kurwov vulnerable to Denial of Service due to improper data sanitization

Summary

An unsafe sanitization of dataset contents on the MarkovData#getNext method used in Markov#generate and Markov#choose allows a maliciously crafted string on the dataset to throw and stop the function from running properly.

Details

https://github.com/xiboon/kurwov/blob/0d58dfa42135ab40e830e92622857282f980ca89/src/MarkovData.ts#L38-L44

If a string contains a forbidden substring (i.e. __proto__) followed by a space character, the second line will access a special property in MarkovData#finalData by removing the last character of the string, bypassing the dataset sanitization (as it is supposed to be already sanitized before this function is called).

data is then defined as the special function found in its prototype instead of an array.

On the last line, data is then indexed by a random number, which is supposed to return a string but returns undefined as it's a function. Calling endsWith then throws.

PoC

https://runkit.com/embed/m6uu40r5ja9b

Impact

Any dataset can be contaminated with the substring making it unable to properly generate anything in some cases.