In shiftfs, a non-upstream patch to the Linux kernel included in the Ubuntu 5.0 and 5.3 kernel series, several locations which shift ids translate user/group ids before performing operations in the lower filesystem were translating them into inituserns, whereas they should have been translated into the suserns for the lower filesystem. This resulted in using ids other than the intended ones in the lower fs, which likely did not map into the shifts suserns. A local attacker could use this to possibly bypass discretionary access control permissions.