Ransomware developers make mistakes in their code. These errors could help victims regain access to their original files after a WannaCry infection. Wannacry has a bug in its read-only file processing. It will only create an encrypted copy of each original file, while the original files themselves only get the hidden attribute. If there are such files on the infected machine, then the ransomware wont encrypt them at all. When this happens, it is simple to find them and restore their normal attributes. To restore files you can use the free utilities available for file recovery.”]
Source: https://securelist.com/wannacry-mistakes-that-can-help-you-restore-files-after-infection/78609/