Matthias Krause reports:

modprobe uses the "basename" of the module argument as the module to load, as
can be seen here:

bbox:~# lsmod | grep vfat
bbox:~# modprobe foo/bar/baz/vfat
bbox:~# lsmod | grep vfat
vfat                   17135  0
fat                    61984  1 vfat
bbox:~# find /lib/modules/`uname -r` -name vfat.ko
/lib/modules/3.18.0-rc5+/vfat.ko

It should instead fail to load the module -- actually fail to *find* the
module.

This can even be abused to load arbitrary modules by nullifying enforced module
prefixes some of the Linux kernel's subsystems try to apply to prevent just
that:

bbox:~# lsmod | grep usb
bbox:~# ifconfig /usbserial up
ifconfig: SIOCGIFFLAGS: No such device
bbox:~# lsmod | grep usb
usbserial              32201  0

The actual modprobe invocation, done by the kernel was:
/sbin/modprobe -q -- netdev-/usbserial

Due to the bug, the "netdev-" prefix including the "/" are ignored and the
usbserial.ko module gets loaded.

The same works for filesystems, e.g.:

bbox:~# lsmod | grep snd_pcm
bbox:~# mount -t /snd_pcm none /
mount: mounting none on / failed: No such device
bbox:~# lsmod | grep snd_pcm
snd_pcm                88826  0
snd_timer              26606  1 snd_pcm
snd                    61141  2 snd_pcm,snd_timer

This time the kernel called out to:
/sbin/modprobe -q -- fs-/snd_pcm

Note the "fs-" prefix.

External reference:
https://bugs.busybox.net/show_bug.cgi?id=7652 (cert maybe expired)