Asking Let's Encrypt when to renew: an ARI request built by hand in Python
Let's Encrypt's 7 October post moves the default certificate lifetime from 90 days to 64 days on 10 February 2027, and the post scheduled staging to switch on 14 October. The advice in it comes in two parts. If your client supports ACME Renewal Information (ARI), "you should be all set". If it doesn't, renew "at approximately ⅔ of the lifetime instead". I wanted to see what "all set" means on the wire: what the client asks, what the CA answers, and how close the answer is to the two-thirds rule.
🎯 Vulnerability: Web Security Vulnerability
💡 Defense Fix: Implement strict input validation, contextual output encoding, and enforce least-privilege authorization controls.