The algorithm is part of a malformed FileZilla.exe binary, therefore sending stolen log in details which bypasses the firewall. Malware authors abuse open source code and add their own stealer function to the main code. We found a hardcoded connection detail stealer after deeper analysis. Any attempt to update the application fails, which is most likely a protection to prevent overwriting of malware binaries. The only differences that can be seen at first glance are smaller filesize of filezilla.exe (~6,8 MB), 2 dll libraries ibgcc_s_dw2-1.dll and libstdc++-6.dll (not included in the official version) and information in "About FileZilla" window indicates the use of older SQLite/GnuTLS versions. The installed malware FTP client looks like the official version and it is fully functional! You can't find any suspicious behavior, entries in the system registry, communication or changes in application GUI. All other elements like texts, buttons, icons and images are the same. The only slight difference is version of NullSoft installer where malware uses 2.46.3-Unicode and the official installer uses v2.45-Unicode. Malware installer GUI is almost identical to the official version. As you can see, the installer is mostly hosted on hacked websites with fake content (for example texts and user comments are represented by images.) The first suspicious signs are bogus download URLs. We have noticed an increased presence of these malware versions of famous open source FTP clients. Malformed FileZilla FTP client with login stealerīeware of malformed FileZilla FTP client versions 3.7.3 and 3.5.3.
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