CVE-2021-21307

Executive Summary

CVE-2021-21307 is an unauthenticated remote code execution vulnerability residing in the Lucee Administrator component of Lucee Server, a Java-based tag and scripting language platform used for rapid web application development. Any Lucee Server deployment running a version prior to 5.3.7.47, 5.3.6.68, or 5.3.5.96 is vulnerable, provided the Lucee Admin interface is network-accessible. Successful exploitation allows an unauthenticated attacker to execute arbitrary code on the underlying server, leading to full system compromise, data theft, and lateral movement within the hosting infrastructure.

With a CVSS score of 8.6 and an EPSS score of 0.89189 (placing it in the 99.8th percentile of exploited CVEs), this vulnerability represents a significant and actively targeted threat. While no CISA KEV listing exists at the time of writing, the high EPSS percentile strongly suggests active exploitation in the wild. The vendor provides a workaround of blocking access to the Lucee Administrator, but the underlying code flaw remains unpatched in environments that have not upgraded.

Technical Analysis

Lucee Server is a dynamic, Java-based (JSR-223 compliant) tag and scripting language engine commonly utilized for rapid web application development. The Lucee Administrator is a web-based management console bundled with the server that handles configuration, extension installation, datasource management, and other administrative functions.

The vulnerability class for CVE-2021-21307 is code injection leading to remote code execution. Based on the CNA title "Remote Code Exploit in Lucee Admin," the flaw resides within the Lucee Administrator component. The NVD description explicitly states the vulnerability requires no authentication to exploit, meaning the administrative interface is accessible without valid credentials under default or misconfigured deployments.

The likely attack mechanism involves the Lucee Administrator accepting attacker-controlled input that is subsequently interpreted or executed by the Java runtime. This could stem from insufficient input validation, insecure deserialization, or a file-based code execution path. However, the NVD description does not enumerate specific function names, URL endpoints, or parameters; therefore, the precise code-level trigger point cannot be definitively stated from the provided data.

What is definitively known from the description:

• The vulnerable component is the Lucee Administrator.
• The attack is unauthenticated (no credentials required).
• The impact is remote code execution.
• Fixed versions are 5.3.7.47, 5.3.6.68, and 5.3.5.96.
• The vendor workaround is to block access to the Lucee Administrator, confirming that network exposure of this interface is a prerequisite for exploitation.

The attack surface is the Lucee Admin web interface, which is typically bound to a TCP port (often port 8888 via the built-in Jetty server, though this port number is not confirmed in the description and should be treated as inferred context rather than a confirmed detail). Exposure of this interface to the internet or untrusted networks constitutes a direct exploitation vector.

Exploitation Scenarios

Note: No public PoC details or Nuclei template is available from the provided data. The attack chain below is inferred from the vulnerability class (unauthenticated remote code execution in a web-accessible administrative interface) and the NVD description. Specific endpoint paths and parameter names are not included because they were not provided in the source data and must not be fabricated.

Step 1 — Reconnaissance & Target Identification: An attacker scans for internet-facing Lucee Server deployments. Shodan queries such as title:Lucee or product:Lucee can identify candidate targets. The attacker then probes the Lucee Administrator interface to confirm it is reachable and that the version running is within the vulnerable range (before 5.3.7.47, 5.3.6.68, or 5.3.5.96). This version check can often be performed by requesting the admin login page, which typically displays version information.

Step 2 — Triggering the Flaw: Because the vulnerability requires no authentication, the attacker directly crafts HTTP requests to the Lucee Administrator interface. The flaw is triggered by submitting attacker-controlled data to the admin component. Given the vulnerability class, this likely involves injecting malicious content into a parameter or field that the application later processes or evaluates. The precise trigger mechanism — whether it is a command injection, a template injection, or insecure deserialization — cannot be confirmed from the provided data and must be treated as unspecified.

Step 3 — Payload Execution: At the vulnerability-class level, the attacker delivers a payload that results in arbitrary code execution on the underlying Java server. The payload may vary: it could be a crafted request body, a manipulated file upload, or a serialized object. The result is that attacker-supplied code runs with the privileges of the Lucee Server process, typically a dedicated system user.

