CVE-2023-48128

Executive Summary

CVE-2023-48128 exposes a critical information disclosure vulnerability in the UNITED BOXING GYM mini-app on the Line messaging platform (v13.6.1). An attacker can exploit a leaked channel access token to send crafted malicious notifications to users, potentially leading to phishing, social engineering, or further compromise of the mini-app ecosystem. The vulnerability stems from improper handling of the channel access token, which is a secret credential used to authenticate API requests to the Line Messaging API. With this token, an attacker can impersonate the mini-app and send arbitrary push messages, bypassing user consent or security controls. The CVSS score is currently unassigned, but the impact is significant due to the trust users place in official mini-app notifications. No patch is available from the vendor, and no exploit code or detection templates have been published in public repositories.

Supply Chain & Package Security

This vulnerability does not involve a package manager (NPM, PyPI, Go, Ruby) but rather a Line mini-app. The supply chain risk is limited to the Line platform’s ecosystem, where third-party mini-apps may mishandle API credentials. Organizations using Line for business communications should audit all mini-apps for similar token leakage issues.

CVSS Breakdown & Threat Level

The CVSS score is currently 0.0 (unassigned). Based on the vulnerability characteristics, a reasonable estimated score would be:

• Attack Vector (AV): Network (N) – The attacker can send malicious notifications remotely.
• Attack Complexity (AC): Low (L) – Exploitation requires only the leaked token.
• Privileges Required (PR): None (N) – No authentication needed beyond the token.
• User Interaction (UI): None (N) – The attack does not require user action.
• Scope (S): Changed (C) – The attacker can impact resources beyond the mini-app (e.g., user devices).
• Confidentiality (C): Low (L) – Limited data exposure (token).
• Integrity (I): High (H) – Attacker can send arbitrary messages.
• Availability (A): None (N) – No denial of service.

Estimated CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:H/A:N
Estimated Score: 8.6 (High)

Threat Level: High – The vulnerability allows unauthorized message injection, which can be used for phishing or malware distribution.

Deep Dive & Mechanics

The Line Messaging API uses a channel access token to authenticate requests from a mini-app to the Line platform. This token is a long-lived secret that should be stored securely on the server side. In the UNITED BOXING GYM mini-app, the token is leaked, likely through client-side exposure (e.g., embedded in JavaScript, exposed in API responses, or stored insecurely in local storage). An attacker who obtains this token can:

1. Send Push Messages: Use the Line Messaging API endpoint https://api.line.me/v2/bot/message/push to send messages to any user who has interacted with the mini-app.
2. Craft Malicious Content: Include links to phishing sites, malware downloads, or social engineering lures.
3. Bypass User Consent: The token allows the attacker to act as the mini-app, so users see the message as coming from a trusted source.

The vulnerability is classified under CWE-200 (Exposure of Sensitive Information to an Unauthorized Actor). The token leakage could occur via:

• Hardcoded token in client-side code.
• Token exposed in HTTP headers or responses.
• Token stored in insecure local storage (e.g., localStorage, sessionStorage).
• Token transmitted over unencrypted channels.

Vulnerable vs. Patched Code

No patched version is available. The vulnerable code likely contains the token in a client-side file (e.g., JavaScript). A secure implementation would store the token server-side and use a proxy to interact with the Line API.

Vulnerable Code Example (Client-Side):

// Vulnerable: Token exposed in client-side code
const CHANNEL_ACCESS_TOKEN = 'YOUR_CHANNEL_ACCESS_TOKEN';
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: 'USER_ID',
    messages: [{ type: 'text', text: 'Hello' }]
  })
});

Patched Code Example (Server-Side Proxy):

// Server-side: Token stored securely, client calls proxy
app.post('/send-message', async (req, res) => {
  const token = process.env.CHANNEL_ACCESS_TOKEN; // Stored in environment variable
  const response = await fetch('https://api.line.me/v2/bot/message/push', {
    method: 'POST',
    headers: {
      'Authorization': `Bearer ${token}`,
      'Content-Type': 'application/json'
    },
    body: JSON.stringify(req.body)
  });
  res.json(await response.json());
});

Exploitation & Lab Setup

Red Team

Objective: Send a malicious notification to a user who has interacted with the UNITED BOXING GYM mini-app.

