CVE-2026-99213 in Linux
Summary
by MITRE • 10/28/2026
In the Linux kernel, the following vulnerability has been resolved:
vsock: treat TCP_CLOSING as once-established
Handle the TCP_ESTABLISHED -> TCP_CLOSING transition on OP_RST, which can race with the connect loop. Immediately break and return 0 on ESTABLISHED/CLOSING. The return value of connect() should reflect what happened up to the point the connection was established or failed. Events that occur afterwards must not affect it. E.g. even if OP_RW has already set sk_err, connect() should still return 0, not ENOBUFS because of it. Adapt the inaccurate comment above signal_pending().
Resetting a socket that is still present in connected_table can lead to memory corruption. The reporter noted lost transports for in-flight skbs, and I have reproduced crashes caused by re-insertion into connected_table.
list_add double add: new=, prev=, next=. kernel BUG at lib/list_debug.c:35! Oops: invalid opcode: 0000 [#1] SMP KASAN NOPTI
Workqueue: vsock-loopback vsock_loopback_work RIP: 0010:__list_add_valid_or_report+0x11f/0x130 Call Trace: vsock_insert_connected.cold+0xe/0x13 virtio_transport_recv_pkt+0x10e9/0x1460 vsock_loopback_work+0x305/0x480 process_one_work+0xe4c/0x1560 worker_thread+0x4f1/0xd60 kthread+0x36e/0x470 ret_from_fork+0x47b/0x6b0 ret_from_fork_asm+0x1a/0x30
This fix is supplementary to commit 002541ef650b ("vsock: Ignore signal/timeout on connect() if already established"). Details under Link.
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Analysis
by VulDB Data Team • 10/28/2026
The Linux kernel vsock subsystem contains a race condition vulnerability related to the handling of TCP connection state transitions, specifically during the shift from ESTABLISHED to CLOSING states triggered by an OP_RST packet. This flaw arises because the connect loop does not properly account for connections that have already been established or are in the process of closing when reset signals arrive. The core issue lies in the failure to immediately break and return success upon detecting these terminal connection states, leading to inconsistent socket management logic where subsequent events incorrectly influence the outcome of prior operations.
From a technical perspective, the vulnerability stems from an inaccurate implementation of state checking within the vsock connect loop. When a remote peer sends a reset (OP_RST), the local stack transitions the socket to TCP_CLOSING. However, if this transition occurs while the kernel is still processing or re-evaluating the connection status in connected_table, it can lead to memory corruption. Specifically, resetting a socket that remains present in the connected table causes double insertion errors into linked lists used for tracking active connections. This manifests as a list_add double add bug, resulting in invalid opcode exceptions and kernel panics due to corrupted data structures within lib/list_debug.c.
The operational impact of this vulnerability is severe, primarily affecting system stability rather than confidentiality or integrity directly. Attackers with the ability to send crafted reset packets can trigger these race conditions remotely if they have network access to the vsock interface. The immediate consequence is a kernel crash, leading to denial of service for any services relying on that host's virtual socket communications. Additionally, the reporter noted lost transports for in-flight skbs, indicating potential data loss or communication failures even before a full system crash occurs. This instability undermines the reliability of inter-VM and container-to-host communication channels provided by vsock.
This issue is categorized under CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition'). The vulnerability allows an attacker to exploit timing differences in state transitions, which aligns with MITRE ATT&CK technique T1499: Endpoint Denial of Service. By forcing the kernel into a corrupted state through rapid or timed reset packets, an adversary can disrupt service availability on vulnerable Linux systems running virtualized environments that utilize vsock for internal networking.
Mitigation strategies involve applying the upstream kernel patch that corrects the connect loop logic to properly handle ESTABLISHED and CLOSING states by returning zero immediately upon detection of these conditions. This ensures that the return value reflects only events up to the point of establishment or failure, ignoring subsequent state changes like OP_RST for the purpose of connection status reporting. Administrators should ensure their systems are updated with kernel versions containing this fix, which supplements earlier commits addressing signal and timeout handling in connect operations. Regular patching is essential as vsock is critical infrastructure for many cloud-native and virtualization deployments.