Advent of CTF - Challenge 18

“Evaluations”

Challenge

For today the challenge centers around NodeJS and the ever interesting Javascript.

Today you will learn:

  • How to identify a Javascript injection
  • How to exploit a Javascript injection

Solution

We created a calculator for Santa to figure out how many days until Christmas remain. It is not finished yet, it will only return what you give it. Sort of. The flag is in flag.txt.

The challenge starts with an edit box which asks you to enter the number of days until Christmas. Entering any number will reflect this number back to you,

challenge-start.png
Figure 1: Entering a number in the challenge

Entering any text in the challenge will result in a big error message. This error message reveals a lot about the challenge. It lists pretty much all the modules used to build the challenge. Clearly it is a NodeJS application that passes the input to the eval function.

error.png
Figure 2: Entering text in the challenge

Testing the eval feature with a simple addition shows that valid code is returned evaluated. Lets try 1+1 and indeed it returns 2.

1plus1.png
Figure 3: Math

Looking around for eval and payload on google several resources can be found that have great introductions. You will notice it is vary similar to template injections. Lets build a sample first based off of Payload all the Things again.

{'test'}

As we know it is a NodeJS application the fs.readFileSync functionality, which is handily available in the app, can be used as well.

{fs.readFileSync('flag.txt', {encoding:'utf8', flag:'r'})}

Don’t forget to also grab the badge.

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Figure 4: The badge

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