CVE-2024-36905 is a vulnerability identified in the Linux kernel's TCP networking stack, specifically related to the handling of TCP sockets in the TCP_SYN_RECV state. This state is a transitional phase during the TCP three-way handshake, primarily used for cross-SYN connections, often encountered in fuzz testing scenarios. The vulnerability arises from an improper state transition triggered during a socket shutdown operation. Normally, a socket transitions through states such as TCP_CLOSE, TCP_SYN_SENT, TCP_SYN_RECV, and TCP_FIN_WAIT1 during connection establishment and teardown. However, in this case, the kernel incorrectly allows a transition from TCP_SYN_RECV directly to TCP_FIN_WAIT1 upon a shutdown call, bypassing necessary initialization functions like tcp_init_transfer() and tcp_init_buffer_space(). This leads to uninitialized buffer space and ultimately causes a divide-by-zero error in the tcp_rcv_space_adjust() function, resulting in a kernel crash. The root cause is the tcp_shutdown() function performing an invalid state transition. The fix involves deferring the shutdown until the socket reaches the TCP_ESTABLISHED state, ensuring that tcp_send_fin() is called from a valid context with appropriate memory allocation flags (GFP_ATOMIC) to avoid race conditions or deadlocks. The vulnerability was discovered via syzbot fuzzing and affects Linux kernel version 6.9.0-rc6 and possibly other recent versions. No known exploits are currently reported in the wild. The issue is technical and low-level, impacting kernel stability and potentially leading to denial of service (DoS) through kernel crashes triggered by crafted TCP connection sequences.

For European organizations, this vulnerability poses a risk primarily to systems running vulnerable Linux kernel versions, including servers, network appliances, and embedded devices. The impact is mainly a denial of service condition caused by kernel crashes, which can disrupt critical services, degrade network availability, and potentially cause downtime. Organizations relying on Linux-based infrastructure for web hosting, cloud services, or internal networking could experience service interruptions if exploited. Although the vulnerability does not appear to allow privilege escalation or remote code execution directly, the induced kernel panic could be leveraged in targeted denial of service attacks, especially in environments with high network traffic or exposure to untrusted networks. This could affect sectors such as finance, telecommunications, government, and critical infrastructure in Europe, where Linux servers are widely deployed. The lack of known exploits reduces immediate risk, but the presence of a kernel-level bug means that attackers with network access could craft packets or connection sequences to trigger the crash. The vulnerability's exploitation does not require user interaction but does require the ability to initiate TCP connections to the target system, which may be mitigated by network controls.

European organizations should prioritize updating their Linux kernel to the latest patched versions that include the fix for CVE-2024-36905. Kernel updates should be applied promptly, especially on internet-facing servers and critical infrastructure. Network administrators should implement strict ingress filtering and firewall rules to limit unsolicited TCP connection attempts, particularly from untrusted sources, reducing the attack surface. Monitoring network traffic for unusual SYN flood patterns or abnormal TCP handshake sequences can help detect attempts to exploit this vulnerability. For environments where immediate patching is not feasible, consider isolating vulnerable systems behind additional network segmentation or proxy layers that can filter or block suspicious TCP connection attempts. Additionally, organizations should review and harden kernel parameters related to TCP connection handling and shutdown behavior if configurable. Regularly auditing kernel versions and maintaining an up-to-date inventory of Linux systems will facilitate timely remediation. Finally, engage with Linux distribution vendors and subscribe to security advisories to stay informed about patches and related vulnerabilities.