A race condition flaw was found in the N_HLDC Linux kernel driver when accessing n_hdlc.tbuf list that can lead to double free. A local, unprivileged user able to set the HDLC line discipline on the tty device could use this flaw to increase their privileges on the system. #### Mitigation The n_hdlc kernel module will be automatically loaded when an application attempts to use the HDLC line discipline from userspace. This module can be prevented from being loaded by using the system-wide modprobe rules. The following command, run as root, will prevent accidental or intentional loading of the module. Red Hat Product Security believe this method is a robust method to prevent accidental loading of the module, even by privileged users. ​# echo "install n_hdlc /bin/true" >> /etc/modprobe.d/disable-n_hdlc.conf The system will need to be restarted if the n_hdlc modules are already loaded. In most circumstances, the n_hdlc kernel modules will be unable to be unloaded if in use and while any current process using this line discipline is required. Exploiting this flaw does not require Microgate or SyncLink hardware to be in use. If further assistance is needed, see this KCS article ( <https://access.redhat.com/solutions/41278> ) or contact Red Hat Global Support Services.