CVE-2026-72246 in Linux
Summary
by MITRE • 08/15/2026
In the Linux kernel, the following vulnerability has been resolved:
netfilter: flowtable: use correct direction to set up tunnel route
The layer 2 encapsulation and layer 3 tunnel information in the xmit path is taken from the other tuple, because the tunnel information that is included in the tuple for hashtable lookups is also used to perform the egress encapsulation in the transmit path.
This patch uses the correct direction when setting up the tunnel, the original proposed patch to address this fix uses the reversed direction.
While at it, remove the redundant check to call dst_release() to drop the reference on the dst that was obtained from the forward path, which is not useful in the direct xmit path unless tunneling is performed.
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Analysis
by VulDB Data Team • 08/16/2026
The Linux kernel netfilter subsystem contains a critical logic error within its flowtable implementation regarding the handling of layer two encapsulation and layer three tunnel information during packet transmission. This vulnerability stems from an incorrect directional assignment when setting up routes for tunneled traffic, specifically affecting the transmit path where egress encapsulation is performed. The root cause lies in the fact that the tuple used for hashtable lookups also dictates how packets are encapsulated upon exit. In the flawed implementation, the system incorrectly utilizes tunnel information derived from the opposite direction of the flow rather than the correct forward or reverse direction appropriate for the current packet's traversal path. This misalignment means that when a packet is prepared for transmission through a tunnel, such as VXLAN, GRE, or IP-in-IP encapsulations, the kernel applies routing and encapsulation parameters intended for traffic moving in the opposite direction of the flow table entry.
This directional confusion leads to significant operational impacts on network performance and stability within environments utilizing netfilter flow offloading with tunnels. When packets are encapsulated using incorrect route information, they may be directed toward unreachable next hops or configured with invalid tunnel endpoints. This results in packet loss for tunneled traffic flowing through the affected kernel version, effectively breaking connectivity for services relying on these specific network configurations. Furthermore, because the routing decision is based on mismatched tuple data, it can cause asymmetric routing issues where return traffic does not align with the established flow state, leading to connection drops or severe latency spikes. In more complex scenarios involving multiple tunnels or overlapping IP ranges, this error could potentially lead to packets being encapsulated and sent into black holes, degrading overall network throughput and reliability for applications dependent on stable tunnel connectivity.
From a vulnerability classification perspective, this issue aligns with CWE-823, which covers the use of pointers to private data structures without proper access control or validation, although more accurately it reflects CWE-691 regarding improper control flow sequencing and logic errors in state management. The flaw represents a failure in correctly mapping network flow states to their corresponding transmission actions, violating the principle that packet processing paths must strictly adhere to the defined directionality of the connection tuple. In terms of MITRE ATT&CK framework relevance, while this is primarily an operational stability issue rather than a direct exploitation vector for unauthorized access, it impacts Availability by causing service degradation or denial of service through network disruption. Attackers could potentially exploit this instability in targeted environments to induce persistent connectivity failures without needing to bypass traditional security controls, effectively achieving availability impact through resource exhaustion via failed packet transmissions and subsequent retransmissions.
The resolution involves correcting the directional logic used when setting up tunnel routes within the flowtable xmit path. The patch ensures that the correct direction is applied during egress encapsulation, aligning the route setup with the actual traffic flow rather than relying on reversed or incorrect tuple data. Additionally, the fix removes redundant code related to dst_release calls for destinations obtained from the forward path in direct transmit scenarios where tunneling is not performed. This cleanup improves efficiency by eliminating unnecessary reference counting operations that provided no functional benefit and could have contributed to subtle memory management overheads. By rectifying these logic errors, the kernel ensures that tunneled packets are encapsulated with accurate destination information, restoring proper connectivity for flow-offloaded traffic.
To mitigate this vulnerability in affected systems, administrators should apply the latest available kernel updates or specific patches provided by their distribution vendors that address netfilter flowtable tunneling issues. For environments where immediate patching is not feasible, temporary workarounds may include disabling hardware offloading features related to flow tables if they are actively causing connectivity disruptions for tunneled traffic. Monitoring network logs for increased packet drops on interfaces involved in tunneling operations can help identify instances of this flaw manifesting as operational anomalies. Long-term mitigation strategies should involve ensuring that all kernel components, particularly those handling netfilter and networking stack interactions, remain up to date with security patches released by the Linux community. Regular auditing of network configuration changes involving tunnels is also recommended to ensure compatibility with patched code paths and to detect any residual misconfigurations that might mimic or exacerbate this vulnerability's effects.