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backdoor CTF 2015: NONAME

Category: Exploit Points: 200 Author: Amanpreet Singh Difficulty: Solves: 25 Description:

Intrestingly enough, even though it was not expected, Chintu found a cool website to play with, though he can't get the flag. Can you? Visit this. Submit the SHA-256 hash of the flag obtained.


by polym

We are given a Clojure sandbox and have to find the flag and secret of an user named admin.

Entering valid clojure code, we can execute several functions, e.g. printing the version:

Give me some code:
> (clojure-version)

After learning about clojure, we first print the complete namespace to find any suspicious imported functions or namespaces using the all-ns function:

> (all-ns)
[..see all-ns file..]

We see a namespace named noname.people.admin, so we try to find any variables within this namespace after importing this namespace:

> (require '[noname.people.admin :as adm])
> (adm/flag)
Ma flag is : [REDACTED]
> (adm/secret)
java.lang.IllegalStateException: var: #'noname.people.admin/secret is not public

Mhh, we can access the variable flag, but can't access secret. Secret must be private. Fortunately, there is a way to read private variables described here:

> (#'adm/secret)
"Ma secret is: [REDACTED]"

We concat these two strings and make a sha256 hash out of them and get the flag!

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