CVE-2026-72320 in Linux
Summary
by MITRE • 08/15/2026
In the Linux kernel, the following vulnerability has been resolved:
netfilter: nft_lookup: fix catchall element handling with inverted lookups
nft_lookup_eval() decides whether a lookup matched (`found`) from the direct set lookup and priv->invert before falling back to the catchall element used by interval sets (e.g. nft_set_rbtree) for the open-ended default range. Since `found` is never recomputed after `ext` is replaced by the catchall lookup, inverted lookups (NFT_LOOKUP_F_INV, "!= @set") can wrongly match or wrongly skip the catchall element, producing the wrong verdict. Fold the catchall lookup into `ext` before computing `found`, matching the order already used by nft_objref_map_eval().
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Analysis
by VulDB Data Team • 08/15/2026
The vulnerability resides in the Linux kernel's netfilter subsystem within the nft_lookup module where a critical flaw exists in how catchall elements are handled during inverted lookups. This issue specifically affects the nft_lookup_eval() function which processes network filter rules involving set lookups. The problem manifests when the kernel evaluates whether a lookup operation should match against a catchall element for interval sets such as those implemented using nft_set_rbtree data structures. The core technical flaw occurs because the function fails to properly recalculate the match status after replacing the original extension with the catchall lookup result, leading to incorrect verdict determination during packet filtering operations.
The operational impact of this vulnerability stems from the fundamental nature of network filtering where incorrect match decisions can lead to unauthorized access or denial of service conditions. When inverted lookups are performed using the NFT_LOOKUP_F_INV flag which implements "!= @set" semantics, the system may either incorrectly match against catchall elements when it should not, or conversely skip necessary catchall matches entirely. This behavior directly violates the expected packet filtering logic and can result in security policy bypasses where packets that should be rejected based on set membership are instead allowed through the network filter chain. The vulnerability affects all systems running Linux kernels with netfilter support, particularly those implementing complex packet filtering rules using interval sets.
This flaw aligns with CWE-125 Out-of-bounds Read and CWE-284 Improper Access Control patterns commonly found in network security implementations. The issue demonstrates poor state management during conditional logic execution where the function's internal variables do not properly account for the transition from direct set lookup to catchall element evaluation. From an ATT&CK perspective, this vulnerability could be leveraged by adversaries to perform evasion techniques under the T1071.004 Application Layer Protocol and T1566 Phishing attack vectors where network filtering is used to block malicious traffic patterns. The fix implemented addresses this by ensuring that catchall lookup operations are properly integrated into the extension processing before computing the final match status, maintaining consistency with how similar operations are handled in nft_objref_map_eval() function, thereby restoring proper logical flow and security boundary enforcement.
The resolution involves restructuring the evaluation order within nft_lookup_eval() to ensure that catchall element handling occurs before the final determination of match status. This approach prevents the scenario where inverted lookup logic could incorrectly interpret catchall results due to state inconsistencies between direct set lookups and fallback catchall evaluations. The fix maintains the existing API compatibility while correcting the fundamental logic error that allowed incorrect verdict generation during complex network filtering operations involving interval sets. This remediation ensures that packet filtering decisions accurately reflect the intended security policies and prevents potential exploitation through malformed or maliciously crafted network traffic that could trigger the buggy code path.