CVE-2025-21896 is a use-after-free (UAF) vulnerability in the Linux kernel's FUSE (Filesystem in Userspace) implementation, specifically related to the readahead mechanism. The vulnerability originated from a code change in commit 3eab9d7bc2f4, which converted the readahead logic to use folios, a newer memory management abstraction. This change dropped the explicit reference count on folios once they were locked, relying instead on an inferred reference. Previously, the code held a reference on the folio for the entire duration of the readpages call, ensuring the folio was not freed prematurely. The issue arises in the splice pipe response path, where the old folio is removed and replaced with a new folio (via fuse_try_move_page()). The code incorrectly assumes that a reference is still held on the folio in ap->folios, but since the reference was dropped earlier, this leads to a use-after-free condition. This can cause memory corruption, kernel crashes, or potentially allow an attacker to execute arbitrary code in kernel space. The fix involved reverting to the older __readahead_folio() method, which maintains the reference on the folio for the duration of the readpages call until it is explicitly dropped, thereby preventing the UAF. This vulnerability affects Linux kernel versions containing the problematic commit and is relevant to systems using FUSE filesystems. No known exploits are currently reported in the wild, and no CVSS score has been assigned yet.

For European organizations, this vulnerability poses a significant risk especially for those relying on Linux-based systems with FUSE filesystems, which are common in enterprise environments for mounting user-space filesystems, network filesystems, or virtual filesystems. Exploitation could lead to kernel crashes causing denial of service, or in worst cases, privilege escalation allowing attackers to gain root-level access. This could compromise confidentiality, integrity, and availability of critical systems. Given the widespread use of Linux servers in European data centers, cloud providers, and critical infrastructure, the impact could be severe if exploited. Organizations running containerized workloads or cloud-native applications on Linux hosts using FUSE are particularly at risk. The absence of known exploits suggests a window of opportunity to patch before active exploitation occurs, but the complexity of kernel vulnerabilities means exploitation could be targeted and stealthy.

European organizations should prioritize applying the patch that reverts the readahead logic to __readahead_folio(), ensuring the reference counting on folios is correctly maintained. Since the vulnerability is in the Linux kernel, updating to the latest stable kernel version that includes this fix is critical. For environments where immediate kernel upgrades are challenging, consider isolating or limiting the use of FUSE filesystems, especially untrusted or external mounts. Employ kernel hardening techniques such as Kernel Address Space Layout Randomization (KASLR), Kernel Page Table Isolation (KPTI), and enable security modules like SELinux or AppArmor to reduce exploitation risk. Monitoring kernel logs for unusual crashes or anomalies related to FUSE operations can provide early detection. Additionally, review and restrict user permissions to mount or interact with FUSE filesystems to trusted users only. Coordination with Linux distribution vendors for timely patch deployment is essential.