For Salsburg, I think Real Madrid wouldn't find it difficult to beat them but considering Brest's performance in the UEFA Champions League competition this season, I think they'll pose a major threat when they host Real Madrid on 29th January for their last game of the group stage. The French club have been one of the dark horses of the competition this season and since that game will be played in their own home ground, I expect them to go all out for a win knowing fully well they have nothing to lose since they're playing against the reigning champions but at the end, I predict a Real Madrid win regardless
In the Champions League, Salzburg will be a bonus opponent for Real Madrid; in fact, they will just give Real Madrid a point. Salzburg has lost five of their previous six Champions League games and won one. However, I don't believe Brest has won against any of the best teams in the Champions League this season; the only top club I can remember of that Brest has met was Barcelona, and they lost at Barcelona's home field. Since Real Madrid is back to their winning ways and isn't in their worst shape, we shouldn't have any doubts about them against Brest.
Furthermore, I think you're not sure Real Madrid can defeat Brest because they've already lost to one French league team. Brest looks like a strong team this season, but they're not as good as Lille. Lille had an incredible season, defeating both Real Madrid and Atletico Madrid in the Champions League. Regardless of how defensive Brest plays against Real Madrid, I think the favorite team will win the match.