CVE-2025-37890 is a use-after-free (UAF) vulnerability identified in the Linux kernel's network scheduler subsystem, specifically involving the Hierarchical Fair Service Curve (hfsc) queuing discipline (qdisc) when used in conjunction with the netem child qdisc. The vulnerability arises because the hfsc class incorrectly assumes that checking if cl->qdisc->q.qlen == 0 guarantees the class has not been inserted into the vttree or eltree data structures. However, this assumption fails in the case of netem duplicates, leading to a scenario where the class can be inserted twice. This double insertion can cause a use-after-free condition when the kernel attempts to access or manipulate freed memory. The patch addressing this vulnerability adds a check on the n_active class variable to prevent multiple insertions of the same class into the scheduling trees, thereby mitigating the reentrancy issue that leads to the UAF. This vulnerability affects Linux kernel versions identified by the commit hash 37d9cf1a3ce35de3df6f7d209bfb1f50cf188cea and potentially other versions containing the same flawed logic. Exploitation of this vulnerability could allow an attacker to execute arbitrary code in kernel space, cause kernel crashes, or escalate privileges by manipulating kernel memory. However, as of the published date, no known exploits are reported in the wild. The vulnerability was responsibly disclosed and patched promptly, with detailed technical discussion available in the linked kernel mailing list post.

For European organizations, this vulnerability poses a significant risk particularly to those relying on Linux-based infrastructure for networking, servers, and embedded systems. The Linux kernel is widely used across European enterprises, government agencies, and critical infrastructure providers. Exploitation could lead to kernel-level code execution, allowing attackers to bypass security controls, gain elevated privileges, or cause denial of service through system crashes. This could impact confidentiality, integrity, and availability of sensitive data and services. Organizations operating network appliances, routers, or virtualized environments using Linux with hfsc and netem qdiscs are especially vulnerable. Given the kernel-level nature of the flaw, successful exploitation could undermine trust in critical systems and disrupt business operations. The absence of known exploits currently reduces immediate risk, but the complexity of the vulnerability and potential for future weaponization necessitate urgent attention. Additionally, the vulnerability could be leveraged in targeted attacks against high-value European targets, including telecom providers, financial institutions, and government networks that heavily depend on Linux networking stacks.

European organizations should prioritize updating their Linux kernel to the patched version that addresses CVE-2025-37890 as soon as possible. Kernel upgrades should be tested in controlled environments to ensure compatibility and stability before deployment. Network administrators should audit configurations to identify usage of hfsc and netem qdiscs and consider temporarily disabling or replacing these with alternative queuing disciplines if patching is delayed. Employing kernel hardening techniques such as Kernel Address Space Layout Randomization (KASLR), Kernel Page Table Isolation (KPTI), and enabling security modules like SELinux or AppArmor can reduce exploitation risk. Monitoring kernel logs and network scheduler behavior for anomalies may help detect exploitation attempts. Organizations should also review and tighten access controls to limit unprivileged user ability to manipulate network qdiscs. Finally, maintaining an up-to-date inventory of Linux kernel versions in use across all systems will facilitate rapid response to this and future vulnerabilities.