CVE-2026-80846 in Linux
Summary
by MITRE • 09/04/2026
In the Linux kernel, the following vulnerability has been resolved:
xfrm: drop ESP-in-TCP packets with no ingress device
ESP-in-TCP receives records through the TCP strparser. handle_esp() restores skb->dev from the saved skb_iif before passing the packet into the XFRM input path.
Queued TCP data can be processed after the original ingress device has been removed, for example during veth or net namespace teardown. In that case dev_get_by_index_rcu() returns NULL. The XFRM IPv4 and IPv6 input paths both expect skb->dev to be valid while building the route lookup, so queued ESP-in-TCP data can dereference a NULL device.
Drop the packet if the saved ingress device can no longer be resolved. Such a packet can no longer be routed through the normal XFRM receive path, and this preserves the existing behaviour for packets whose ingress device still exists.
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Analysis
by VulDB Data Team • 09/04/2026
The Linux kernel vulnerability identified in the xfrm subsystem involves a critical NULL pointer dereference that occurs during the processing of Encapsulating Security Payload (ESP) data encapsulated within Transmission Control Protocol (TCP) streams. This issue specifically affects systems utilizing TCP stream parsing to handle ESP traffic, where packets are processed through the strparser mechanism. The core technical flaw lies in how the kernel handles the network device context for these queued packets. When an ESP-in-TCP packet is received, the function handle_esp() attempts to restore the original ingress device pointer by retrieving it from a saved interface index stored within the socket buffer structure. This step is crucial because subsequent processing stages rely on this device reference to perform routing lookups and other network stack operations that are dependent on the presence of a valid network interface.
The vulnerability manifests when queued TCP data is processed after the original ingress device has been removed from the system, such as during the teardown of virtual Ethernet interfaces or net namespaces. In these scenarios, the kernel function dev_get_by_index_rcu(), which is responsible for resolving the saved interface index back to a valid network device structure, returns NULL because the device no longer exists in the global list of active devices. Consequently, when this NULL pointer is passed into the XFRM IPv4 and IPv6 input paths, it causes a crash during the route lookup phase. The XFRM subsystem strictly expects skb->dev to be a valid reference while building routing decisions for incoming encrypted packets. Without this valid device context, the kernel cannot determine how to process or forward the packet, leading to an immediate NULL pointer dereference that can result in system instability or a denial of service condition depending on the execution context and memory layout at the time of the crash.
From a security impact perspective, this vulnerability allows for potential local denial of service attacks if an attacker can trigger the creation and subsequent removal of network namespaces or virtual interfaces while ESP-in-TCP traffic is being processed in the background queue. Although the attack vector requires specific conditions involving TCP stream parsing and device lifecycle management, it represents a significant stability risk for systems relying on complex networking configurations such as containerized environments or software-defined networks where interface churn is common. The vulnerability aligns with CWE-476, which describes NULL Pointer Dereference vulnerabilities, indicating that improper handling of memory references leads to unpredictable system behavior. Furthermore, the exploitation technique relates to resource management and lifecycle issues often associated with CWE-362, as it involves race conditions or timing dependencies between packet processing queues and device teardown operations.
To mitigate this vulnerability, kernel developers have implemented a safeguard within the handle_esp() function that explicitly checks whether the saved ingress device can still be resolved before proceeding with XFRM input path operations. If dev_get_by_index_rcu returns NULL, indicating that the original network interface has been removed, the packet is dropped immediately rather than being passed to the vulnerable routing logic. This approach preserves existing behavior for packets where the ingress device remains valid while preventing crashes in scenarios involving device teardown. System administrators should ensure their Linux kernels are updated with patches addressing this xfrm subsystem flaw. Additionally, monitoring logs for unexpected network interface removals or TCP stream parsing errors can help identify potential exploitation attempts or misconfigurations that might trigger this condition in production environments.