The overlayfs filesystem does not correctly check file permissions when creating new files in the upper filesystem directory. This can be exploited by an unprivileged process in kernels with CONFIG_USER_NS=y and where overlayfs has the FS_USERNS_MOUNT flag, which allows the mounting of overlayfs inside unprivileged mount namespaces. This is the default configuration of Ubuntu 12.04, 14.04, 14.10, and 15.04. Included is a full exploit demonstration root code execution.