CVE-2023-53121 is a vulnerability identified in the Linux kernel's TCP stack, specifically involving the functions tcp_make_synack() and tcp_rtx_synack(). The issue arises because tcp_rtx_synack() can be invoked in process context, which was not the original design assumption. When tcp_rtx_synack() calls tcp_make_synack(), the latter accesses per-CPU variables using __this_cpu_add() with preemption enabled. This is problematic because __this_cpu_add() is unsafe to use in preemptible code, leading to kernel BUGs and potential system instability or crashes. The root cause is the incorrect use of __TCP_INC_STATS(), which touches per-CPU variables in a context where preemption is enabled. The recommended fix is to replace __TCP_INC_STATS() with TCP_INC_STATS(), which is safe to be called from contexts where preemption can occur. This vulnerability does not appear to allow direct code execution or privilege escalation but can cause kernel panics or denial of service due to BUGs triggered by improper per-CPU variable access in process context. The vulnerability affects Linux kernel versions identified by the commit hash 8336886f786fdacbc19b719c1f7ea91eb70706d4 and was published on May 2, 2025. There are no known exploits in the wild at this time, and no CVSS score has been assigned yet.

For European organizations, this vulnerability primarily presents a risk of denial of service (DoS) through kernel panics or system crashes on Linux-based systems, which are widely used in servers, cloud infrastructure, and embedded devices. Systems handling critical network traffic, such as firewalls, routers, or servers running TCP/IP stacks, may experience instability or unexpected reboots, potentially disrupting business operations or services. Although it does not directly lead to privilege escalation or data breaches, the availability impact can be significant, especially for organizations relying on high uptime and network reliability. The vulnerability could be exploited by sending crafted TCP packets that trigger the problematic code path, causing the kernel BUG. This may affect network-facing systems and services, increasing the risk of targeted DoS attacks. Given the widespread use of Linux in European data centers, cloud providers, and enterprise environments, the impact could be broad if unpatched. However, the lack of known exploits and the technical complexity of triggering the bug may limit immediate risk.

European organizations should prioritize updating their Linux kernel to versions that include the fix for CVE-2023-53121, ensuring that tcp_make_synack() no longer uses __this_cpu_add() in preemptible contexts. Kernel updates should be applied promptly, especially on network-facing servers and infrastructure devices. In environments where immediate patching is not feasible, administrators should monitor kernel logs for BUG messages related to tcp_make_synack and tcp_rtx_synack to detect potential exploitation attempts or instability. Network-level mitigations could include filtering or rate-limiting suspicious TCP SYN-ACK retransmissions that might trigger the vulnerability. Additionally, organizations should review their incident response plans to handle potential DoS scenarios caused by kernel crashes. For embedded or specialized Linux devices, coordination with vendors to obtain patched firmware or kernel versions is critical. Finally, maintaining robust backup and recovery procedures will help mitigate operational impacts if systems become unstable due to this vulnerability.