Originally, the Dutch press believed that it was the Chinese Government that was spying on ASML, which wouldn't be a too outlandish idea given China's recent actions. But, ASML's CEO, Peter Wennink denied that it was China, and instead said it was "our biggest [South] Korean customer". Wennink refused to say that it was Samsung by name, and instead said that it was their biggest Korean customer a second time. Which basically points to Samsung, seeing as that is the only chip maker (that's big enough) in South Korea that ASML works with.
According to Wennink, this conclusion came from the company hiring an external company, to find out who was behind the espionage and theft of its software. The external company was able to conclude that it was not state espionage, but rather "ordinary" business espionage.
Apparently, the spies are all Chinese former employees who stole the software and created their own company to compete with ASML, Xtal. It is in China and the US.
The spying took place in 2015, and the new company, Xtal was created in 2016. that year, Samsung became a 30-percent shareholder of Xtal, then ASML lost Samsung as a customer for this particular software. The software in question is used to set up chip machines for optimum production of electronic chips. But interestingly enough, Samsung is already a customer of ASML, once again.
ASML Was Spied On By Its '''Biggest South Korean Customer''', Samsung [Updated] | Android Headlines