CVE-2025-21967 in Linux
Summary
by MITRE • 04/01/2025
In the Linux kernel, the following vulnerability has been resolved:
ksmbd: fix use-after-free in ksmbd_free_work_struct
->interim_entry of ksmbd_work could be deleted after oplock is freed. We don't need to manage it with linked list. The interim request could be immediately sent whenever a oplock break wait is needed.
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Analysis
by VulDB Data Team • 02/01/2026
The vulnerability identified as CVE-2025-21967 affects the Linux kernel's ksmbd implementation, which provides SMB/CIFS file sharing capabilities for Linux systems. This issue resides within the ksmbd_work structure management logic where a use-after-free condition can occur during oplock handling operations. The flaw manifests when the interim_entry associated with a ksmbd_work structure is deleted while the oplock is still in the process of being freed, creating a scenario where memory that has been released is subsequently accessed, potentially leading to system instability or arbitrary code execution.
The technical root cause of this vulnerability stems from improper memory management within the ksmbd subsystem's oplock break handling mechanism. When an oplock break is required, the system attempts to manage interim requests through a linked list structure that contains the interim_entry pointer. However, the current implementation allows for the deletion of this interim_entry before the oplock freeing process is complete, creating a race condition where freed memory is accessed. This pattern violates fundamental memory safety principles and represents a classic use-after-free vulnerability classified under CWE-416. The vulnerability specifically impacts the ksmbd_free_work_struct function where the cleanup process fails to properly synchronize the deletion of interim entries with the oplock release operations.
The operational impact of this vulnerability extends beyond simple system crashes, as it represents a potential attack vector that could be exploited by malicious actors to gain unauthorized access to systems running affected kernel versions. When exploited, the use-after-free condition could allow for privilege escalation, denial of service attacks, or potentially remote code execution depending on the system configuration and attack surface. The vulnerability affects Linux systems that utilize the ksmbd kernel module for SMB file sharing services, making it particularly concerning for enterprise environments that rely on SMB/CIFS protocols for file access and sharing. Organizations running affected systems may experience unexpected service interruptions or security breaches if this vulnerability is exploited.
Mitigation strategies for CVE-2025-21967 should focus on immediate patch application from trusted sources, as the vulnerability requires kernel-level fixes that cannot be effectively addressed through configuration changes alone. System administrators should prioritize updating their kernel versions to include the patched ksmbd implementation that properly synchronizes interim_entry management with oplock freeing operations. Additional protective measures include monitoring SMB traffic for unusual patterns, implementing network segmentation to limit access to SMB services, and ensuring that only necessary systems have SMB services enabled. The fix implemented addresses the underlying race condition by eliminating the need to manage interim entries through linked lists and instead allows for immediate sending of interim requests when oplock breaks are required, aligning with best practices for concurrent programming and memory management. This vulnerability also highlights the importance of proper synchronization mechanisms in kernel space operations and demonstrates how seemingly minor memory management issues can have significant security implications. Organizations should also consider implementing runtime protections such as kernel address space layout randomization and stack canaries to further mitigate potential exploitation attempts, while adhering to the principle of least privilege for SMB service configurations.