CVE-2025-21873 is a vulnerability identified in the Linux kernel specifically within the SCSI UFS (Universal Flash Storage) core's block storage generic (bsg) interface. The flaw arises when the kernel attempts to handle an ARPMB (Authenticated Replay Protected Memory Block) command that fails on devices that do not support this command. The vulnerability manifests as a crash caused by improper handling of user data copying in the function bsg_transport_sg_io_fn(). More precisely, when the function ufs_bsg_exec_advanced_rpmb_req() returns an error, the kernel erroneously sets the job's reply length, leading to a memory crash. The crash backtrace indicates that invalid operations and usercopy aborts occur, which are symptomatic of kernel memory corruption or invalid memory access. This vulnerability can cause a denial of service (DoS) by crashing the kernel, potentially leading to system instability or reboot. The issue affects Linux kernel versions identified by the commit hash 6ff265fc5ef660499e0edc4641647e99eed3f519 and was published on March 27, 2025. There are no known exploits in the wild at this time, and no CVSS score has been assigned yet. The vulnerability is rooted in the kernel's failure to correctly handle error conditions from UFS devices that do not support the ARPMB command, resulting in unsafe memory operations and kernel crashes.

For European organizations, this vulnerability poses a risk primarily to systems running affected Linux kernel versions with UFS storage devices, which are common in embedded systems, mobile devices, and some server environments. The impact is mainly a denial of service condition caused by kernel crashes, which can disrupt critical services, cause data loss, or require system reboots. Organizations relying on Linux-based infrastructure for critical applications, especially those using UFS storage, may experience operational downtime. Although no remote code execution or privilege escalation is indicated, the instability introduced can be exploited in targeted attacks to degrade service availability. This is particularly relevant for sectors such as telecommunications, manufacturing, and IoT deployments prevalent in Europe, where embedded Linux systems are widespread. The lack of known exploits reduces immediate risk, but the vulnerability's presence in the kernel means that any unpatched system remains susceptible to crashes triggered by malformed or unsupported ARPMB commands.

To mitigate this vulnerability, European organizations should prioritize updating their Linux kernel to the latest patched version that addresses CVE-2025-21873. Since the issue arises from improper error handling in the UFS bsg interface, applying vendor-supplied kernel patches or upgrading to a kernel version that includes the fix is essential. Organizations should audit their systems to identify devices using UFS storage and verify kernel versions. For embedded or specialized devices where kernel upgrades are challenging, consider disabling or restricting access to the bsg interface or UFS devices if feasible. Additionally, implementing kernel crash monitoring and automated recovery mechanisms can reduce downtime impact. Network segmentation and strict access controls can limit exposure by preventing untrusted users or processes from issuing ARPMB commands. Finally, maintain vigilance for any emerging exploit reports and coordinate with Linux kernel maintainers and vendors for timely updates.