CVE-2026-98441 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
net/sched: act_ct: avoid modifying shared unconfirmed ct entry
In a case where skb with an unconfirmed ct entry gets cloned, we may end up processing both again but with different sets of extensions.
The series of events:
1. The first clone wants to commit and runs the helpers wiring up the extension pointer into the expectation list. 2. Then it looses the confirmation keeping the entry unconfirmed. 3. Second clone now wants to commit labels or run NAT and adds the new extension for that breaking the pointer in the expectation list causing UAF on the destruction path later.
While this is possible to trigger, there should be no practical network pipeline where we need to process both clones without modifications in the same zone. So, let's just reset the entry in case for some reason we got an skb with a shared one. This doesn't affect any known use cases, but avoids any potential problems with sharing and modification of the unconfirmed ct entry.
Unlike openvswitch module, act_ct allows for NAT without commit. Changing that would be a uAPI break. So, act_ct needs to reset on NAT regardless of the commit flag to avoid reallocation of the extension space. This, however, doesn't really change the picture for sensible networking cases as there should be no need to run the same packet twice (before and after the clone) through conntrack without packet header or zone changes and without commit.
The fixes tag points to the introduction of helpers, since that's the main UAF trigger for the sharing.
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Analysis
by VulDB Data Team • 10/18/2026
A critical use-after-free vulnerability has been identified within the Linux kernel networking subsystem, specifically affecting the net/sched: act_ct component which handles connection tracking actions in traffic control policies. This flaw arises from improper handling of shared unconfirmed connection track entries when network packets are cloned during processing. The core technical issue stems from a race condition involving memory management and pointer integrity within the conntrack extension framework. When an skb containing an unconfirmed connection track entry is duplicated, both the original packet buffer and its clone may attempt to process or modify that same shared state concurrently. This scenario creates a dangerous situation where one instance of the processing logic modifies the internal structures while another retains references to those now-invalidated memory regions.
The specific sequence leading to exploitation begins when an initial cloned packet attempts to commit connection tracking information, which involves wiring up extension pointers into the expectation list for protocol helpers. If this first clone subsequently loses its confirmation status without properly cleaning up or detaching from the shared entry, it leaves behind stale references. Meanwhile, a second clone of the same original packet proceeds with different processing requirements, such as applying network address translation or setting connection labels. To accommodate these new operations, the system allocates and attaches new extensions to the shared unconfirmed entry. This reallocation effectively invalidates the pointer previously stored in the expectation list by the first clone because the underlying memory structure has been modified or moved. Consequently, when the destruction path for the original state is executed later, it attempts to access these dangling pointers, resulting in a use-after-free condition that can lead to kernel panic, information disclosure, or potentially arbitrary code execution depending on heap layout and exploitation techniques.
From an operational impact perspective, this vulnerability allows for potential denial of service through system crashes if triggered repeatedly under specific traffic conditions. While the likelihood of triggering this flaw is considered low because typical network pipelines do not require processing identical clones without modifications in the same zone simultaneously, the existence of such a path represents a significant security risk. The act_ct module differs from openvswitch by allowing NAT operations to occur without an explicit commit step, which necessitates careful management of extension space reallocation. Without this fix, any scenario involving packet cloning followed by divergent processing paths that involve connection tracking modifications could destabilize the kernel network stack. This is particularly relevant in complex traffic shaping or firewalling environments where packets might be mirrored, cloned for analysis, and then subjected to different policy rules.
Mitigation strategies primarily focus on applying the upstream Linux kernel patch that addresses this logic error by resetting the shared entry when an skb with a shared unconfirmed connection track context is encountered. This reset ensures that subsequent modifications do not corrupt existing pointers in expectation lists. Administrators should ensure their systems are updated to include kernels where this fix has been integrated, as it resolves the root cause of the pointer invalidation during extension reallocation. Additionally, network architects can review traffic control configurations to minimize unnecessary packet cloning operations within zones where connection tracking is active, thereby reducing the attack surface for potential exploitation attempts that rely on concurrent modification of shared state.
This vulnerability aligns with CWE-416, Use After Free, as it involves accessing memory after it has been freed or invalidated due to improper pointer management during dynamic allocation changes. In terms of adversary behavior and detection frameworks, this flaw relates to ATT&CK technique T1059, Command and Scripting Interpreter, if exploited for code execution, but more broadly falls under system resource manipulation aspects found in lateral movement or persistence phases where kernel stability is compromised. The fix specifically targets the introduction point of helpers that trigger the sharing mechanism, ensuring that the integrity of connection tracking state is maintained even when packets are duplicated and processed with varying parameters such as NAT rules or label assignments without a formal commit operation.