The bad guys are getting smarter unfortunately.

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.”

Weaknesses in versions of TeslaCrypt allowed researchers to create tools including TeslaCrack, Tesladecrypt and TeslaDecoder for people to decrypt their files without paying a ransom.

That encryption weakness has now been closed.