CVE-2023-53843: 
Linux Kernel vulnerability analysis and mitigation

CVE-2023-53843 is a vulnerability in the Linux kernel's Open vSwitch (OVS) subsystem where the net/openvswitch module fails to validate ifindex values, allowing the creation of network device ports with negative ifindex values. This issue was surfaced by syzbot following changes introduced by commit 759ab1edb56c ("net: store netdevs in an xarray") in net-next, which refactored pre-assigned ifindex handling. The vulnerability affects the Linux kernel and was published on December 9, 2025. Feedly estimates the severity as Medium, with an EPSS score of 0.000170 (Feedly).

The root cause is improper input validation (CWE-20) in the Open vSwitch vport creation path within the Linux kernel. When a user creates a new OVS vport via Netlink and supplies a large unsigned integer as the ifindex (e.g., 4294901760, which wraps to -65536 as a signed 32-bit integer), the kernel does not reject the value, resulting in a netdev being registered with a negative ifindex. This was made exploitable by the xarray-based netdev storage refactor, which exposed the latent lack of bounds checking. The fix adds validation to reject out-of-range ifindex values, returning ERANGE to the caller (Feedly).

Exploitation of this vulnerability allows a local user with sufficient privileges to create network device ports with negative ifindex values, leading to undefined or unexpected kernel behavior. This could result in system instability, kernel crashes, or memory corruption depending on how downstream code handles the malformed ifindex, affecting availability and potentially integrity of the kernel's networking subsystem. The scope is limited to the local system; there is no known direct path to remote code execution or lateral movement (Feedly).

1. Prerequisite: Obtain local access to a Linux system running a vulnerable kernel with Open vSwitch enabled and sufficient privileges to interact with OVS Netlink sockets.
2. Create an OVS datapath: Use the YNL CLI or a Netlink client to create a new OVS datapath: ./cli.py --spec netlink/specs/ovs_datapath.yaml --do new --json '{"upcall-pid": 1, "name":"my-dp"}'
3. Submit malformed ifindex: Create a new OVS vport with an out-of-range ifindex value (e.g., 4294901760, which wraps to -65536 as a signed integer): ./cli.py --spec netlink/specs/ovs_vport.yaml --do new --json '{"upcall-pid": "00000001", "name": "some-port0", "dp-ifindex":3,"ifindex":4294901760,"type":2}'
4. Observe result: On unpatched kernels, ip link show will display a network interface with a negative ifindex (e.g., -65536: some-port0), indicating successful exploitation of the missing validation (Feedly).

• Logs: Kernel logs (dmesg) showing unexpected netdev registration with negative ifindex values, or OVS-related Netlink errors.
• Network: Presence of network interfaces with negative ifindex values visible via ip link show.
• Process: Unusual use of OVS Netlink socket operations (e.g., OVS_VPORT_CMD_NEW) with out-of-range ifindex parameters from unprivileged or unexpected processes.

The fix is to apply the upstream Linux kernel patch that adds ifindex validation in the OVS vport creation path, rejecting values outside the valid signed 32-bit integer range and returning ERANGE. Patches have been committed to stable kernel trees (referenced at git.kernel.org stable commits 881faff9e548, a552bfa16bab, and c965a58376146). Users should update to a patched kernel version as provided by their Linux distribution (e.g., SUSE has issued advisories addressing this CVE). No configuration-based workaround is known; upgrading the kernel is the recommended remediation (Feedly, Linux Security SUSE).