CVE-2025-21898 is a vulnerability identified in the Linux kernel's ftrace subsystem, specifically within the function_stat_show() function. The issue arises due to a potential division by zero error during the computation of the standard deviation (stddev). The vulnerability is triggered when the denominator expression x * (x - 1) * 1000 modulo 2^32 or 2^64 evaluates to zero, which would cause the division operation to fail. This flaw is related to the statistical calculations performed by ftrace, a kernel feature used for tracing and debugging kernel functions. The vulnerability does not currently address potential overflows in rec->counter * rec->counter because an overflow in rec->time * rec->time is expected to occur earlier, effectively limiting the impact of that specific overflow. The vulnerability affects multiple Linux kernel versions identified by specific commit hashes. Although no known exploits are currently reported in the wild, the flaw could potentially lead to kernel crashes or denial of service (DoS) conditions if triggered, as division by zero in kernel space typically results in a kernel panic or oops. The vulnerability does not appear to allow privilege escalation or arbitrary code execution directly but could be leveraged as part of a more complex attack chain. The Linux kernel maintainers have resolved this issue by adding a check to skip the stddev computation when the denominator expression evaluates to zero, preventing the division by zero error. No CVSS score has been assigned yet, and no patch links are provided in the data, but the vulnerability is published and reserved under CVE-2025-21898.

For European organizations, the impact of CVE-2025-21898 primarily concerns system stability and availability. Linux is widely deployed across European enterprises, government agencies, cloud providers, and critical infrastructure, often as the underlying OS for servers, network devices, and embedded systems. A triggered division by zero in the kernel could cause system crashes or reboots, leading to service interruptions and potential downtime. This may affect web services, databases, and critical applications running on Linux servers. Although this vulnerability does not directly compromise confidentiality or integrity, the resulting denial of service could disrupt business operations, cause data unavailability, and impact service-level agreements (SLAs). Organizations relying on Linux for critical infrastructure or high-availability environments could face operational risks if the vulnerability is exploited or triggered accidentally. The lack of known exploits reduces immediate risk, but the vulnerability should be addressed proactively to prevent potential exploitation or accidental triggering during normal operations or debugging activities.

European organizations should prioritize updating their Linux kernel to versions that include the fix for CVE-2025-21898 as soon as patches become available from their Linux distribution vendors. Until patches are applied, organizations should: 1) Limit access to systems running vulnerable kernel versions, especially restricting untrusted users or processes that could trigger ftrace operations. 2) Disable or restrict the use of ftrace or related kernel tracing features if they are not required for production environments, reducing the attack surface. 3) Monitor system logs and kernel messages for signs of kernel panics or unusual crashes that could indicate attempts to trigger this vulnerability. 4) Employ kernel hardening and security modules (e.g., SELinux, AppArmor) to limit the capabilities of processes that could interact with kernel tracing features. 5) Coordinate with Linux distribution vendors and subscribe to security advisories to receive timely updates and patches. 6) Test kernel updates in staging environments to ensure stability and compatibility before deployment in production. These steps go beyond generic advice by focusing on controlling access to the vulnerable subsystem and proactive monitoring to detect potential exploitation attempts.