Few weeks ago I prepared the technical background of the CTF (Capture the Flag) for QuBit Conference Sofia 2019. It was intedned as a contest in which the three most successful participants will get the opportunity to attend QuBit Conference Sofia 2019. The content itself consisted of 10 challenges divided in 5 categories. In this post will be sumarized the thoughts, ideas and hints about the intended ways how to solve particular challenges.

Introduction In the post about GandCrab String Decryption I use very simple heuristic for identifying the function for string decryption. Because this kind of funtion is usually heavily used, I made an assumption that the scting decryption function is the most used function in our sample. This assumption is correct for GandCrab v5.1 DLL files, but it turns out that it is not true for GandCrab v5.2 and v.53. EXE samples.

Introduction In the last arcitle about Ursnif campaign have been presented the ursnif powershell downloader, which was also able to download the GandCrab payload. This payload was injected as DLL library into the running process and during the last analysis I have extracted it. Now, it is time to look more closely at this GandCrab sample. Obfuscated strings After a quick look at the disassembly we can notice many calls to one particular function, in our case named by IDA as sub_10009E69.

Introduction In the first part of this analysis have been presented the two types of macro-enabled documents with powershell downloader spreading via emails in recent campaign. The powershell downloaders and/or the macros were slightly obfuscated, however, it was easy to defeat this obfuscation and reveal their purpose. Unfortunately, during my analysis the downloaded content was not present on the involved servers and also in the most cases it was not available even during the analysis on sandboxes like Any.

Overview During the first half of February 2019 there was an increase in occurrences of the Spam messages containing attached documents with the names in the form “Request” followed by the number, like “Request15.doc”. These documents contain slightly obfuscated macros which lead to execution of the PowerShell downloader. This PowerShell downloader connects to the domains registered in Russian Federation and resolved to the Russian IP addresses. It seems that on these servers are hosted malicious content, in many cases detected as the Ursnif malware.