Do SN2 reactions always invert absolute configuration?
For the ease of thinking about this, let's imagine $\mathrm{S_N2}$ attack of $\ce{CN-}$ on bromochlorofluoroiodomethane, where iodide is being displaced.



Absolute stereochemistry is retained in this case because the highest priority substituent is becoming the lowest priority substituent. If you were to find the absolute stereochemistry of $\ce{2 -&gt;3-&gt;4}$ in (R)-bromochlorofluoroiodomethane, which becomes $\ce{1-&gt;2-&gt;3}$ after attack, you would see that it is S and after attack becomes R. You can see that here: