Several Indian banks have preemptively asked customers to either replace their debit cards or change their PINs as the threat of a data security breach looms large. The breach, which is being touted as the biggest in India, took place between May and July, and could have affected up to 3.2 million people, reported PTI. However, it came to fore only after several customers complained to banks that their cards have been used in China at various ATMs and point of sale terminals, according to the Economic Times.

State Bank of India's Chief Technology Officer Shiv Kumar Bhasin told The Times of India, "A few ATMs have been affected by a malware. When people use their card on infected switches or ATMs, there is a high probability that their data will be compromised". The banks affected include Axis Bank, HDFC Bank, ICICI Bank, State Bank of India and YES Bank. However, all banks have claimed that the breach has not led to any monetary loss to anyone.

On Wednesday, SBI said that it had blocked nearly six lakh debit cards after a suspected "malware-related security breach" in an ATM network outside the bank's purview was detected. SBI deputy managing director and chief operating officer Manju Agarwal told PTI, “We are now issuing EMV-based debit cards which cannot be compromised."

HDFC Bank has asked customers to change their PINs and urged people not to use ATMS of any other banks, reported PTI. YES Bank has clarified that did a comprehensive audit of ATMs and found no problems in their system.

Meanwhile, the Payments Council of India has ordered for a forensic audit of bank servers and systems to detect the origin of the fraud. "We have received complaints from banks about debit cards being used in China which aroused suspicion," NPCI Managing Director AP Hota told Economic Times. An official from the Reserve Bank of India also told PTI that they are investigating this.