The latest version of the TeslaCrypt ransomware has tidied up a weakness in previous versions that in some cases allowed victims to recover their files without paying a ransom.
Cisco's Talos research group found that TeslaCrypt 3.0.1 has improved its implementation of a cryptographic algorithm making it impossible now to decrypt files.
"We can not say it loud and often enough, ransomware has become the black plague of the internet," wrote Andrea Allievi and Holger Unterbrink, both security researchers with Cisco, in a blog post on Wednesday. "The adversaries are modifying and improving it in every version."
That encryption weakness has now been closed.
"Unfortunately, so far we are not aware of any tool which can do the same for this variant of TeslaCrypt," the Cisco reseachers wrote.
Ransomware schemes have become one of the most common scams on the Internet. The malware encrypts a user's files, then displays instructions for how victims can pay to obtain the decryption key.
Although ransomware has been around for more than a decade, the schemes have proliferated in the last couple of years, striking consumers and businesses.
Antivirus programs often miss ransomware, as its authors make minor tweaks to the code to avoid security scanning.
Backing up files is the best defense, but the FBI warned last month that cybercriminals are increasingly aiming "to infect whole networks with ransomware and use persistent access to locate and delete network backups," according to the Security Ledger.