CVE-2025-21667 is a vulnerability identified in the Linux kernel affecting 32-bit kernel versions, specifically related to the iomap subsystem used for managing file I/O operations. The flaw arises in the function iomap_write_delalloc_scan(), which is responsible for scanning and managing delayed allocation writes on filesystems, particularly the XFS filesystem. The root cause is that the function uses a 32-bit position value due to the folio_next_index() function returning an unsigned long that is truncated improperly on 32-bit architectures. This truncation leads to an infinite loop condition when writing to an XFS filesystem, as the offset handling does not correctly process 64-bit file offsets. The infinite loop can cause the kernel process handling the write operation to hang indefinitely, potentially leading to denial of service (DoS) conditions. This vulnerability is specific to 32-bit Linux kernels and affects systems using the XFS filesystem. Although no known exploits are currently reported in the wild, the nature of the bug suggests that an attacker with write access to an XFS filesystem could trigger the infinite loop by performing specific write operations, causing resource exhaustion or system instability. The issue has been resolved in recent Linux kernel updates by correcting the offset handling to avoid truncation and infinite looping.

For European organizations, the primary impact of CVE-2025-21667 is the risk of denial of service on systems running 32-bit Linux kernels with XFS filesystems. While 64-bit systems are more prevalent, some legacy or embedded systems in industrial, governmental, or critical infrastructure environments may still operate 32-bit kernels. An attacker with write access could exploit this vulnerability to cause system hangs or crashes, disrupting services and potentially leading to operational downtime. This could affect sectors such as manufacturing, telecommunications, and public administration where Linux-based systems are common. The infinite loop could also be leveraged as part of a larger attack chain to degrade system reliability or availability. However, since exploitation requires write access and specific filesystem conditions, the threat is somewhat limited to environments where such access is possible. The absence of known exploits reduces immediate risk but does not eliminate the need for remediation, especially in sensitive or high-availability environments.

To mitigate CVE-2025-21667, European organizations should: 1) Identify and inventory all systems running 32-bit Linux kernels, particularly those using the XFS filesystem. 2) Apply the latest Linux kernel patches that address this vulnerability as soon as they become available from trusted Linux distributions or kernel maintainers. 3) For systems where patching is not immediately feasible, consider migrating critical workloads to 64-bit kernels or alternative filesystems less affected by this issue. 4) Restrict write access to XFS filesystems to trusted users and processes to minimize the risk of exploitation. 5) Implement monitoring for unusual system hangs or kernel process behavior that could indicate attempts to trigger the infinite loop. 6) In environments with legacy or embedded devices, coordinate with vendors to ensure firmware or kernel updates are provided and applied. 7) Conduct regular security audits to detect unauthorized write operations or anomalous filesystem activity.