CriticalCVE-2021-40539

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

    Published In CISA KEV since Sources: CVE.org

    What is affected, what to do

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

    The vulnerability is being actively exploited; treat it as a priority. CISA lists it as used in ransomware campaigns. The available sources name no fixed version. Check the vendor guidance in the references below. Immediate actions, mitigation and detection rules are in the runbook below.

    Conflicting version data — version display is hidden until the data is resolved.

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    Metrics

    9.8

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: NVD, CVSS v3

    9.8

    CVSS v3

    Severity Critical on the scale from 0 to 10.

    Source: CVE.org

    99.0 %

    EPSS score

    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.

    Source: FIRST EPSS, percentile 99.93

    Show all metrics
    Severity
    Critical
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    253.89 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

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    actively exploited
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

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    9.8 Methodology
    CVSS v3

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    9.8
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

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    99.0 %
    High — model estimates ≥ 50% chance of real-world exploitation within 30 days.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

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    99.93
    Critical — this CVE ranks in the top fifth of all CVEs scored today (rank ≥ 80%).
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

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    Actively exploited
    Published
    In CISA KEV since

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

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    CISA assessment (SSVC)
    Exploitation: active · automatable · Technical impact: total
    Source: CISA Vulnrichment, as of 3 February 2025
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-706

    Response and mitigation

    Why act now?

    Prioritisation rationale

    The flaw is network-reachable without authentication or user interaction and yields direct code execution on a host that, by design, holds privileged write access to Active Directory. That makes it a near-ideal launchpad for full domain takeover. CISA flags known use in ransomware campaigns, and its exploitation likelihood ranks among the very highest of any CVE. Typical targets are organisations that expose the password self-service portal to remote staff, including critical-infrastructure sectors such as energy, healthcare and public administration. Treat any internet-facing unpatched instance as presumed compromised and handle it at top priority. Internal-only instances still rate critical because of their AD reach and should be patched within days, not weeks.

    1. Runbook step 1

      Immediate response (0-24 h)

      • Cut exposure now: Block internet access to ADSelfService Plus instances at the perimeter firewall or reverse proxy until patch level and integrity are confirmed. The default ports are 9251/TCP HTTPS and 8888/TCP HTTP; verify the actual ports in the server config.
      • Patch: Build 6113 and earlier are vulnerable. Upgrade to build 6114 or later, ideally the newest build listed in the vendor advisory rather than the minimum fix.
      • Hunt before you patch: Grep <install path>\logs\access_log_*.txt for requests to /RestAPI/LogonCustomization and /RestAPI/Connection, especially path-normalisation variants such as /./RestAPI/. Run Zoho's exploit detection tool if it is available to you.
      • If you get hits, treat it as an incident: Contain the host via EDR and take a forensic image before rebuilding. Patching does not remove webshells that are already planted.
      • Rotate credentials: Reset the ADSelfService Plus service account and any AD admin credentials stored in the application. Also reset the accounts of every user who changed their password or logged in through the portal since the earliest suspicious request. If there are signs of domain-admin abuse, plan a double krbtgt reset.

    2. Runbook step 2

      Mitigation layers

      • Network segmentation: Publish the portal only behind VPN or a zero-trust gateway that enforces pre-authentication. Egress-filter the server to the destinations it actually needs, which breaks webshell callbacks and tool downloads.
      • WAF/reverse proxy rule: Drop requests whose normalised URI contains /RestAPI/ preceded by /./, //, %2e or ;. Reject unauthenticated POSTs to LogonCustomization and Connection outright.
      • IPS: Enable the vendor-ruleset Suricata/Snort signatures for CVE-2021-40539. They should match the URI fragment RestAPI/LogonCustomization and multipart uploads carrying .cer or .jsp filenames.
      • Endpoint hardening: Use AppLocker/WDAC or an EDR rule to block the ADSelfService Java process from spawning cmd.exe, powershell.exe or certutil.exe, or keytool.exe with unusual arguments. Minimise the service account's write access to webapps\adssp\.
      • IAM/least privilege: Scope the service account's AD delegation to the OUs and attributes it genuinely needs (password reset, unlock). It should have no Domain Admin membership and no interactive logon.

    3. Runbook step 3

      Detection rules

      • Web access logs: Look for the pattern "(POST|GET) /\./RestAPI/(LogonCustomization|Connection)" in the ADSelfService access logs. A Sigma shape would be cs-uri-stem|contains: ['/./RestAPI/', '/RestAPI/LogonCustomization'] combined with cs-method: POST.
      • Sysmon EID 1 / EDR process ancestry: Watch for the Java process under the ADSelfService path spawning keytool.exe, cmd.exe or powershell.exe. In KQL: DeviceProcessEvents | where InitiatingProcessFolderPath has "ADSelfService" and FileName in~ ("keytool.exe","cmd.exe","powershell.exe","whoami.exe","net.exe").
      • Sysmon EID 11 / file creation: Flag new .jsp files under webapps\adssp\ (known drop locations include paths under help\admin-guide\reports\) and a service.cer file in the bin directory. These indicate uploaded webshells or a tampered certificate file.
      • Windows Security log: Check 4624/4625 and 4723/4724 events from the service account at odd hours, and 4688 events for child processes of the service. Also check 4662 events for DCSync-style replication access by accounts tied to the ADSelfService host.
      • Network telemetry (Zeek/Suricata): Alert on outbound connections from the ADSelfService server to unfamiliar external IPs. Also alert on SMB, WinRM or RDP from the server to domain controllers that goes beyond its normal LDAP(S) traffic.

    Detection rules

    No NEOSEC detection rule is available for this vulnerability yet.

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

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    Actively exploited: treat the vulnerability as a priority in vulnerability handling under Art. 21(2)(e) NIS2. If it is exploited in your environment, check the reporting obligation under Art. 23 (early warning within 24 hours).
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    Since 11.09.2026, manufacturers of affected products with digital elements must report actively exploited vulnerabilities under Art. 14 CRA, with an early warning within 24 hours via ENISA's single reporting platform.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    Description

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

    Source: NVD (NIST), cvelistv5, CISA KEV Catalog, CVE.org (MITRE)

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    zohocorpmanageengine_adselfservice_plus
    • 6.1
    • < 6.1
    • —
    • 6.1

    Vendors and products

    Weakness classes (CWE)

    • CWE-706Use of Incorrectly-Resolved Name or Reference

      Class

      The product uses a name or reference to access a resource, but the name/reference resolves to a resource that is outside of the intended control sphere.

      Entry at MITRE CWE-706

    Public exploit references

    Publicly reported proof-of-concepts, exploit modules and detection templates for this vulnerability. NEOSEC links to the original sources only and does not provide any exploit code.

    Public exploit references
    CISA KEV CatalogCISA Known Exploited Vulnerability (opens in a new tab)cisa.govactively exploited
    Packet StormManageEngine ADSelfService Plus Authentication Bypass Code Execution (opens in a new tab)packetstormsecurity.comPoC (publicly reported)reported, not verified—

    References & sources

    • manageengine.comx_refsource_MISC, Product
    • manageengine.comproducts/self-service-password/kb/how-to-fix-authentication-byp…x_refsource_MISC, Patch, Vendor advisory
    • packetstormsecurity.comfiles/165085/ManageEngine-ADSelfService-Plus-Authentication-Byp…x_refsource_MISC, Exploit, Third-party advisory, Vulnerability database
    • cisa.govknown-exploited-vulnerabilities-catalog?field_cve=CVE-2021-40539government-resource, US government resource
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