This was Kali Linux's security package repository: http://security.kali.org/
With Kali Rolling (2016.x), this repository is no longer needed as security
updates come directly from new upstream releases to the Debian Testing
It was kept alive as long as Kali 2.0 "sana" was available on the main mirror,
this is no longer the case and thus this repository is discontinued and will
be dropped in the near future.
You can find a backup of the sana/updates repository on old.kali.org if you
still need it. The corresponding entry in /etc/apt/sources.list would be:
deb http://old.kali.org/kali-security sana/updates main contrib non-free
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