In early 2024, security researchers discovered a major vulnerability, CVE-2024-24681, affecting the Yealink Configuration Encrypt Tool (both AES and RSA versions before 1.2). This critical flaw involves a hardcoded encryption key that's used to secure device provisioning documents. The same key is used for all customers, which means once the key is known, it can decrypt any document encrypted by any user's tool, leading to a massive security risk.

Let’s break down what this means, how an attacker might exploit it, and what you can do about it.

Yealink is a popular manufacturer of Voice over IP (VoIP) phones and devices. These devices require configuration files to set them up on networks. To keep these configuration files secure (think passwords, server details, etc.), Yealink ships an "Encryption Tool" that scrambles the file contents before they’re uploaded to devices.

RSA version *(before 1.2)*

These tools are supposed to make sure only the devices can read the configs, keeping prying eyes out. Sadly, both versions suffered from the same critical problem.

The Problem: One Encryption Key For Everyone

Instead of securely generating or asking the user for a key, Yealink hardcoded a single key into all copies of the Encryption Tool. This means:

- Every single instance of the toolkit (across all companies, all users) used the exact same encryption key.
- Any attacker who extracts this key (which is easy—they just look inside the program) can now decrypt any provisioning file made with the tool.

This breaks the security promise of the encryption tool. Rather than protecting your files, it gives a false sense of security.

How Does an Attacker Take Advantage of This?

Let’s say an attacker gets hold of an encrypted configuration file—maybe by intercepting it or finding a leaked copy. All they need is the encryption tool (or a reverse-engineered key), and they can decrypt the file to see:

Admin credentials.

This is a big deal! It means the attacker gains access to entire phone systems or even corporate networks.

Here's how simple it can be to exploit this issue, assuming the hardcoded key is known (let’s use a made-up example key for demonstration):

from Crypto.Cipher import AES

# Here, key and iv should match the hardcoded values in Yealink's Encrypt Tool.
key = b"YealinkHardcodedK"  # Example 16-byte AES key
iv = b"YealinkHardcodedIV"  # Example 16-byte AES IV

def decrypt_config_file(file_path):
    with open(file_path, 'rb') as f:
        ciphertext = f.read()
    cipher = AES.new(key, AES.MODE_CBC, iv)
    plaintext = cipher.decrypt(ciphertext)
    # Remove PKCS7 padding
    pad_len = plaintext[-1]
    plaintext = plaintext[:-pad_len]
    return plaintext.decode('utf-8')

decrypted = decrypt_config_file("yealink_encrypted.cfg")
print(decrypted)

If you know the key and IV, you can decrypt any config file from any customer.

Real-World Impact

- Corporate Confidentiality: Anyone with this key can read all sensitive settings for Yealink devices used by a company.
- Network Threats: Passwords and server information could let attackers hijack phones, eavesdrop on calls, or attack IT systems.
- Wide Scale: Because it's a universal key, compromising just one tool means all users are at risk.

References & More Details

- NIST National Vulnerability Database entry for CVE-2024-24681
- Yealink Security Advisories
- Exploit Database – Hardcoded Keys
- Discussion: Reddit thread on Yealink Encryption Issue

What Should You Do?

1. Upgrade the tool: Yealink has released new versions (RSA v1.2 and above) that let users create their own encryption keys, so you’re not stuck with a default one.
2. Re-encrypt old configs: After upgrading, regenerate and re-encrypt all configuration files with your own secure key.
3. Audit device deployments: Make sure devices aren’t exposed to the internet and use secure provisioning best practices.
4. Monitor for suspicious logins: Change any credentials that were ever stored in encrypted configs made with the vulnerable tool.

Conclusion

CVE-2024-24681 is a classic case of “security theater”—encryption that looks secure, but is fundamentally broken by a developer shortcut. It's a reminder: always beware of hardcoded secrets, even in products from trusted companies.

If you use Yealink devices, take action now! Rotate keys, re-encrypt configs, and update your tools to keep your communication safe.


*Written exclusively for those who care about VoIP security. Share and spread awareness!*


Related reading:
- Why Hardcoded Keys Are Dangerous (OWASP)
- Yealink Product Security Updates

Timeline

Published on: 02/23/2024 23:15:09 UTC
Last modified on: 02/25/2025 22:56:29 UTC