authentication bypass with resultant remote code execution.

Affected Products

1 product · 10 configurations

Application

manageengine adselfservice pluszohocorp

< 6.1

1 version

6.1

Exploits & PoCs

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.