148. Base64? And What About Base85?

Hello everyone!

Adversaries always abuse Base64 to conseal malicious scripts. For example, abusing PowerShell or Python. But what about other encoding schemes?

Let's look inside this report. The threat actors executed the Python Launcher, py.exe, with an obfuscated Python command as an argument:

"C:/winsystem/py/py.exe" -c exec(__import__('marshal').loads(__import__('zlib').decompress(__import__('base64').b85decode('[redacted]'))))

A Base85-encoded string is decoded, decompressed, deserialized, and then executed as Python code.

As Base85 isn't very common in modern environments, we can use it to build our hunting query:

event_type: "processcreatewin"

AND

cmdline: "b85decode"

See you tomorrow!

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