Ransomware CryptoLocker not intercepted

a member of my network has opened a file with a .zip attachment and started the infection throughout the network. Why the virus has not intercepted since it is a fairly well-known virus since 2013?

Un utente della mia rete ha aperto un file con allegato .zip ed ha iniziato l’infezione per tutta la rete. Perchè l’antivirus non lo ha intercettato dal momento che si tratta di n virus abbastanza noto fin dal 2013?

What is telling you it is a 2013 version ?
The latest CryptoLocker version is from earlier this month.

No, ok, the first version began in 2013. I don’t know if our virus was old or new.
Anyway, now we are protected by the last of one month ago that you said?
Many thanks.

https://www.avast.com/virus-update-history
As you can see there, avast can detect many variants of it.

And it is not a virus, but a trojan.
It uses RSA public-key cryptography.
Not only to encrypt files on a infected system, but also to “crawl” around on networks.
That is why it is really is hard to create a good detection for it.