CVE-2026-89951 in Linux
Summary
by MITRE • 09/16/2026
In the Linux kernel, the following vulnerability has been resolved:
batman-adv: fix stale receive device on merged fragments
Fragment reassembly reuses the skb from the highest-numbered buffered fragment as the merged packet. When that fragment was received on a hard interface which is deleted before the chain completes, the merged skb can re-enter the receive path with a stale skb->dev and skb_iif.
batadv_batman_skb_recv() passes such merged packets through the normal receive handlers again. DAT and bridge loop avoidance both derive the ARP header length from skb->dev, so they can dereference the freed net_device before the packet reaches the local mesh interface.
Refresh the receive device metadata from the current receive device before running the packet handlers. This keeps internally reinjected merged fragments consistent with the normal receive path after hard interface teardown.
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Analysis
by VulDB Data Team • 09/16/2026
The vulnerability identified in the Linux kernel's batman-adv module stems from a critical flaw in how fragmented network packets are reassembled and subsequently processed within the mesh networking stack. Specifically, when multiple packet fragments arrive out of order or with gaps, the system buffers them until all pieces are received to reconstruct the original data stream. The implementation logic dictates that the socket buffer structure for the final merged packet is reused from the fragment possessing the highest sequence number. This optimization reduces memory allocation overhead but introduces a significant risk if the network interface associated with that specific high-numbered fragment undergoes state changes during the reassembly window. If the hard interface on which this last fragment was received is deleted or goes offline before the entire chain of fragments has been successfully collected and merged, the resulting socket buffer retains references to resources belonging to a now-deleted device structure.
This stale reference manifests as an invalid pointer within the skb->dev field and the associated input interface index (skb_iif). When batadv_batman_skb_recv processes these internally reinjected merged fragments, it routes them through standard receive handlers that assume valid network device contexts are present. Critical components such as Distributed ARP Table processing and bridge loop avoidance mechanisms rely heavily on accessing the net_device structure to determine header lengths and validate packet integrity. Because the underlying net_device has been freed due to interface teardown, these modules attempt to dereference memory that is no longer allocated or managed by the kernel's networking subsystem. This scenario creates a classic use-after-free condition where the kernel accesses deallocated memory, leading to potential data corruption, unpredictable system behavior, or complete kernel panic resulting in a denial of service for the affected node.
From a vulnerability classification perspective, this issue aligns with CWE-416, Use After Free, as it involves accessing an object after its lifetime has ended. The attack vector is primarily local or network-based depending on whether an attacker can influence fragment timing and interface state changes, potentially categorized under ATT&CK technique T1059 if leveraged for further exploitation within a compromised mesh node. The operational impact includes severe stability degradation of the batman-adv mesh networking service. In production environments relying on this technology for robust wireless or wired mesh connectivity, such vulnerabilities can cause nodes to crash unexpectedly, disrupting network topology and causing widespread communication failures across the mesh infrastructure.
To mitigate this vulnerability, it is imperative that system administrators ensure their Linux kernels are updated with patches addressing batman-adv fragment reassembly logic. The fix involves refreshing the receive device metadata from the current receiving interface before passing merged packets through subsequent handlers. This ensures that internally reinjected fragments maintain consistency with the normal receive path even after hard interface teardown events occur. By decoupling the packet processing context from the specific physical or virtual interface used for initial reception, the kernel prevents dereferencing freed net_device structures. Regularly applying security updates and monitoring system logs for network device state changes are essential practices to maintain resilience against this class of memory safety errors in Linux-based mesh networking deployments.