The largest cryptocurrency hacks to date have targeted MT Gox, BitFloor, Linode, Bitfinex, Coincheck, and Bitgrail.

MT Gox

Over 850k Bitcoins were stolen from Mt. Gox between 2011 and 2014, making it the biggest cryptocurrency theft in history. According to Mt. Gox, a flaw that led to the loss was brought on by the transaction malleability problem, which is a fundamental flaw in Bitcoin. Changing a digital signature that produces a transaction's unique code is known as "transaction malleability."

MtGox's secret keys were found to have been stolen in September 2011, and the company failed to utilize any auditing procedures to identify the incident. Additionally, the keys were continuously used to rob new deposits because MtGox often reused addresses. As a result, by the middle of 2013, approximately 630k BTC had been stolen from the exchange. Surprisingly, WizSec (a team of Bitcoin security experts) asserts that blockchain transactions can provide evidence of continuing theft to back up this allegation.

Linode

Community whales and Bitcoin exchanges used Linode, a web hosting company, to secure their hot wallets. Unfortunately, the digital services that housed the wallets were attacked during the Linode breach in June 2011.

Unfortunately, this led to over 46k BTC being stolen, yet the exact amount is unclear. 

BitFloor

Despite the less severe nature of these thefts, high-impact Bitcoin break-ins have occurred. In 2012, 24k BTC were taken from BitFloor. During the crime, an attacker acquired access to an unsecured (i.e., unencrypted) duplicate of wallet keys and took virtual money worth around $250,000 in total. Roman Shtylman, the guy behind BitFloor, decided to close the exchange.

Bitfinex

As demonstrated by yet another massive robbery at Bitfinex, which saw 119,756 BTC stolen, multisig (the necessity of several keys to authenticate a BTC transaction) isn't a solution in and of itself.

Additionally, it appears that Bitfinex decided against using cold wallets to qualify for a legal exemption from the Commodities and Exchange Act. Although using threshold signatures is fascinating, it does not ensure that the power to allow transactions is distributed.

Bitgrail

Small-scale Italian exchange Bitgrail dealt in esoteric cryptocurrencies like Nano (XNO), formerly known as RaiBlocks. Nano's value went as low as 20 cents in 2017, but as it hovered at $10 in February 2018, the platform was hacked, causing $146 million in losses for BitGrail.

More than 220,000 people were duped by a coin that was stolen online. Unfortunately, tiny exchanges don't use fundamental security measures like cold storage wallets, which puts a lot of money in danger. Ivano Gabrielli, the head of the national center for cybercrimes, claims that it soon became clear that the CEO was involved in the company's controversy.

Coincheck

NEM (XEM) tokens valued at $530 million were stolen from Japan-based Coincheck in January 2018. However, it is still unknown who the Japanese hackers penetrated the security system.

Following the inquiry, Coincheck disclosed that a personnel shortage at the time allowed hackers to attack their system. In addition, the hackers effectively compromised the system because funds were held in "hot wallets," and there weren't enough security precautions.