FreeBSD VuXML: Documenting security issues in FreeBSD and the FreeBSD Ports Collection

FreeBSD -- Cross-mount links between nullfs(5) mounts

Affected packages
9.1 <= FreeBSD < 9.1_7
8.4 <= FreeBSD < 8.4_4
8.3 <= FreeBSD < 8.3_11


VuXML ID b72bad1c-20ed-11e3-be06-000c29ee3065
Discovery 2013-09-10
Entry 2013-09-19
Modified 2016-08-09

Problem Description:

The nullfs(5) implementation of the VOP_LINK(9) VFS operation does not check whether the source and target of the link are both in the same nullfs instance. It is therefore possible to create a hardlink from a location in one nullfs instance to a file in another, as long as the underlying (source) filesystem is the same.


If multiple nullfs views into the same filesystem are mounted in different locations, a user with read access to one of these views and write access to another will be able to create a hard link from the latter to a file in the former, even though they are, from the user's perspective, different filesystems. The user may thereby gain write access to files which are nominally on a read-only filesystem.


CVE Name CVE-2013-5710
FreeBSD Advisory SA-13:13.nullfs