LUCKY13 is an SSL/TLS protocol vulnerability that uses weakness in CBC-mode cipher padding for attacks. This flaw makes it easy for attackers to perform side-channel attacks that decrypt secret information, including login details, credit cards, and session tokens. This post explains the LUCKY13 vulnerability, its effect on your server, and how to remedy it successfully.
What is the LUCKY13 Attack?
The LUCKY13 attack exploits the CBC (Cipher Block Chaining) mode of encryption in SSL/TLS protocols. It works by exploiting timing differences about how padding errors are handled by servers during decryption, which allows attackers to determine encrypted plaintext by analyzing response times to requests.

Why is the LUCKY13 Attack a Problem?
If your server is susceptible to the LUCKY13 attack, then you may face the following conditions:
- Intercept and decrypt sensitive information transmitted over HTTPS.
- Bypass encryption layers, exposing login credentials, payment information, or confidential data.
- Reduce the overall security of SSL/TLS connections.
Best Practices to Prevent the LUCKY13 Attack
Here’s how you can prevent the LUCKY13 attack vulnerability by updating server configurations and applying fixes.
1. Eliminate CBC-mode Ciphers and Use AEAD-based Ciphers
CBC mode ciphers are vulnerable to timing attacks. Instead use AEAD (Authenticated Encryption with Associated Data) type of ciphers like Galois/Counter Mode which is GCM.
For Ubuntu/NGINX:
- Open the NGINX configuration file located at /etc/nginx/nginx.conf:
ssl_protocols TLSv1.2 TLSv1.3;
ssl_ciphers 'EECDH+AESGCM:EDH+AESGCM:AES256+EECDH:AES256+EDH';
ssl_prefer_server_ciphers on;

- Restart NGINX:
# service nginx restart
For CentOS/Apache:
- Open the Apache SSL configuration file at /etc/httpd/conf.d/ssl.conf:
SSLProtocol all -SSLv2 -SSLv3
SSLCipherSuite HIGH:!aNULL:!MD5:!CBC
SSLHonorCipherOrder on

- Restart Apache:
# sudo systemctl restart httpd
For Windows Server/IIS
- Open Registry Editor: Press Win + R, type regedit, and press Enter Key.
- Disable Weak Protocols:
- Navigate to:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols
SSL 2.0, SSL 3.0, TLS 1.0, TLS 1.1: Add or set Enabled (DWORD) = 0 under Server.
- Enable Strong Protocols:
TLS 1.2, TLS 1.3 (if supported): Add or set Enabled (DWORD) = 1 under Server.
- Disable Weak Ciphers: Navigate to:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Ciphers
Disable weak ciphers (e.g., RC4, Triple DES) by setting Enabled (DWORD) = 0.
- Enable Strong Ciphers: Make sure AES ciphers are enabled:
- Set Enabled (DWORD) = 1 for AES.

- Restart Server: Apply changes by restarting the server.
2. Update Your Server and OpenSSL Updated
Make sure that your server is using the latest version of OpenSSL that has mitigations against the LUCKY13 vulnerability.
For Ubuntu:
- Update OpenSSL:
# sudo apt update && sudo apt install openssl
- Check OpenSSL *Version:
# openssl version
* Make sure the version is at least 1.0.1g or higher.
For CentOS:
- Update OpenSSL:
# sudo yum update openssl
- Verify OpenSSL Version:
# openssl version
For Windows Server:
- Update your Windows to the latest Windows Updates to ensure the built-in TLS library is patched.
3. Verify Your Server’s Security Configuration
Once these changes have been applied, it’s always necessary to test your server for LUCKY13.
How to Use CertPanel SSL Monitor to Prevent LUCKY13 Attacks
CertPanel SSL Monitor is an efficient tool to detect and fix SSL/TLS vulnerabilities, including the LUCKY13 attack. Here’s how you can use it to secure your server:
- Add Your Domain to CertPanel SSL Monitor: Log in to CertPanel, navigate to the SSL Monitor feature, and add your domain for continuous monitoring.

- Initiate SSL Scan: Run a vulnerability scan to identify attacks like the LUCKY13 attack. The tool will analyse your SSL/TLS configuration and mark down all weak ciphers and older protocols.

- Remediate Issues: Follow the remediation instructions offered by CertPanel by disabling CBC-mode ciphers and enabling AEAD-based ciphers.

- Rescan Your Domain: After applying the above remediation, re-run your domain scan to validate if the LUCKY13 vulnerability has been successfully removed.

LUCKY13 attack is another critical vulnerability to SSL/TLS that if not worked upon, compromises sensitive information. You can ensure safe server operation with the exclusion of CBC-mode ciphers, using of AEAD-based encryption, OpenSSL, and CertPanel SSL Monitor tools. Regular check-up as well as prompt update always keep your strong encryption afloat and protected online communications.