CVE-2025-21691 is a vulnerability identified in the Linux kernel related to the 'cachestat()' system call, which was introduced as a more efficient alternative to the 'mincore()' system call. The 'cachestat()' syscall provides page cache statistics without requiring mapping pages into the user virtual address space, improving performance and convenience. However, the initial implementation missed a critical permission check that was previously added to 'mincore()' to verify writability or ownership of the memory region. Specifically, 'mincore()' was updated to be more conservative by ensuring that the caller has the appropriate permissions to query memory mappings, preventing unauthorized information disclosure or potential privilege escalation. The vulnerability arises because 'cachestat()' lacked this permission validation, potentially allowing unprivileged users to obtain page cache statistics for files they do not own or have write access to. This could lead to unauthorized information disclosure about the system's memory usage patterns or file caching behavior. The fix involves adding equivalent permission checks to 'cachestat()', adapted for file context rather than virtual memory areas (vma), thereby enforcing proper access control. The vulnerability affects Linux kernel versions starting from the commit where 'cachestat()' was introduced (commit cf264e1329fb0307e044f7675849f9f38b44c11a). There are no known exploits in the wild at the time of publication, and no CVSS score has been assigned yet. The issue was publicly disclosed on February 10, 2025.

For European organizations, this vulnerability could have several implications. Since Linux is widely used across servers, cloud infrastructure, and embedded devices in Europe, any flaw in the kernel's permission checks can potentially be leveraged by local or remote attackers with user-level access to gain unauthorized insights into system memory usage and file caching. While this vulnerability does not directly allow code execution or privilege escalation, the unauthorized information disclosure could aid attackers in crafting more targeted attacks, such as side-channel attacks or data leakage. Organizations handling sensitive data or operating critical infrastructure could be at risk if attackers use this information to map system behavior or infer confidential operational details. Additionally, multi-tenant environments like cloud service providers or hosting companies in Europe could see increased risk if unprivileged tenants exploit this to glean information about other tenants' workloads. The absence of known exploits reduces immediate risk, but the vulnerability's presence in the widely deployed Linux kernel means that timely patching is essential to prevent future exploitation.

European organizations should prioritize updating their Linux kernel to the patched versions that include the permission check fix for 'cachestat()'. Since the vulnerability stems from a missing permission validation, applying the official kernel patches or upgrading to the latest stable kernel release is the most effective mitigation. System administrators should audit their environments to identify systems running vulnerable kernel versions, especially those exposed to untrusted users or multi-tenant scenarios. Additionally, organizations should implement strict access controls to limit unprivileged user access on critical systems and monitor for unusual system call usage patterns that could indicate attempts to exploit this vulnerability. Employing kernel security modules (e.g., SELinux, AppArmor) to enforce mandatory access controls can further reduce the risk. Finally, organizations should stay informed about any emerging exploit techniques related to this vulnerability and apply security advisories promptly.