CVE-2025-21635 is a vulnerability identified in the Linux kernel specifically related to the Reliable Datagram Sockets (RDS) subsystem's sysctl interface for TCP receive and send buffer sizes (rds_tcp_rcvbuf and rds_tcp_sndbuf). The root cause of the vulnerability lies in the improper use of the 'current->nsproxy' pointer, which references the network namespace proxy of the currently executing task. Using 'current->nsproxy' in this context is problematic for multiple reasons: it can lead to inconsistencies by mixing network namespace contexts between the reader/writer and the opener of a socket, and it can also result in null pointer dereferences (kernel 'Oops') when 'current->nsproxy' is NULL, such as when the current task is exiting. This issue was detected by syzbot, an automated kernel fuzzer, during the execution of the acct(2) system call. The vulnerability is addressed by modifying the code to avoid relying on 'current->nsproxy' and instead obtaining the per-network namespace structure from the table's data using container_of(), and retrieving the 'net' structure from the listen socket if available. This approach ensures consistent and safe access to network namespace data, preventing null pointer dereferences and potential kernel crashes. The vulnerability affects certain Linux kernel versions identified by specific commit hashes and was published on January 19, 2025. No known exploits are reported in the wild at this time, and no CVSS score has been assigned yet.

For European organizations, this vulnerability primarily poses a risk of kernel instability and potential denial of service due to kernel crashes triggered by null pointer dereferences in the RDS sysctl interface. Systems running affected Linux kernel versions with RDS enabled and exposed to untrusted or malicious users or processes manipulating the sysctl parameters could experience unexpected reboots or service interruptions. This can impact critical infrastructure, cloud services, and enterprise environments relying on Linux servers, especially those using RDS for high-performance networking. Although no direct remote code execution or privilege escalation is indicated, the denial of service impact can disrupt business operations, affect availability of services, and potentially cause cascading failures in networked environments. Given the widespread use of Linux in European data centers, cloud providers, and government systems, the vulnerability could affect a broad range of sectors including finance, telecommunications, healthcare, and public administration. The lack of known exploits reduces immediate risk, but the presence of a kernel-level null pointer dereference vulnerability warrants prompt attention to prevent exploitation attempts or accidental crashes.

European organizations should prioritize updating their Linux kernels to versions that include the patch addressing CVE-2025-21635. Since the vulnerability involves kernel-level code, applying vendor-provided kernel updates or recompiling kernels with the fix is essential. Organizations should audit their use of RDS and sysctl interfaces to determine exposure and consider disabling RDS if it is not required, reducing the attack surface. Monitoring kernel logs for Oops or null pointer dereference messages related to RDS sysctl parameters can help detect attempted exploitation or instability. Implementing strict access controls on sysctl interfaces, limiting which users or processes can modify RDS buffer sizes, will reduce risk. For environments using containerization or network namespaces extensively, verifying that network namespace handling follows best practices and is consistent can prevent similar issues. Finally, organizations should maintain robust backup and recovery procedures to mitigate potential service disruptions caused by kernel crashes.