CVE-2026-72254 in Linux
Summary
by MITRE • 08/15/2026
In the Linux kernel, the following vulnerability has been resolved:
netfilter: nft_fib: reject fib expression on the netdev egress hook
A fib expression in a netdev egress base chain dereferences nft_in(pkt), NULL on the transmit path, causing a NULL pointer dereference at eval. nft_fib_validate() masks the hook with NF_INET_* values, but netdev hook numbers are a separate enum that aliases them (NF_NETDEV_EGRESS == NF_INET_LOCAL_IN), so an egress chain passes validation and then faults.
Add nft_fib_netdev_validate() that limits each result/flag to the netdev hook where the device it reads exists: the input-device cases (OIF, OIFNAME, ADDRTYPE with F_IIF) to ingress, the output-device case (ADDRTYPE with F_OIF) to egress, ADDRTYPE with no device flag to both. Also restrict nft_fib_validate() to NFPROTO_IPV4/IPV6/INET so its NF_INET_* masks are not applied to another family's hooks.
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Analysis
by VulDB Data Team • 08/15/2026
The vulnerability resides in the Linux kernel's netfilter subsystem, specifically within the nftables framework's fib expression handling mechanism. This flaw manifests when processing fib expressions within netdev egress base chains, creating a critical NULL pointer dereference condition during packet evaluation. The underlying issue stems from improper validation logic that fails to account for the fundamental differences between netdev hook numbering and the traditional NF_INET_* enumeration system. When a fib expression executes on the transmit path through an egress chain, it attempts to access nft_in(pkt) which resolves to NULL, leading directly to kernel panic conditions that can compromise system stability and availability.
The technical flaw originates from nft_fib_validate() function's inadequate validation approach that masks hook numbers with NF_INET_* values without considering that netdev hooks utilize a separate enumeration system where NF_NETDEV_EGRESS equals NF_INET_LOCAL_IN. This aliasing creates a false validation pass where egress chains are incorrectly permitted to process fib expressions, despite the fundamental incompatibility between ingress and egress processing contexts. The validation mechanism fails to distinguish between the different network device processing paths, allowing malicious or malformed packet flows to trigger the NULL pointer dereference during evaluation.
Operational impact of this vulnerability extends beyond simple system crashes to encompass potential denial of service scenarios that can affect network connectivity and overall system reliability. Attackers could exploit this weakness by crafting specific netfilter rules that target egress chains with fib expressions, potentially causing kernel oops or complete system hangs depending on the packet flow characteristics. The vulnerability particularly affects systems running kernel versions where nftables is actively used for packet filtering and routing decisions, making it a significant concern for network infrastructure and security appliance deployments.
The mitigation strategy involves implementing a new validation function called nft_fib_netdev_validate() that properly restricts fib expression results and flags based on the specific netdev hook context where device information exists. This enhanced validation ensures that input-device cases such as OIF, OIFNAME, and ADDRTYPE with F_IIF are restricted to ingress processing paths, while output-device cases like ADDRTYPE with F_OIF are properly confined to egress contexts. Additionally, the original nft_fib_validate() function is modified to limit its scope to NFPROTO_IPV4, IPV6, and INET protocol families, preventing inappropriate application of NF_INET_* masks to other network protocol family hooks. This dual-layer validation approach addresses both the enumeration mismatch and protocol family isolation issues that enable the vulnerability.
This vulnerability aligns with CWE-476 which describes NULL pointer dereference conditions in software systems, while also demonstrating characteristics consistent with ATT&CK technique T1562.001 related to "Impairing Security Tools" through potential service disruption. The fix implementation follows secure coding principles by enforcing proper validation boundaries and maintaining separation of concerns between different network processing contexts, ensuring that kernel subsystems operate within their intended operational domains without cross-contamination between ingress and egress pathways.