Prerequisites:

• Obtain the leaked channel access token (e.g., via network sniffing, client-side inspection, or public leak).
• Obtain a user ID (e.g., from the mini-app’s database or by observing API responses).

Steps:

1. Extract Token: Use browser developer tools to inspect network requests from the mini-app. Look for Authorization: Bearer <token> headers.
2. Craft Payload: Create a JSON payload with a malicious message.
3. Send Request: Use curl or a script to send a push message.

curl -X POST https://api.line.me/v2/bot/message/push \
  -H "Authorization: Bearer LEAKED_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{
    "to": "USER_ID",
    "messages": [
      {
        "type": "text",
        "text": "Click here for a free gift: https://malicious-site.com"
      }
    ]
  }'

Expected Outcome: The user receives a notification from the UNITED BOXING GYM mini-app with the malicious link.

Blue Team

Objective: Detect and prevent exploitation of the leaked token.

Steps:

1. Monitor API Calls: Log all requests to the Line Messaging API from your infrastructure. Look for unusual patterns (e.g., high volume, unexpected user IDs).
2. Token Rotation: Immediately rotate the channel access token if leakage is suspected.
3. User Education: Warn users about unexpected notifications from the mini-app.
4. Network Segmentation: Ensure the mini-app’s backend is isolated and tokens are not exposed to clients.

Detection Rule (Sigma):

title: Suspicious Line Messaging API Push Requests
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects unauthorized push messages via Line API using leaked tokens
logsource:
  category: proxy
  product: generic
detection:
  selection:
    http.url|contains: "api.line.me/v2/bot/message/push"
    http.method: "POST"
  condition: selection
falsepositives:
  - Legitimate mini-app operations
level: high

Indicators of Compromise (IOCs)

• Network Indicators:
  • API endpoint: https://api.line.me/v2/bot/message/push
  • Suspicious user IDs in push requests (e.g., not in the mini-app’s user database)
• File Indicators:
  • Client-side JavaScript files containing hardcoded CHANNEL_ACCESS_TOKEN
  • Configuration files with channel_access_token in plaintext
• Behavioral Indicators:
  • Unexpected push notifications from the UNITED BOXING GYM mini-app
  • High volume of push requests from a single IP

Detection Engineering

YARA Rule

rule Detect_Line_Token_Leak {
  meta:
    description = "Detects hardcoded Line channel access tokens in source code"
    author = "Valters IT"
    date = "2025-04-10"
  strings:
    $token = /[A-Za-z0-9+/=]{40,}/  // Base64-like token pattern
    $header = "Authorization: Bearer"
  condition:
    $header and $token
}

Sigma Rule

title: Line API Token Leak Detection
id: 87654321-4321-4321-4321-210987654321
status: experimental
description: Detects potential leakage of Line channel access tokens in logs or files
logsource:
  category: file_event
  product: windows
detection:
  selection:
    TargetFilename|contains: "config.js"
    TargetFilename|endswith: ".env"
  condition: selection
falsepositives:
  - Legitimate configuration files
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
  msg:"Potential Line API Token Leak - Authorization Header";
  flow:to_server;
  http.header_names; content:"Authorization";
  http.header_value; content:"Bearer";
  sid:1000001;
  rev:1;
)

Remediation & Mitigation

1. Immediate Actions:

   • Rotate the channel access token immediately.
   • Revoke any existing tokens and issue new ones.
   • Audit all mini-app code for hardcoded secrets.

2. Long-Term Fixes:

   • Move token storage to server-side environment variables.
   • Implement a proxy server that handles Line API calls without exposing tokens to clients.
   • Use short-lived tokens with automatic rotation.

3. Detection & Monitoring:

   • Deploy the YARA, Sigma, and Suricata rules provided.
   • Monitor Line API logs for unauthorized push requests.
   • Set up alerts for unusual API call patterns.

4. User Communication:

   • Notify users of the vulnerability and advise them to ignore suspicious notifications.
   • Provide a mechanism for users to report unexpected messages.

Valters IT
DevSecOps & Cyber Threat Intelligence Expert