CVE-2026-98306 in Linux
Summary
by MITRE • 10/06/2026
In the Linux kernel, the following vulnerability has been resolved:
seg6: set IPSKB_L3SLAVE from IP6SKB_L3SLAVE on IPIP decapsulation
When an SRv6 packet arrives on an interface enslaved to a VRF, vrf_ip6_rcv() sets IP6SKB_L3SLAVE in IP6CB, but decap_and_validate() has never set IPSKB_L3SLAVE in IPCB. The bit stayed clear in the common case, and with CONFIG_IPV6_MIP6 the leftover frag_max_size of a reassembled outer packet could even set it, with no VRF involved. Commit 44930446dde4 ("ipv6: seg6: clear IPv4 control block on IPIP decapsulation") then made the unreliable bit reliably clear.
The effect of the missing flag is visible with End.DX4 when a delivery to a local address of the node reaches the socket lookup. For example, a UDP socket bound to the enslaved ingress interface does not receive any of the decapsulated packets, while an unbound socket outside the VRF does. This contradicts Documentation/networking/vrf.rst: by default the scope of an unbound UDP or TCP socket is limited to the default VRF.
Set IPSKB_L3SLAVE for IPv4 in decap_and_validate(), which already does the same for IPv6. The socket lookup then matches the decapsulated packet like any other packet received on that enslaved interface. Such a packet matches an unbound UDP or TCP socket only when udp_l3mdev_accept or tcp_l3mdev_accept is set.
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Analysis
by VulDB Data Team • 10/06/2026
The vulnerability resides within the Linux kernel's Segment Routing over IPv6 (SRv6) implementation, specifically affecting the IPIP decapsulation process for packets arriving on interfaces enslaved to a Virtual Routing and Forwarding instance. The core technical flaw stems from an inconsistency in how network control block flags are propagated during packet processing. When an SRv6 packet is received on such an interface, the function vrf_ip6_rcv correctly sets the IP6SKB_L3SLAVE flag within the IPv6 control block to indicate that the packet originated from a slave interface associated with a VRF. However, upon decapsulation of this outer IPv6 header into an inner IPv4 packet via the IPIP mechanism, the function decap_and_validate fails to set the corresponding IPSKB_L3SLAVE flag in the newly created IPv4 control block. This omission means that for the vast majority of cases, the L3 slave status is lost during the transition from outer to inner protocol headers, effectively stripping the packet of its context regarding which VRF it belongs to.
The operational impact of this missing flag manifests primarily when handling End.DX4 SRv6 endpoints, where the decapsulated IPv4 packet must be delivered locally to a socket on the node. Because the IPSKB_L3SLAVE bit remains clear in the IPv4 control block, the kernel's socket lookup logic does not associate the incoming packet with the specific VRF of the ingress interface. Consequently, UDP or TCP sockets bound specifically to that enslaved interface fail to receive any decapsulated packets. This behavior directly contradicts standard networking expectations and documentation which state that unbound sockets should be limited to the default VRF scope unless explicitly configured otherwise via parameters like udp_l3mdev_accept or tcp_l3mdev_accept. In practice, this results in a denial of service for applications relying on interface-bound socket semantics within VRF environments using SRv6 decapsulation, while packets may incorrectly match unbound sockets outside the intended VRF context if specific kernel configurations are active.
From a security and standards perspective, this issue aligns with CWE-20 Improper Input Validation as it involves incorrect handling of packet metadata that dictates routing scope. It also relates to ATT&CK technique T1564 Hidden Execution through process or service manipulation in the sense that network traffic intended for specific services is silently dropped due to internal kernel state mismanagement, potentially masking connectivity issues from administrators while disrupting legitimate application functionality. The root cause was further complicated by previous commits aimed at clearing IPv4 control blocks during decapsulation, which inadvertently made this unreliable bit consistently clear rather than occasionally set based on leftover fragment sizes in scenarios involving CONFIG_IPV6_MIP6.
The resolution involves modifying the decap_and_validate function to explicitly set IPSKB_L3SLAVE for IPv4 packets when they are derived from an outer packet that originated on a VRF enslaved interface, mirroring the existing logic already present for IPv6 control blocks. By ensuring this flag is correctly propagated, the socket lookup mechanism can accurately determine the scope of the incoming packet. This allows decapsulated traffic to be delivered to sockets bound to the correct slave interface, restoring expected behavior where unbound sockets only match packets if global acceptance flags are enabled. Administrators should apply kernel patches that include this fix and verify network connectivity in VRF environments using SRv6 End.DX4 endpoints after updating their systems to ensure proper socket binding and packet delivery semantics are restored.