A significant security flaw has been identified in the Linux kernel’s `ksmbd` module, potentially enabling authenticated local users to escalate their privileges to root level. This vulnerability, designated as CVE-2025-37947, is an out-of-bounds write issue that poses a substantial risk to systems utilizing the affected module.
Understanding the Vulnerability
The `ksmbd` module is an in-kernel Server Message Block (SMB) server introduced to enhance file-sharing performance by operating within the kernel space. The vulnerability resides in the `ksmbd_vfs_stream_write()` function, which manages write operations to file streams using extended attributes. Specifically, the flaw is triggered when an authenticated user interacts with a system where `ksmbd` is configured with a writable share and the `streams_xattr` Virtual File System (VFS) module is active.
The core issue arises from improper validation of input sizes. When a user-supplied position and data count exceed the `XATTR_SIZE_MAX` limit of 65,536 bytes, the system truncates the buffer allocation size but fails to adjust the `memcpy` operation accordingly. This oversight allows an attacker to write data beyond the allocated buffer, leading to memory corruption in adjacent regions.
Exploitation and Impact
Security researchers at Doyensec have demonstrated how this out-of-bounds write can be exploited to achieve full root access on affected systems, such as Ubuntu 22.04.5 LTS. The exploitation process involves several stages:
1. Heap Manipulation: The attacker carefully allocates and frees kernel objects to control the memory layout, positioning a target object (e.g., a `msg_msg` kernel message structure) adjacent to the vulnerable buffer.
2. Memory Corruption: By exploiting the out-of-bounds write, the attacker corrupts the `msg_msg` header, creating a use-after-free (UAF) condition.
3. Information Leak: The UAF condition is leveraged to leak kernel memory addresses, effectively bypassing Kernel Address Space Layout Randomization (KASLR).
4. Privilege Escalation: With KASLR defeated, the attacker overwrites a function pointer in a `pipe_buffer` object, redirecting the kernel’s control flow to execute a Return-Oriented Programming (ROP) chain that grants root privileges.
The public release of a proof-of-concept (PoC) exploit underscores the practical risk posed by this vulnerability. While the current exploit requires local access, the potential for remote exploitation cannot be dismissed, especially if combined with other vulnerabilities that could disclose information or facilitate reliable heap manipulation.
Mitigation Measures
System administrators are strongly advised to assess their use of the `ksmbd` module and apply patches addressing CVE-2025-37947 as they become available from their Linux distribution providers. In the interim, consider the following actions:
– Disable `ksmbd`: If the module is not essential to your operations, disabling it can mitigate the risk.
– Restrict Access: Limit access to the `ksmbd` service to trusted users and networks to reduce the attack surface.
– Monitor Systems: Implement monitoring to detect unusual activities that may indicate exploitation attempts.
This discovery is part of a broader security audit of `ksmbd` by Doyensec, which has previously identified other critical vulnerabilities, including unauthenticated race conditions and memory exhaustion flaws. The findings highlight the importance of continuous security assessments and prompt patch management to safeguard systems against emerging threats.
