The hackers who stole US $81 million from Bangladesh's central bank likely used custom malware designed to interfere with the SWIFT transaction software used by many financial institutions.The attackers attempted to transfer $951 million out of Bangladesh Bank's account at the Federal Reserve Bank of New York in February, but most of the transfers were blocked before completion. The attackers did manage to send $81 million to accounts in the Philippines, and that money is still missing.Researchers from BAE Systems have recently come across several malware components that they believe are part of a custom attack toolkit that was likely used in the heist.The samples were uploaded to online malware repositories by someone in Bangladesh and they are designed to monitor, delete and alter transaction records in the database used by the SWIFT client software.Brussels-based SWIFT, or the Society for Worldwide Interbank Financial Telecommunication, is a cooperative society owned by thousands of financial institutions that runs the world's largest secure financial messaging network.One of the malware components bypasses a validation check in a library that's part of SWIFT's Alliance software suite, which stores transaction records in an Oracle Database, the BAE researchers said in a blog post Monday.The rogue program then monitors SWIFT Financial Application messages for strings defined in an encrypted configuration file. When it detects a matching one, it identifies the corresponding database entry and deletes it.The malware also monitors for login and logout events in the application and notifies command-and-control servers when they are detected.Finally, the program can manipulate confirmation SWIFT messages, which are automatically printed out on paper by the bank's systems. Altering database records is not enough to hide fraudulent transactions\u00a0because these printed confirmation messages could give an attack away, the BAE researchers said.There are still many unknowns about the well-planned Bangladesh Bank heist, such as who was behind it, how they got\u00a0into the bank's network in the first place, and how they initiated the rogue transfers. However, the existence of this custom malware toolkit should serve as a\u00a0warning to other financial institutions."This malware was written bespoke for attacking a specific victim infrastructure, but the general tools, techniques and procedures used in the attack may allow the gang to strike again," the BAE researchers said. "All financial institutions who run SWIFT Alliance Access and similar systems should be seriously reviewing their security now to make sure they too are not exposed."SWIFT is aware of the malware's existence. However, the program doesn't impact SWIFT's network or core messaging services, the organization said in an emailed statement."We understand that the malware is designed to hide the traces of fraudulent payments from customers\u2019 local database applications and can only be installed on users\u2019 local systems by attackers that have successfully identified and exploited weaknesses in their local security," SWIFT said."We have developed a facility to assist customers in enhancing their security and to spot inconsistencies in their local database records," the organization added. "However, the key defence against such attack scenarios remains for users to implement appropriate security measures in their local environments to safeguard their systems -- in particular those used to access SWIFT -- against such potential security threats."According to investigators, one factor that contributed to the success of the attack against Bangladesh's central bank was the lack of proper segmentation between the bank's SWIFT systems and the rest of its network. The SWIFT-connected computers were linked to the network via cheap switches with no management capability and lacking a firewall, Reuters reported Friday.