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Linux Kernel's KSMBD Component Exposes Servers to Resource Exhaustion by Remote Assailants

Linux kernel's KSMBD (SMB Direct) subsystem vulnerability sparks concern within the open-source community due to potential denial-of-service issues.

, and Administrator

2025 September 18 . 2:54 AM

2 min read

Remote attackers can exploit a weakness in the KSMBD subsystem of the Linux kernel, resulting in... — Remote attackers can exploit a weakness in the KSMBD subsystem of the Linux kernel, resulting in server resource exhaustion.

Linux Kernel's KSMBD Component Exposes Servers to Resource Exhaustion by Remote Assailants

In a significant development for the cybersecurity community, a denial-of-service vulnerability (CVE-2025-38501) has been identified in the Linux kernel's SMB Direct (KSMBD) subsystem. This flaw could potentially render SMB shares inaccessible and halt file transfers and authentication services.

The vulnerability was introduced in Linux kernel version 5.3 and has since been addressed in commit e6bb9193974059ddbb0ce7763fa3882bd60d4dc3. This commit adds a configurable backlog limit and enforces a shorter threshold for half-open sockets, mitigating the risk posed by the denial-of-service attack.

As of September 17, 2025, distributions with updated kernels incorporating these fixes have released updated packages. However, specific distributions are not named explicitly in the available sources. Users are advised to apply the fix by upgrading to Linux 6.1.15 or later.

Security teams are urged to monitor for an abnormal number of SYN packets, a telltale sign of an attack exploiting this vulnerability. Additionally, adjusting KSMBD's user-space settings to lower and limit backlog counts can further enhance protection against such attacks.

In environments where an immediate kernel upgrade is impractical, network-level rate limiting on TCP port 445 and stricter firewall rules can help mitigate exploitation.

A public proof-of-concept exploit, KSMBDrain, demonstrates how attackers can overwhelm a KSMBD server. This Python-based exploit initiates thousands of TCP three-way handshakes and fails to complete the session, causing the server to hold sockets indefinitely. This allows a remote, unauthenticated adversary to exhaust all available SMB connections.

It is crucial to defend against resource exhaustion attacks that leverage protocol-level quirks, such as CVE-2025-38501. Continuous monitoring and maintaining up-to-date kernel versions will significantly reduce the risk posed by this vulnerability.

SMB services remain a critical component for file sharing and authentication in enterprise networks. Ensuring their security is paramount in today's digital landscape. Stay vigilant and keep your systems updated.