The authors of Crypren, a new data-encoding trojan sample, appear to have fairly modest mercantile appetites as they extort an unusually low ransom of 0.1 Bitcoins, which converts to about 40 USD. This feature, though, doesn’t make such an attack incident any less abominable than the rest of the crypto malware assaults out there. The online criminals’ trump card in defrauding their victims of some savings is a rather strong cryptosystem leveraged in the course of the compromise.
The offending program makes use of a blend of AES-256 and RSA-2048 to turn one’s personal data into an array of inaccessible entities. It targets both the files stored locally and those residing on mapped network shares as well as external media that’s currently inserted into the infected computer running Windows or Linux. The range of file formats at risk isn’t very plentiful, covering objects with about 40 different extensions, as opposed to some ransomware variants that lock hundreds of types of data.
A straightforward symptom of the Crypren attack is the .encrypted string being attached to every mutilated file. An item named “resume.docx”, for instance, will therefore become “resume.docx.encrypted”, which certainly adds a bit of confusion to the mix. However, this isn’t the main thing that makes the victim realize something went wrong. The ransomware brings up a warning message in READ_THIS_TO_DECRYPT.html. It says:
“Your personal files have been encrypted. Your data (photos, documents, databases, etc.) have been encrypted with a private and unique key generated for this computer. This means that you will not be able to access your files anymore until they are decrypted. The private key is stored in our servers and the only way to receive your key to decrypt your files is making a payment.”
The racketeers provide seven days for the user to send the payment and get the private key in return, indicating a unique Bitcoin address at the bottom. If they don’t receive the ransom within the specified period of time, the information necessary for decryption will be allegedly erased from their C2 server. In fact, it will most likely stay there but the buyout amount may increase. Although Crypren doesn’t seem to be a super-complex ransomware sample, it does take one’s files hostage and its crypto routine is difficult to get around.
When a contamination instance occurs, most of the affected users should blame it on their own curiosity because they shouldn’t have opened an email attachment received from some unknown sender. This social engineering trick is by far the most heavily used attack vector in campaigns like this. The emails look interesting as the subjects are CVs, payroll notifications, subpoenas and similar. The enclosed ZIP archives or PDF documents instantly execute the ransom trojan on the system once opened. In some cases, these are Microsoft Office files that encourage users to enable macros, which is a nasty recommendation to follow.
In the event Crypren has crept into a computer and wreaked havoc with data on it, there are a couple of tips that will come in handy as far as the damage mitigation and file recovery are concerned.