Disable the 3DES cipher
Why it matters
3DES uses a small block size that lets attackers recover data from long connections (the Sweet32 attack), and it is weaker than the 128-bit minimum used today. Current browsers and clients do not need it.
Staged change. Do this in stages and check each one before the next, so a mistake does not break your site or your mail.
The fix
- Replace the cipher list with a modern one made of ECDHE key exchange and AES-GCM or ChaCha20, as in the snippets for your server.
- Stage it. Apply the change, test with the commands below, and be ready to restore the previous list if an old client you must support (very old Windows or Java) breaks.
- Confirm 3DES is refused.
bash
# lists every cipher the server accepts per protocol version; 3DES (DES-CBC3-SHA) must not appear nmap --script ssl-enum-ciphers -p 443 example.com # alternative: testssl.sh --sweet32 example.com
The examples use example.com. Replace it with your own domain.
Related
Scan your domain to see whether this applies to you.