FUSE through version 3.2.4 is vulnerable to a bypass of the 'user_allow_other' restriction that allows, when SELinux is active, non-root users to mount FUSE file systems with the 'allow_other' mount option. Local users can exploit this with the 'fusermount' command, bypassing the system configuration. This results in a mounted file system accessible by all other users including root.
This flaw allows a local attacker to mount a FUSE file system with the 'allow_other' option, even if they should not be able to do so. As a consequence, if the attacker can trick other users to access his FUSE mount point, he can cause Denial of Services or other unspecified effects. Accesses to the FUSE mount point could be slowed down or delayed indefinitely. Moreover the FUSE file system could present files or directories of unlimited length, or directory structures of unlimited depth, thus causing DoS or other unspecified effects on other programs which do not consider this possibility.
Patches: https://github.com/libfuse/libfuse/commit/28bdae3d113ef479c1660a581ef720cdc33bf466 https://github.com/libfuse/libfuse/commit/5018a0c016495155ee598b7e0167b43d5d902414
When checking whether allow_other/allow_root are allowed, fusermount lets users escape commas, even when the mount options do not permit backslashes. This allows a malicious user to hide options forbidden by fusermount. By using specially crafted values in the `context` mount option interpreted by SELinux, it is possible to use a backslash and smuggle the `allow_other` mount option.
Flaw was introduced by https://github.com/libfuse/libfuse/commit/555d6b504308eac6b976321ce938ee4bec62c354.
Created fuse tracking bugs for this issue: Affects: fedora-all [bug 1607854]
References: https://sourceforge.net/p/fuse/mailman/message/36374753/
Statement: This issue did not affect the versions of fuse as shipped with Red Hat Enterprise Linux 6 as they did not consider escaped characters when checking mount options. This issue is present in fuse packages included with Red Hat Virtualization, however it is not exploitable under any supported configuration.
This issue has been addressed in the following products: Red Hat Enterprise Linux 7 Via RHSA-2018:3324 https://access.redhat.com/errata/RHSA-2018:3324