From UK gov website: Thieves and pickpockets operate on the Paris underground and R E R lines. There have been several victims of serious assault on the R E R line B, which serves Paris Charles de Gaulle and Orly airports and Paris Gare du Nord Eurostar terminus. There have also been serious assaults on RER line D, which serves the Stade de France.
http://www.fco.gov.uk/en/travel-and-living-abroad/travel-advice-by-country/europe/france1#safetySecurity