RTF (Rich Text Format) files have been used before by cybercriminals, but of late it seems their use of this format is becoming more creative.
We’d earlier talked about how CPL files were being embedded in RTF files and sent to would-be victims as an e-mail attachment. These CPL files would then proceed to download malicious files which would be run on the affected samples.
Earlier samples used instructions in Portuguese, but newer samples now use German:
Figure 1. German-language RTF document
Overall, the tactics are still the same – the RTF file contains an embedded “receipt” with instructions to double-click the receipt. Double-clicking this file runs the CPL malware, which downloads the payload.
Figure 2. Code of RTF document
In this particular case, the URL is no longer accessible so we cannot be 100% sure what the payload was. However, previous incidents have used information stealers, so in all likelihood that would have been the case here as well. We detect this variant of CPL malware as TROJ_CHEPRTF.SM2.
A separate case also embedded malware into a RTF file, but this time the embedded malware belonged to the ZBOT malware family. This ZBOT variant is detected as TSPY_ZBOT.KVV; this variant has the capability to steal user names and passwords such as from various sources such as email, FTP and online banking.
These incidents highlight how cybercrime techniques are always improving. RTF files may have been used in these cases because users may not know that RTF files can be used to spread malware, and even if they do know they may not be able to easily determine which files are malicious and which are not.
In addition, using RTF files to spread ZBOT is unusual, as it’s typically spread via other means such as downloaders, malicious sites, or spam. This shows how cybercriminals are willing to embrace new tactics to achieve their goals.
We encourage users to be careful when opening email messages and attachments. Never download and open attachments unless they can be verified. Businesses should employ a mail scanning solution implemented on the network and enable the scanning of email messages.
The Trend Micro™ Smart Protection Network™ protects users from this threat by blocking access to all related malicious URLs, and preventing the download and execution of the malicious file.
Update as of 7:00 PM PST, March 6, 2014
The hashes of the files involved in this attack are: