Why was the Constantinople hard fork postponed?

Question

The Constantinople hard fork — really a network upgrade — was scheduled to happen today (January 16) at block 7,800,000.

Instead the upgrade was postponed.

What are the technical reasons for the postponement?

Answer 1

Have a look here: https://medium.com/chainsecurity/constantinople-enables-new-reentrancy-attack-ace4088297d9

TLDR;

Repricing SSTORE defeats the re-entrancy protection that the gas stipend is meant to provide. That would be a breaking change to some deployed contracts.

Hope it helps.