UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:56.0) Gecko/20100101 Firefox/56.0

Steps to reproduce the problem:
I haven't tested this yet, but I think an attacker with control over a sandboxed renderer on OS X can get MAC addresses via getifaddrs() or equivalent.

What is the expected behavior?
The sandbox would hide personally identifying network addresses if possible.  For example, the Linux sandbox unshares the network namespace, so from its point of view the physical network interfaces don't exist.

What went wrong?
 https://crbug.com/11325  added (allow sysctl-read), which allows all sysctls, including the one used by getifaddrs().  Even if this doesn't expose MAC addresses directly, there are also non-privacy-enabled IPv6 addresses which encode it.

Did this work before? No 

Chrome version: 59.0.3071.109  Channel: n/a
OS Version: OS X 10.10
Flash Version: 

This is also described in https://bugzilla.mozilla.org/show_bug.cgi?id=1376976, which was briefly public before being restricted.  (Firefox sandboxing isn't yet at a point where something this minor would normally be considered security-sensitive for us.)

Newer versions of OS X allow forms like (allow sysctl-read (sysctl-name "hw.physicalcpu_max")) but this might not be present on older versions.