CVE-2026-99211 in Linux
Summary
by MITRE • 10/28/2026
In the Linux kernel, the following vulnerability has been resolved:
netlink: reset skb headers before unicast and multicast filtering
Socket filters can use SKF_NET_OFF and SKF_LL_OFF to read relative to an skb's network and MAC headers. Netlink producers may reserve headroom without resetting those headers, allowing a receiving socket's filter to read data before the initialized message.
For example, IWPM HELLO replies are allocated with dev_alloc_skb(), which reserves NET_SKB_PAD bytes. An unprivileged NETLINK_RDMA user can attach a classic socket filter and disclose this headroom through the length of the delivered reply. The resets in netlink_dump() do not help because IWPM sends a separately allocated skb through netlink_unicast().
Reset both headers before running the receiver's filter in unicast and broadcast delivery, as is already done for dump skbs. This also covers other netlink producers that reserve headroom without changing their allocation or message-building code.
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Analysis
by VulDB Data Team • 10/28/2026
The Linux kernel contains a vulnerability within its Netlink subsystem related to improper handling of socket buffer headers during packet filtering operations. Specifically, the issue arises when unprivileged users can attach classic socket filters to Netlink sockets and exploit reserved headroom in allocated sk_buff structures. The core technical flaw lies in the fact that certain Netlink producers reserve space for network layer padding without resetting the skb's network and link-layer header pointers after allocation. This oversight allows receiving socket filters, which utilize instructions such as SKF_NET_OFF and SKF_LL_OFF to read data relative to these headers, to access uninitialized or stale memory regions preceding the actual initialized message content.
This vulnerability is particularly relevant in scenarios involving devices like wireless interface power management (IWPM) components that allocate sk_buffs using functions like dev_alloc_skb(). These allocation routines typically reserve NET_SKB_PAD bytes of headroom for alignment and protocol overhead purposes. When an unprivileged user with NETLINK_RDMA access attaches a socket filter, they can trigger the delivery of replies where the header pointers have not been properly adjusted to account for this reserved space. Consequently, the filter reads data from before the start of the valid message payload, effectively exposing internal kernel memory structures that were intended to remain opaque or unused in that specific context.
The operational impact of this flaw is primarily information disclosure through side-channel analysis via packet length and content inspection. Although existing code paths such as netlink_dump() correctly reset headers for dump operations, other delivery mechanisms like netlink_unicast(), which are used by IWPM HELLO replies, do not perform this reset before passing the buffer to the receiver's filter logic. This inconsistency creates a vector where an attacker can distinguish between different memory states or potentially leak kernel pointers and configuration data based on how much headroom is present in the delivered packet. The severity is heightened because it requires only unprivileged access to specific Netlink families, making it accessible to local attackers without elevated privileges.
To mitigate this vulnerability, developers have implemented a fix that ensures both network and link-layer headers are reset before executing receiver-side filters during unicast and broadcast deliveries. This change aligns the behavior of netlink_unicast() with that of dump operations, ensuring consistent header management across all Netlink message delivery paths. By resetting these pointers to reflect the actual start of the valid payload rather than the beginning of the allocated buffer space, the system prevents socket filters from accessing unintended memory regions. This patch addresses not only the specific IWPM case but also covers other Netlink producers that may reserve headroom without adjusting their allocation or message-building code accordingly.
From a classification perspective, this vulnerability aligns with CWE-200: Information Exposure and CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer. It represents an out-of-bounds read condition where the bounds are defined by the logical start of the message rather than the physical allocation boundary. In terms of attack tactics, this relates to ATT&CK technique T1083: File and Directory Discovery or more broadly T1074: Data Staged if used for exfiltration preparation, though primarily it serves as a reconnaissance vector for further exploitation attempts against kernel memory integrity. The fix emphasizes the importance of consistent state management in network stack components to prevent leakage through seemingly benign filtering mechanisms.