Note to fellow-HTBers: Only write-ups of retired HTB machines or challenges are allowed.

Challenge info

Inferno [by yuntao ]
Find the flag.

The challenge

We start of by downloading the inferno.zip file and verifying it’s sha256sum with the hash displayed on the challenge page.

$ echo "b178fc8605169bf0d9fd7ba74cd172a193db867d0057be74b5ad6f103d22dc80 inferno.zip" | sha256sum -c -
inferno.zip: OK

We then proceed to unzip this file using the password provided on the challenge page. This will give us a txt file.

$ unzip inferno.zip 
Archive:  inferno.zip
[inferno.zip] inferno.txt password: 
  inflating: inferno.txt

Looking at the content of the file, we see what looks like base64 encoded text.

$ cat inferno.txt 
RCdgXyReIjdtNVgzMlZ4ZnZ1PzFOTXBMbWwkakdGZ2dVZFNiYn08eyldeHFwdW5tM3Fwb2htZmUrTGJnZl9eXSNhYFleV1Z6PTxYV1ZPTnJMUUpJTkdrRWlJSEcpP2MmQkE6Pz49PDVZenk3NjU0MzIrTy8uJyYlJEgoIWclJCN6QH59dnU7c3JxdnVuNFVxamlubWxlK2NLYWZfZF0jW2BfWHxcW1pZWFdWVVRTUlFQMk5NRktKQ0JmRkU+JjxgQDkhPTw1WTl5NzY1NC0sUDAvby0sJUkpaWh+fSR7QSFhfXZ7dDpbWnZvNXNyVFNvbm1mLGppS2dgX2RjXCJgQlhXVnpaPDtXVlVUTXFRUDJOR0ZFaUlIR0Y/PmJCJEA5XT08OzQzODFVdnUtMiswLygnSysqKSgnfmZ8Qi8=

Decoding the text, we get what looks like random gibberish.

$ base64 -d inferno.txt 
D'`_$^"7m5X32Vxfvu?1NMpLml$jGFggUdSbb}<{)]xqpunm3qpohmfe+Lbgf_^]#a`Y^WVz=<XWVONrLQJINGkEiIHG)?c&BA:?>=<5Yzy765432+O/.'&%$H(!g%$#z@~}vu;srqvun4Uqjinmle+cKaf_d]#[`_X|\[ZYXWVUTSRQP2NMFKJCBfFE>&<`@9!=<5Y9y7654-,P0/o-,%I)ih~}${A!a}v{t:[Zvo5srTSonmf,jiKg`_dc\"`BXWVzZ<;WVUTMqQP2NGFEiIHGF?>bB$@9]=<;4381Uvu-2+0/('K+*)('~f|B/

I tried a few things like ROT13, base64 decoding the output again, hexdump, … to no avail.

Getting the flag

Then it struck me: this might be very similar to that Art challenge!

So I went back to the Esoteric programming languages Wikipedia page .
And look at what I found in this list. Do you see the reference with “Inferno”?

Malbolge (the 8th circle of Hell)

The an online interpreter .

Entering the “gibberish” into the interpreter resulted in the flag:

HTB{!1t_1s_just_M4lb0lg3_l4ngu4g3!}