Analysis Summary

A newly disclosed denial-of-service vulnerability in the Linux kernel’s KSMBD (SMB Direct) subsystem, tracked as CVE-2025-38501, has raised concerns across the open-source community. The flaw, introduced in kernel version 5.3, allows a remote, unauthenticated attacker to exhaust all available SMB connections by exploiting how KSMBD handles half-open TCP sessions. By sending repeated SYN packets without completing the three-way handshake, attackers can force the server to retain incomplete connections indefinitely, preventing legitimate SMB traffic from being processed.

A publicly available proof-of-concept exploit, dubbed KSMBDrain, demonstrates the attack’s simplicity. Written in Python, the script uses raw sockets to spawn mass handshake attempts, quickly overwhelming the KSMBD server’s connection pool. Since the subsystem does not enforce an upper limit on pending SYN–ACK sockets, even a single attacker with basic network connectivity to port 445 can exhaust server resources, halting file transfers, authentication, and access to SMB shares. Configurations such as setting a handshake_timeout only delay the attack rather than preventing it, as adversaries can continuously open new half-open sessions.

Upstream maintainers have patched the flaw in Linux kernel 6.1.15 and later, addressing the issue through commit e6bb9193974059ddbb0ce7763fa3882bd60d4dc3. The fix introduces a configurable backlog limit and enforces a shorter tcp_synack_retries threshold for incomplete sockets. Linux distributions have already begun shipping updated packages, and administrators are strongly advised to upgrade their systems to mitigate the risk. Without patching, environments running vulnerable versions remain susceptible to low-effort, high-impact denial-of-service attacks.

For organizations unable to immediately update their kernels, mitigations include applying network-level rate limiting on TCP port 445, enforcing stricter firewall rules, and tuning KSMBD settings to reduce handshake timeouts and backlog counts. Security teams should also monitor for unusual volumes of SYN packets, which may indicate ongoing exploitation attempts. Given the critical role of SMB services in enterprise networks, prompt patching and proactive monitoring are essential. The release of the KSMBDrain exploit highlights how protocol-level resource exhaustion flaws, while less sophisticated than code execution bugs, can be just as disruptive when left unaddressed.

Impact

• Denial of Service
• Gain Access

Indicators of Compromise

CVE

• CVE-2025-38501

Affected Vendors

Affected Products

• Linux Kernel - 5.15 - 6.1 - 6.6 - 6.12 - 6.15 - 6.16

Remediation

• Upgrade to Linux kernel 6.1.15 or later, where the flaw is patched.
• Apply distribution updates as vendors are rolling out fixed kernel packages.
• Enable backlog limits and configure shorter tcp_synack_retries thresholds if running a patched version.
• Rate-limit TCP port 445 at the network level to reduce the impact of SYN floods.
• Implement stricter firewall rules to restrict unnecessary external access to SMB services.
• Tune KSMBD settings by lowering handshake_timeout and limiting backlog counts.
• Monitor network traffic for abnormal spikes in SYN packets or incomplete connections.
• Segment and restrict SMB exposure, ensuring only trusted internal hosts can connect to port 445.