Major Indian bitcoin exchange Unocoin went offline on Monday because of a serious security vulnerability.

Unocoin CEO and co-founder Sathvik Vishwanath told the Economic Times India in a recent interview,

Unocoin will return to regular operations by the “end of day Tuesday”. “We identified that there was an issue with the way the files were being stored and are resolving that part. We are also moving the servers to a more secure location,”

the issue is that someone's unocoin account was hacked that person received an email that his password was changed after being hacked he found that there was two transiction made 0.3 btc and 0.4 btc then he qucickly contacted te unicoin support team and unocoin able to locked his account.
but the article from bitcoin.com says:

Unocoin has seen an increase in transactions, account verification requests and customer support queries following the increased media exposure of bitcoin heading into June. Along with the increase in customer base, Unocoin has also seen an increase in reported wallet hacks – with the company claiming that it saw nine cases last month, more than double the monthly average of just 3 or 4. As a precautionary measure, the company has temporarily reduced its automatic transaction approval threshold to 0.1btc, as opposed to the normal limit at 0.5btc.

you can check full article on: https://news.bitcoin.com/major-indian-bitcoin-exchange-unocoin-offline-after-discovering-major-security-flaw/