CVE-2026-90237 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
netfilter: nft_ct: move custom expectation support to helper
Originally, the ct expectation support called nf_ct_helper_ext_add() for confirmed conntracks, which is invalid, triggering a splat. This was fixed by commit 1710eb913bdc ("netfilter: nft_ct: skip expectations for confirmed conntrack") which restricted it to unconfirmed conntracks.
However, early insertion of expectations into the expectations list when the conntrack is unconfirmed leads to stale entries pointing to the wrong hlist_head through .pprev due to ct extension reallocation.
Commit 7c9664351980 ("netfilter: move nat hlist_head to nf_conn") moved the nat hlist_head to nf_conn for this reason:
1. ... 2. When reallocation of extension area occurs we need to fixup the bysource hash head via hlist_replace_rcu.
I'd rather not increase the size of the struct nf_conn for this feature has very limited scope: only one expectation can be created at a time given expect_clash() will make nf_ct_expect_related() reports EBUSY. For this reason, relax nf_ct_expect_related() not to drop packets in case expectation creation fails, therefore, expectation creation becomes best effort.
To address this issue, add an internal ct helper and attach it to the conntrack entry to streamline the custom ct expectation support with existing ct helpers.
Expose a new nf_conntrack_helper_release() function to release the internal helper that is allocated and attached to the conntrack entry to create the custom expectations. The nft_ct module removal always waits for rcu grace period, then the NULL helper callback is observed after this.
This patch also restricts the creation of expectations to different helpers other than this custom helper that is created for this type of expectations.
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Analysis
by VulDB Data Team • 09/17/2026
The vulnerability described involves a logic error in the Linux kernel's netfilter subsystem, specifically within the nft_ct module responsible for connection tracking and expectation management. The core issue stems from how custom connection track expectations are handled during the lifecycle of a network flow. Originally, the code attempted to add helper extensions using nf_ct_helper_ext_add() even for confirmed conntracks, which was an invalid operation that triggered kernel splats or crashes. While a previous fix restricted this behavior to unconfirmed conntracks, it introduced a new class of bugs related to memory management and pointer integrity during reallocation events.
The technical flaw arises from the interaction between expectation insertion timing and connection track extension reallocation. When expectations are inserted early into the expectations list while a conntrack entry is still in an unconfirmed state, they can become stale if the underlying nf_conn structure undergoes reallocation. This reallocation causes pointers within the data structures, specifically the .pprev field used for hash list traversal, to point to incorrect memory locations or hlist_heads. Although previous kernel commits moved certain hash heads like nat_hlist_head into the main nf_conn struct to mitigate similar issues by allowing proper fixup via hlist_replace_rcu during reallocation, applying this same structural change to custom expectation support would unnecessarily increase the size of the critical nf_conn structure. Given that only one expectation can typically be created at a time due to conflict detection mechanisms like expect_clash(), expanding the base struct is deemed inefficient and undesirable for performance reasons.
To resolve these stability issues without altering the fundamental memory layout of connection tracking structures, the fix implements an architectural shift by moving custom expectation support into a dedicated internal helper mechanism. This approach attaches the necessary state directly to the conntrack entry through an existing ct helper framework rather than relying on fragile direct list manipulations that are susceptible to reallocation side effects. The solution introduces a new function nf_conntrack_helper_release() which ensures proper cleanup of this internal helper when it is no longer needed. By leveraging the rcu grace period during module removal, the system guarantees that any references to the NULLed helper callback are safely observed before memory is freed, preventing use-after-free scenarios and ensuring thread-safe access patterns across different CPU cores handling concurrent network packets.
From a security perspective, this vulnerability falls under CWE-120 Buffer Copy without Checking Size of Input in Classic C contexts if it leads to buffer overflows via malformed expectations, or more accurately CWE-675 Duplicate Operations on Resource which can lead to resource exhaustion or inconsistent state. In the MITRE ATT&CK framework, such kernel-level logic errors that allow for denial of service through system crashes are categorized under T1499 Endpoint Denial of Service. The improper handling of memory pointers and race conditions during reallocation represents a significant risk to system stability, as an attacker could potentially trigger these code paths by crafting specific network traffic patterns designed to force rapid conntrack creation and expectation insertion cycles.
The operational impact of this vulnerability includes kernel panics or splats that can result in service disruption for the host machine running the Linux kernel with affected netfilter configurations. Systems relying on complex NAT rules, connection tracking expectations for protocols like FTP or SIP, or those using nftables for advanced packet filtering are particularly susceptible to these stability issues under high load or specific traffic patterns that trigger frequent conntrack state transitions and reallocations. The fix ensures that expectation creation becomes a best-effort operation where failures do not drop packets unnecessarily but also prevents the system from entering an inconsistent state due to stale pointers.
Mitigation strategies primarily involve applying the kernel patch associated with commit 7c9664351980 or later updates that incorporate this logic change. Administrators should ensure their systems are updated to versions of the Linux kernel where nft_ct properly utilizes internal helpers for expectation management rather than direct manipulation of unconfirmed conntrack lists. Additionally, monitoring system logs for netfilter-related splats can help identify instances where older vulnerable code paths might still be active in custom or out-of-tree modules that have not yet been updated to follow this new helper-based paradigm.