CVE-2024-31400

Executive Summary

CVE-2024-31400 is an information disclosure vulnerability in Cybozu Garoon versions 5.0.0 through 5.15.0. The flaw allows sensitive data to be unintentionally included in forwarded emails due to improper handling of message content during the forwarding process. This can lead to the exposure of confidential information to unintended recipients. No public exploit code or proof-of-concept is currently available, and the vendor has not released a patch. The vulnerability is classified under CWE-201 (Insertion of Sensitive Information Into Sent Data) and aligns with MITRE ATT&CK tactic T1567 (Exfiltration Over Web Service).

CVSS Breakdown & Threat Level

The CVSS score is listed as 0.0, indicating insufficient data for scoring. However, based on the nature of the vulnerability, a reasonable assessment would be:

• Attack Vector (AV): Network (exploitable remotely)
• Attack Complexity (AC): Low (no special conditions)
• Privileges Required (PR): Low (attacker needs a valid user account)
• User Interaction (UI): Required (victim must forward an email)
• Scope (S): Unchanged
• Confidentiality (C): High (sensitive data exposed)
• Integrity (I): None
• Availability (A): None

Estimated CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:H/I:N/A:N
Estimated Base Score: 5.7 (Medium)

Threat Level: Medium – Exploitation requires user interaction but can lead to significant data leakage.

Deep Dive & Mechanics

The vulnerability resides in the email forwarding functionality of Cybozu Garoon. When a user forwards an email, the application fails to properly sanitize or filter the original message content. This results in the inclusion of unintended data—such as hidden metadata, previous attachments, or internal headers—in the forwarded message. The issue is triggered during the normal forwarding workflow, meaning any authenticated user can inadvertently expose sensitive information.

Root Cause: The application does not strip or redact sensitive fields (e.g., BCC recipients, internal notes, or embedded objects) when constructing the forwarded email payload. This is a classic case of CWE-201, where the system sends data that should remain confidential.

Attack Scenario: An attacker with a valid Garoon account sends an email containing hidden sensitive data (e.g., a maliciously crafted email with embedded metadata). When the victim forwards this email, the sensitive data is included in the forwarded copy, potentially reaching unintended recipients controlled by the attacker.

Vulnerable vs. Patched Code

No official patch is available. The vulnerable code likely resides in the email forwarding module, where the original message object is copied without filtering. A patched version would need to:

• Strip internal headers (e.g., X-Original-From, Bcc)
• Remove embedded objects or attachments not explicitly selected for forwarding
• Sanitize the message body to exclude hidden fields

Example of vulnerable pseudo-code:

def forward_email(original_email):
    forwarded = Email()
    forwarded.to = get_forward_recipients()
    forwarded.subject = "Fwd: " + original_email.subject
    forwarded.body = original_email.body  # Vulnerable: includes all original content
    forwarded.attachments = original_email.attachments  # Vulnerable: includes all attachments
    send(forwarded)

Patched pseudo-code:

def forward_email(original_email):
    forwarded = Email()
    forwarded.to = get_forward_recipients()
    forwarded.subject = "Fwd: " + original_email.subject
    forwarded.body = sanitize_body(original_email.body)  # Remove hidden data
    forwarded.attachments = filter_attachments(original_email.attachments)  # Only selected ones
    send(forwarded)

Exploitation & Lab Setup

Red Team

Prerequisites:

• Valid user account on a vulnerable Cybozu Garoon instance (5.0.0–5.15.0)
• Ability to send emails to the victim

Steps:

1. Craft an email containing hidden sensitive data (e.g., embedded metadata in HTML, hidden fields, or a specially crafted attachment).
2. Send the email to the victim.
3. Wait for the victim to forward the email to an external address (or an address you control).
4. The forwarded email will include the hidden data, which you can then extract.

Example payload (hidden data in HTML comment):

<html>
<body>
<!-- SENSITIVE_DATA: internal_api_key=abc123 -->
<p>Please forward this to the team.</p>
</body>
</html>

Blue Team

Detection:

• Monitor for unusual email forwarding patterns (e.g., forwarding to external domains).
• Inspect forwarded emails for unexpected content (e.g., HTML comments, hidden fields).
• Use email content analysis to detect embedded metadata.

Prevention:

• Implement email content sanitization at the gateway.
• Restrict forwarding to external addresses.
• Educate users on the risks of forwarding emails without reviewing content.

Indicators of Compromise (IOCs)

• Network: Emails containing unexpected HTML comments or hidden fields in forwarded messages.
• Host: Log entries showing forwarding actions to external recipients.
• File: Emails with unusually large size due to embedded metadata or attachments.

Example IOC (email header):

X-Forwarded-By: user@example.com
X-Original-Message-ID: <original@example.com>

Detection Engineering

YARA Rule

rule CVE_2024_31400_Garoon_Forwarded_Data_Leak {
    meta:
        description = "Detects emails with hidden data in forwarded messages (CVE-2024-31400)"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2024-31400"
    strings:
        $hidden_comment = /<!--\s*SENSITIVE_DATA:.*?-->/ nocase
        $hidden_field = /<input[^>]*type=["']hidden["'][^>]*value=["'][^"']*["']/ nocase
        $metadata = /X-Original-From:|X-Original-To:|Bcc:/ nocase
    condition:
        any of them
}

Sigma Rule

title: CVE-2024-31400 - Cybozu Garoon Email Forwarding Data Leak
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects forwarded emails containing hidden sensitive data
author: Valters IT
date: 2025-04-10
logsource:
    category: email
    product: generic
detection:
    selection:
        Subject|contains: "Fwd:"
        Body|contains: "<!-- SENSITIVE_DATA:"
    condition: selection
falsepositives:
    - Legitimate use of HTML comments in forwarded emails
level: medium

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET 25,587,465 (msg:"CVE-2024-31400 - Cybozu Garoon Email Forwarding Data Leak"; flow:to_server,established; content:"Fwd:"; http_header; content:"<!-- SENSITIVE_DATA:"; http_client_body; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable email forwarding functionality if not essential.
2. Implement a web application firewall (WAF) rule to inspect forwarded emails for hidden data.
3. Educate users to manually review forwarded emails before sending.

Long-term Fix:

• Apply vendor patch when released.
• Upgrade to a patched version of Cybozu Garoon (none available as of this report).
• Consider migrating to an alternative email platform if patch is delayed.

Workaround:

• Use a proxy or email gateway that strips HTML comments and hidden fields from outgoing emails.

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert