CVE-2023-28842

Moby) is an open source container framework developed by Docker Inc. that
is distributed as Docker, Mirantis Container Runtime, and various other
downstream projects/products. The Moby daemon component (dockerd), which
is developed as moby/moby is commonly referred to as Docker. Swarm Mode,
which is compiled in and delivered by default in dockerd and is thus
present in most major Moby downstreams, is a simple, built-in container
orchestrator that is implemented through a combination of SwarmKit and
supporting network code. The overlay network driver is a core feature of
Swarm Mode, providing isolated virtual LANs that allow communication
between containers and services across the cluster. This driver is an
implementation/user of VXLAN, which encapsulates link-layer (Ethernet)
frames in UDP datagrams that tag the frame with the VXLAN metadata,
including a VXLAN Network ID (VNI) that identifies the originating overlay
network. In addition, the overlay network driver supports an optional,
off-by-default encrypted mode, which is especially useful when VXLAN
packets traverses an untrusted network between nodes. Encrypted overlay
networks function by encapsulating the VXLAN datagrams through the use of
the IPsec Encapsulating Security Payload protocol in Transport mode. By
deploying IPSec encapsulation, encrypted overlay networks gain the
additional properties of source authentication through cryptographic proof,
data integrity through check-summing, and confidentiality through
encryption. When setting an endpoint up on an encrypted overlay network,
Moby installs three iptables (Linux kernel firewall) rules that enforce
both incoming and outgoing IPSec. These rules rely on the u32 iptables
extension provided by the xt_u32 kernel module to directly filter on a
VXLAN packet’s VNI field, so that IPSec guarantees can be enforced on
encrypted overlay networks without interfering with other overlay networks
or other users of VXLAN. The overlay driver dynamically and lazily
defines the kernel configuration for the VXLAN network on each node as
containers are attached and detached. Routes and encryption parameters are
only defined for destination nodes that participate in the network. The
iptables rules that prevent encrypted overlay networks from accepting
unencrypted packets are not created until a peer is available with which to
communicate. Encrypted overlay networks silently accept cleartext VXLAN
datagrams that are tagged with the VNI of an encrypted overlay network. As
a result, it is possible to inject arbitrary Ethernet frames into the
encrypted overlay network by encapsulating them in VXLAN datagrams. The
implications of this can be quite dire, and GHSA-vwm3-crmr-xfxw should be
referenced for a deeper exploration. Patches are available in Moby releases
23.0.3, and 20.10.24. As Mirantis Container Runtime’s 20.10 releases are
numbered differently, users of that platform should update to 20.10.16.
Some workarounds are available. In multi-node clusters, deploy a global
‘pause’ container for each encrypted overlay network, on every node. For a
single-node cluster, do not use overlay networks of any sort. Bridge
networks provide the same connectivity on a single node and have no
multi-node features. The Swarm ingress feature is implemented using an
overlay network, but can be disabled by publishing ports in host mode
instead of ingress mode (allowing the use of an external load balancer),
and removing the ingress network. If encrypted overlay networks are in
exclusive use, block UDP port 4789 from traffic that has not been validated
by IPSec.