Linux Kernel KSMBD Flaw Lets Remote Attackers Drain Server Resources

A critical vulnerability in the Linux kernel’s KSMBD implementation has been discovered that allows remote attackers to completely exhaust server connection resources through a simple denial-of-service attack.

The flaw, tracked as CVE-2025-38501 and dubbed “KSMBDrain,” enables malicious actors to render SMB services unavailable by consuming all available connections.

How the Attack Works

The vulnerability exploits KSMBD’s connection handling mechanism through a straightforward attack vector, as per a report by Github.

Remote attackers can establish TCP connections to the KSMBD server by completing the initial three-way handshake, but then deliberately stop responding to subsequent packets.

The server continues holding these abandoned connections indefinitely by default, allowing attackers to systematically consume all available connection slots.

This attack method is particularly dangerous because it requires minimal resources from the attacker while causing maximum disruption to the target server.

Even when administrators configure connection timeouts in the user-space configuration file, the minimum timeout setting of one minute still provides attackers with sufficient time to launch repeated connection exhaustion attempts from a single IP address.

The vulnerability affects all Linux kernel versions from 5.3 onward, marking the point when KSMBD was merged into the mainline kernel.

This encompasses virtually all modern Linux distributions running SMB services through the in-kernel KSMBD implementation, making the potential impact extremely broad across enterprise and personal computing environments.

Organizations relying on Linux-based file servers, network-attached storage devices, and any system providing SMB/CIFS services through KSMBD face immediate risk.

The attack can be executed remotely without authentication, making it accessible to both sophisticated threat actors and opportunistic attackers scanning for vulnerable systems.

Linux kernel developers have addressed the issue in commit e6bb9193974059ddbb0ce7763fa3882bd60d4dc3, implementing proper connection management to prevent resource exhaustion.

System administrators should prioritize updating to patched kernel versions and review their SMB service configurations to implement appropriate connection limits and timeouts as additional protective measures.

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