"taintedness" vs. "taintness"
Which should you use?  Neither.  Personally, I think the words "taintness" and "taintedness" both look awkward.  I would recommend rephrasing the sentence to avoid using either of those words.   Instead, you might try to use something like "whether the variable is tainted".  Or, if you desperately need a noun phrase and can't make the prior suggestion work, try something like "taint status".

For example, I might rewrite the quoted sentence as something like:


  To achieve that, we propose to perform a taint dependency analysis whose objective will be twofold: 1) to compute which variables are tainted at each program location; 2) to extract from the program source the whole dependency chains between each vulnerable statement and the corresponding inputs leading to this vulnerability.


(I added italics to highlight the part that I changed.)