Multiple versions of Open vSwitch are vulnerable to remote buffer
overflow attacks, in which crafted MPLS packets could overflow the
buffer reserved for MPLS labels in an OVS internal data structure.
The MPLS packets that trigger the vulnerability and the potential for
exploitation vary depending on version:

    - Open vSwitch 2.1.x and earlier are not vulnerable.

    - In Open vSwitch 2.2.x and 2.3.x, the MPLS buffer overflow can be
      exploited for arbitrary remote code execution.

    - In Open vSwitch 2.4.x, the MPLS buffer overflow does not
      obviously lead to a remote code execution exploit, but testing
      shows that it can allow a remote denial of service.

    - Open vSwitch 2.5.x is not vulnerable.

Mitigation
==========

For any version of Open vSwitch, preventing MPLS packets from reaching
Open vSwitch mitigates the vulnerability.  We do not recommend
attempting to mitigate the vulnerability this way because of the
following difficulties:

    - Open vSwitch obtains packets before the iptables host firewall,
      so iptables on the Open vSwitch host cannot ordinarily block the
      vulnerability.

    - If Open vSwitch is configured to support tunnels, MPLS packets
      encapsulated within tunnels must also be prevented from reaching
      the host.

    - If Open vSwitch runs on a hypervisor, MPLS packets from VMs can
      also trigger the vulnerability.

We believe that Open vSwitch 2.4 is subject to denial of service only
when debug logging is enabled.  By default, debug logging is not
enabled.  Users most commonly enable debug logging at runtime using
the "ovs-appctl" utility.  When this is the case, the buffer overflow
will crash the ovs-vswitchd daemon once, and then when it
automatically restarts debug logging will be disabled; thus, in this
situation, the vulnerability can only cause a single, brief
interruption in service.  Debug logging can also be enabled
persistently using a command-line flag; in this situation, a stream of
crafted MPLS packets could cause an extended denial of service.

Acknowledgments:

Name: the Open vSwitch project
Upstream: Kashyap Thimmaraju, Bhargava Shastry