qemu-bridge-helper.c in QEMU 3.1 and 4.0.0 does not ensure that a network interface name (obtained from bridge.conf or a --br=bridge option) is limited to the IFNAMSIZ size, which can lead to an ACL bypass.
This flaw can only be exploited if /etc/qemu*/bridge.conf
contains a line containing allow all
or at least one line with a bridge name of at least 15 characters.