A Cryptographic Flaw in Zerocoin (and Two Critical Coding Issues) - https://www.chaac.tf.fau.eu/2018/04/12/zerocoinzcoinpivxzoinsmartcashhexxcoin-attack/

In this article, we present a cryptographic flaw in the Zerocoin cryptographic scheme (not Zerocash), which allows an attacker to burn coins of honest users. On the way, we identified two more critical coding issues in a software library implementing Zerocoin, allowing an attacker to create money out of thin air and stealing coins from honest users. Zerocoin is used by multiple cryptocurrencies (Zcoin, PIVX, SmartCash, Zoin, and Hexxcoin), and some of them are still vulnerable to specific attacks.

Zcoin, PIVX, SmartCash, Zoin, and Hexxcoin have been vulnerable to the denial-of-spending attack. Of those currencies, Zcoin, and Zoin are still vulnerable at the time of writing.