CVE-2026-23270 in Linux
Summary
by MITRE • 03/18/2026
In the Linux kernel, the following vulnerability has been resolved:
net/sched: Only allow act_ct to bind to clsact/ingress qdiscs and shared blocks
As Paolo said earlier [1]:
"Since the blamed commit below, classify can return TC_ACT_CONSUMED while the current skb being held by the defragmentation engine. As reported by GangMin Kim, if such packet is that may cause a UaF when the defrag engine later on tries to tuch again such packet."
act_ct was never meant to be used in the egress path, however some users are attaching it to egress today [2]. Attempting to reach a middle
ground, we noticed that, while most qdiscs are not handling TC_ACT_CONSUMED, clsact/ingress qdiscs are. With that in mind, we address the issue by only allowing act_ct to bind to clsact/ingress qdiscs and shared blocks. That way it's still possible to attach act_ct to egress (albeit only with clsact).
[1] https://lore.kernel.org/netdev/674b8cbfc385c6f37fb29a1de08d8fe5c2b0fbee.1771321118.git.pabeni@redhat.com/
[2] https://lore.kernel.org/netdev/[email protected]/
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Analysis
by VulDB Data Team • 05/23/2026
The vulnerability described in CVE-2026-23270 represents a critical issue within the Linux kernel's networking subsystem, specifically affecting the traffic control framework's classification and action handling mechanisms. This flaw manifests in the net/sched component where the act_ct (connection tracking) action is improperly allowed to operate in contexts where it can cause memory safety violations. The vulnerability stems from the fact that act_ct was never intended to function in the egress path of network packet processing, yet certain users were attaching it to egress qdiscs, creating a dangerous operational scenario.
The technical root cause of this vulnerability lies in the interaction between the connection tracking action and the packet defragmentation engine within the kernel's networking stack. When packets are processed through the classification system, the act_ct action can return TC_ACT_CONSUMED status codes while the packet is still being held by the defragmentation engine. This creates a race condition where the defragmentation engine attempts to access packet data that has already been marked for consumption, leading to potential use-after-free conditions. The vulnerability specifically exploits the fact that while most qdisc implementations do not properly handle TC_ACT_CONSUMED return values, clsact and ingress qdiscs are designed to manage these conditions correctly, making them safe contexts for act_ct operations.
The operational impact of this vulnerability extends beyond simple memory corruption, as it represents a potential vector for privilege escalation and system instability. Attackers could potentially leverage this flaw to cause kernel crashes, memory corruption, or even execute arbitrary code within the kernel context. The vulnerability affects systems running Linux kernels where the traffic control subsystem is actively used, particularly those implementing complex network filtering and connection tracking policies. Given that connection tracking is fundamental to many network security implementations, the potential for widespread impact is significant, especially in environments where network traffic is heavily filtered and monitored.
The resolution implemented for CVE-2026-23270 follows a careful balancing approach that maintains backward compatibility while preventing the dangerous usage patterns that lead to the vulnerability. The fix restricts act_ct operations to only bind to clsact and ingress qdiscs, along with shared blocks, effectively preventing its use in egress paths where the memory safety issues occur. This approach aligns with the principle of least privilege and follows established security patterns for kernel subsystem design. The solution addresses the underlying issue identified in the Linux kernel's traffic control framework while maintaining the functionality that legitimate users require. This fix demonstrates the importance of proper qdisc isolation and the careful consideration of action contexts within kernel networking subsystems, following principles that align with CWE-476 (NULL Pointer Dereference) and ATT&CK techniques related to kernel exploitation and privilege escalation through memory corruption vulnerabilities. The patch ensures that connection tracking actions operate only in contexts where the kernel's packet handling mechanisms can safely manage the TC_ACT_CONSUMED return values without risking memory safety violations.