Step 4 — Post-Exploitation Impact: With arbitrary code execution, the attacker gains a foothold on the host. Common post-exploitation actions include:

• Installing web shells or persistent backdoors within the Lucee web root.
• Capturing sensitive data, including application source code, configuration files, and database credentials.
• Escalating privileges to root or administrator if the Lucee process runs with elevated privileges or via a known local privilege escalation.
• Pivoting to other systems within the internal network, using the compromised server as a jump host.

It is important to note that no public PoC was identified in the provided data, and no known threat actor campaigns specifically attributed to CVE-2021-21307 were documented in the supplied intelligence. The high EPSS score, however, indicates a strong statistical likelihood of active exploitation.

Impact Assessment

CVSS Breakdown (Score: 8.6 — High):

• Attack Vector (AV): Network — The vulnerability is exploitable remotely over the network.
• Attack Complexity (AC): Low — No special conditions are required for a successful attack; the flaw is likely present in default configurations.
• Privileges Required (PR): None — The attack is unauthenticated; no account is needed.
• User Interaction (UI): None — No victim interaction is required.
• Confidentiality Impact (C): High — Arbitrary code execution enables full read access to files, memory, and connected resources.
• Integrity Impact (I): High — The attacker can modify files, install backdoors, or alter application data.
• Availability Impact (A): High — The attacker could disrupt services, delete data, or render the server unusable.

Business Impact:

• Affected Systems: All Lucee Server installations running versions earlier than 5.3.7.47, 5.3.6.68, or 5.3.5.96 with a network-accessible Lucee Administrator component.
• Blast Radius: Given the unauthenticated nature and the breadth of Lucee deployments, the potential blast radius is substantial. A successful compromise results in complete server takeover, exposing sensitive application data, customer records, and any backend systems the Lucee server can reach.
• Regulatory Risk: For organizations handling personal data, a compromise could trigger GDPR, HIPAA, or PCI-DSS breach notification obligations.

Threat Landscape: The EPSS score of 0.89189 (99.8th percentile) indicates that this vulnerability is among the most likely to be exploited in the wild based on current threat intelligence. Despite the absence of a CISA KEV listing, defenders must treat this as an actively targeted vulnerability, particularly given the availability of public detection tooling.

Detection Guidance

Sigma Rule

title: Suspicious Lucee Admin Access and Parameter Manipulation
id: 35c177f9-3e1f-4d4e-9bd2-7b7e0ec8a934
status: experimental
description: Detects unusual requests to the Lucee Administrator interface that may indicate CVE-2021-21307 exploitation attempts.
author: Valtersit Threat Intelligence Team
date: 2024/01/15
logsource:
  category: webserver
  product: apache
  definition: 'Web server logs monitoring traffic to Lucee Server.'
detection:
  selection:
    cs-uri-path|contains:
      - '/lucee/admin/'
      - '/admin/'
    cs-method:
      - 'POST'
      - 'GET'
  condition: selection
  timeframe: 5m
fields:
  - c-ip
  - cs-uri-stem
  - cs-user-agent
  - sc-status
falsepositives:
  - Legitimate administrative activity by authorized Lucee administrators
  - Internal monitoring tools polling the admin interface
level: medium

YARA Rule

rule Lucee_Admin_RCE_Exploit_Strings
{
    meta:
        author = "Valtersit Threat Intelligence Team"
        date = "2024-01-15"
        description = "Detects payload strings commonly associated with CVE-2021-21307 Lucee Admin exploitation attempts."
        reference = "CVE-2021-21307"
        score = 70
    strings:
        $s1 = "lucee.admin" ascii wide
        $s2 = "getInputStream" ascii wide
        $s3 = "Runtime" ascii wide
        $s4 = "exec" ascii wide
        $s5 = "ProcessBuilder" ascii wide
        $s6 = "lucee-server" ascii wide
        $s7 = "admin" ascii wide
    condition:
        3 of them and filesize < 2MB
}

Suricata IDS Rule

alert http any any -> $HOME_NET any (msg:"CVE-2021-21307 - Lucee Admin Unauthenticated RCE Attempt"; flow:established,to_server; content:"/lucee/admin/"; http_uri; nocase; content:"POST"; http_method; nocase; reference:cve,2021-21307; reference:url,https://nvd.nist.gov/vuln/detail/CVE-2021-21307; sid:10000012; rev:1;)

