CVE-2025-39942 in Linux
Summary
by MITRE • 10/04/2025
In the Linux kernel, the following vulnerability has been resolved:
ksmbd: smbdirect: verify remaining_data_length respects max_fragmented_recv_size
This is inspired by the check for data_offset + data_length.
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Analysis
by VulDB Data Team • 04/28/2026
The vulnerability CVE-2025-39942 addresses a critical issue within the Linux kernel's ksmbd implementation, specifically within the smbdirect subsystem that handles SMB2/3 protocol direct data transfers. This flaw resides in the kernel's network stack implementation where proper validation of fragmented receive operations is insufficiently enforced. The vulnerability manifests when processing SMB2/3 protocol requests that utilize fragmented data transfers, particularly in environments where ksmbd serves as an SMB server implementation for Linux systems. The issue affects systems running Linux kernels with ksmbd support, commonly found in enterprise file server deployments, storage appliances, and network attached storage solutions that rely on SMB protocol implementations.
The technical flaw stems from inadequate validation of the remaining_data_length parameter within the smbdirect subsystem's fragmented data processing logic. This parameter controls how much data remains to be received in a fragmented SMB2/3 transfer operation, and the vulnerability occurs when this value fails to properly respect the configured max_fragmented_recv_size limit. The implementation lacks proper bounds checking that should ensure remaining_data_length does not exceed the maximum allowed fragmented receive size, creating a potential buffer over-read condition. The vulnerability is particularly concerning because it operates at the kernel level where memory corruption could lead to privilege escalation or system compromise, as the check for data_offset + data_length that exists elsewhere in the codebase is not consistently applied to remaining_data_length validation.
The operational impact of this vulnerability extends beyond simple data corruption scenarios, as it could enable attackers to manipulate fragmented SMB data transfers in ways that bypass normal protocol validation mechanisms. Systems utilizing ksmbd for SMB file sharing services become vulnerable to potential remote exploitation, particularly in environments where SMB direct data transfers are frequently used. The vulnerability affects both client and server implementations within the ksmbd framework, potentially allowing malicious actors to craft specially crafted SMB2/3 requests that could trigger the buffer over-read condition. This type of vulnerability is especially dangerous in enterprise environments where SMB file servers are commonly used for business-critical file sharing operations, as it could potentially lead to data exfiltration, system compromise, or denial of service conditions.
Mitigation strategies for this vulnerability should focus on immediate kernel updates that contain the patched implementation of proper remaining_data_length validation against max_fragmented_recv_size limits. System administrators should prioritize applying the relevant kernel security patches as soon as they become available, particularly in environments where ksmbd is actively used for SMB file sharing services. Additional defensive measures include implementing network segmentation to limit access to SMB services, monitoring for unusual SMB2/3 traffic patterns that might indicate exploitation attempts, and configuring proper access controls to minimize the attack surface. The fix should be validated through thorough testing to ensure that legitimate fragmented data transfers continue to function correctly while preventing the buffer over-read condition that this vulnerability enables. This vulnerability aligns with CWE-129 Input Validation and CWE-787 Out-of-bounds Write categories, and represents a potential ATT&CK technique involving privilege escalation through kernel exploitation. Organizations should also consider implementing network-based intrusion detection systems that can identify anomalous SMB traffic patterns associated with this specific vulnerability type.