Category Archives: analysis

Shell hiding in image files

One day, we noticed strange GET request towards our JBoss server:

From the request above, you’ll quickly noticed that this attack leveraging Apache Struts vulnerability from CVE-2017-5638.

The request tried to execute command below:

“-O” : writes the documents to file.
“-” : if is used as file, documents will be printed to standard output, disabling link conversion.
“-q” : quiet (no output)

As you see, it tried to fetch image (jpeg file) from 91.230.47.41. Seems normal right?
We fetch the file & take a look at the jpg file:

ASCII?? Not JPEG?? hmm..
Here’s whats inside the “logo.jpg” file:

We noticed there are several other file fetched; possibly a config file & bin file.
Let’s fetch those file!

Here is the config file:
http://91.230.47.41/pics/kworker.conf

Not sure it is. Maybe bin file to run a process:
http://91.230.47.41/pics/kworker

Lets see if the file is packed:

Yup. So lets unpacked the file using UPX:

http://91.230.47.41/pics/kworker_na

Overall, looks like the attacker want to hack our servers & turn it into his own crypto currency mining machine.
Typical behavior of attack we see in this time where the crypto currency is rising. People hack to make profit. 🙂

Here the MD5 for file above: