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Coding mistakes in a malicious program that encrypts data mean anyone hit by the Power Worm virus will not be able to recover files, say security experts.

Usually viruses known as ransomware decrypt files when victims have paid a substantial fee.

But one variant of Power Worm destroys keys that could help recover any data that it scrambled.

The news comes as hackers produce ransomware that is aimed at websites and encrypts data sitting on servers.

Power Worm infects Microsoft Word and Excel files but the latest poorly written update of it goes after many more types of data files it finds on a victim's machine.

Malware researcher Nathan Scott discovered the variant and uncovered the mistakes its creator made when updating it.

Mr Scott believes the errors arose when the creator tried to simplify the decryption process. They tried to make it use just one decryption key but mangled the process of generating it. As a result, there is no key created for the files it encrypts when it compromises a computer.

"There is unfortunately nothing that can be done for victims of this infection," . "If you have been affected by this ransomware, your only option is to restore from a back-up."

Mr Abrams said anyone hit by Power Worm should not pay the 2 bitcoin (about £500) ransom it asks for because they will not get any data back.

Ransomware is proving increasingly popular with hi-tech thieves and one group has now extended its list of potential targets to web servers that run Linux.

Russian anti-virus firm Dr Web has discovered a novel r that tries to infect sites via add-ons such as shopping systems that many of them use.

Once it lands on a server, the software encrypts any files, images, pages, scripts and stored source code it finds on the machine's main and back-up directories. Linux.encoder leaves behind a text file detailing how victims can pay the 1 bitcoin ransom required to recover their data.

"In the volume cybercrime space, ransomware is one of the most prolific problems we face," said Greg Day, chief security officer for Europe at Palo Alto Networks.

"Credit card theft is getting to the point where the value of each card is very low. As a result ransomware has stepped into that gap and gives a higher value for each victim."

Research by Palo Alto Networks and industry partners suggests the well-known Crypto Wall family of ransomware has generated about $325m (£215m) for the gang behind it.

"The return is so much better," Mr Day said. "That's why it's escalated to such a level."

He said regularly backing up data would help people and companies avoid having to pay criminals if they got caught out by ransomware.