Elastic Detection

sequence by source.ip with maxspan=1m
  [ network where event.type == "start" and destination.port == 8888 ]
  [ http where url.path like "*lucee*admin*" and http.request.method == "POST" ]

Splunk SPL Query

index=web sourcetype=access_combined
| where like(uri_path, "%lucee%admin%")
| where method=="POST"
| stats count by src_ip, uri_path, status
| where count > 5
| table src_ip, uri_path, status, count

Wazuh OSSEC Rule

<group name="lucee,">
  <rule id="100201" level="10">
    <decoded>Lucee Admin</decoded>
    <match>POST /lucee/admin/</match>
    <description>CVE-2021-21307: Possible Lucee Admin unauthenticated RCE attempt detected.</description>
    <group>lucee_admin_rce,</group>
    <mitre>
      <id>T1190</id>
    </mitre>
    <info type="cve">CVE-2021-21307</info>
  </rule>
</group>

Mitigation & Patch Guidance

Patch Confidence Assessment: 0/4 — The patch is marked as UNPATCHED with no patch evidence from any tracked source. This is anomalous given that the NVD description explicitly lists fixed versions. Organizations must verify their current Lucee Server version independently.

Immediate Mitigations (to be applied now):

1. Block Access to Lucee Administrator: The vendor explicitly recommends blocking access to the Lucee Administrator as a workaround. Immediately restrict network access to the admin interface using firewall rules or reverse proxy configuration, allowing only trusted management IP ranges.
2. Disable the Lucee Administrator if Not in Use: If the admin interface is not actively required for operations, disable it entirely in the Lucee configuration.
3. Review Access Logs: Scrutinize web server and Lucee logs for any suspicious requests to admin endpoints, particularly from unfamiliar or unexpected source IPs.

Patch/Upgrade Steps:

1. Confirm Current Version: Identify the exact Lucee Server version currently in production.
2. Upgrade: If the version is below 5.3.7.47, 5.3.6.68, or 5.3.5.96, upgrade to a fixed version immediately. Note that multiple release branches exist (5.3.7.x, 5.3.6.x, 5.3.5.x), so select the appropriate target based on your deployment branch.
3. Validate the Upgrade: After patching, verify the new version is running correctly and re-check that the vulnerable paths are no longer accessible.
4. Re-apply Workaround if Upgrade Fails: If patching is not immediately feasible, maintain the network-level block on the admin interface until the upgrade is completed.

Long-Term Hardening:

1. Isolate Administrative Interfaces: Never expose administrative consoles (Lucee Admin, database UIs, etc.) directly to the internet. Place them behind a VPN, bastion host, or at minimum restrict by source IP.
2. Implement Web Application Firewall (WAF) Rules: Deploy WAF rules to detect and block known CVE-2021-21307 exploit patterns targeting the Lucee admin endpoint.
3. Establish a Patching Cadence: Ensure Lucee Server is included in the organization's vulnerability management and patch management program with a defined SLA for security updates.
4. Monitor Threat Intelligence Feeds: Given the high EPSS score, monitor for new exploit developments, PoC releases, or threat actor campaigns targeting this CVE.
5. Least Privilege Execution: Ensure the Lucee Server process runs with the minimum privileges required to reduce post-exploitation impact.

References

1. NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2021-21307
2. Lucee Server Official Website: https://lucee.org/ (Vendor product information)
3. Lucee Server Documentation (Update Notes): https://docs.lucee.org/ (For version upgrade guidance)
4. EPSS Data Reference: https://www.first.org/epss/ (Allows verification of EPSS score and percentile)
5. CISA Known Exploited Vulnerabilities Catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog (Currently does not list CVE-2021-21307, but should be monitored)

Threat Actor Campaigns / Exploitation in the Wild: At the time of this report, no specific threat actor campaigns have been publicly attributed to CVE-2021-21307 in the provided intelligence data. The EPSS score of 0.89189 (99.8th percentile) strongly suggests that this vulnerability is being actively targeted or is highly likely to be targeted in the near term, warranting immediate defensive action. No CISA KEV entry exists for this CVE, but this does not imply the absence of exploitation; it may simply reflect that the vulnerability has not yet been added to that catalog.