Kim Thanh had no idea why she kept losing money from her mobile phone account on a daily basis despite the fact that she did not make any calls or send any texts at all.

The Ho Chi Minh City resident then contacted MobiFone, her network operator, and was told that her account was constantly sending texts to a call center that charged her VND15,000 for each message.

At first, Thanh was not convinced by the explanation, but she was later shocked to discover hidden malware installed in her handset, which was secretly and automatically sending the costly messages every day without her knowledge.

Thanh is only one of some 262,000 victims of malicious software that is programmed to take money from their prepaid phones, according to a recent report by Bkav, a Hanoi-based Internet security firm.

“With 262,000 infected phones forced to send SMS to the paid call center, and each message costing VND15,000, Vietnamese subscribers are losing more than VND3.9 billion (US$183,564) to the hackers behind the malware every day,” the report concludes.

Malware, short for malicious software, is any software used to disrupt computer or mobile phone operations, gather sensitive information, or gain access to private computer and phone systems.

Bkav Mobile Security, an antivirus app developed by Bkav, has found 621,000 new kinds of malware in the first five months of this year, according to company statistics.

“The real number of infected phones could be much higher than reported because many users are not aware that their devices have malware,” the report says.

While most smartphone users in Vietnam can easily download new apps using WiFi or the 3G network, few of them have adequate knowledge of malware risks and preventive solutions.

“It only takes the malware five to ten minutes to begin charging money from the victim’s phone balance,” an Internet security expert warned.

“Millions of subscribers are ‘pickpocketed’ through their phones and do not have the slightest idea.”

Smartphone users are advised to protect themselves by carefully considering a new app before installation.

“It is also advisable that they only download apps from trusted sources like Google Play or the Apple App Store,” experts said.