External risk intelligence

Linux Kernel SMB Client Use-After-Free Vulnerability

CVE advisorySeverity: CRITICAL (CVSS 9.8)

CVE-2025-40320

This vulnerability is located deep within the Linux kernel's CIFS/SMB client implementation. It is triggered during local filesystem operations, such as statfs, which are initiated by local processes or users. It does not represent a network-accessible service or endpoint, making public internet exposure through this component not possible in standard deployments.

Use After Free

Halo Surface Signal: 1 out of 5 — much less likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

A vulnerability was identified in the Linux kernel's handling of SMB client operations, specifically a use-after-free issue that could arise during retries of file information queries. While the technical details involve memory management within the kernel, the broader implication is a potential for instability or unpredictable behavior in systems utilizing the SMB client.

Use-after-free flaw in kernel's SMB client.
Potential for system instability or unexpected behavior.
Confirm relevance and exposure to affected systems.

Attack Path

How an attacker could exploit the issue

An attacker could trigger this vulnerability by interacting with the Linux kernel's SMB client. This interaction, specifically when a file system query is retried, could lead to a use-after-free condition within the `smb2_query_info_compound` function. If successful, this could allow an attacker to compromise the system.

Requires local access to the system.
Triggered by retrying file system queries.
Allows for arbitrary code execution.

Live Threat

Current exploitation, exposure, and threat context

This vulnerability could affect system stability when the Linux kernel's SMB client attempts to retrieve file system information. A use-after-free condition may occur during retry operations, potentially leading to unexpected behavior or crashes. No sensitive data types are specified as at risk by the advisory.

Kernel process stability.
Use-after-free on retry operations.
System instability or crashes.

Operational Fix

Recommended remediation, mitigation, and detection steps

This use-after-free vulnerability in the Linux kernel's SMB client could impact systems performing SMB file operations. Platform or infrastructure teams managing Linux systems with the CIFS module loaded are likely responsible for addressing this. The first practical step involves identifying systems where the affected kernel component is active, assessing their exposure to potential exploitation (though the provided context suggests this is unlikely to be externally exploitable), and confirming business criticality before planning remediation.

Identify affected Linux systems.
Verify SMB client usage and reachability.
Plan kernel updates or mitigation.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

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Frequently asked questions

What is the software component affected by CVE-2025-40320?

This vulnerability resides within the Linux kernel's CIFS/SMB client. This client allows Linux systems to connect to and interact with remote Windows-style file shares. It is a fundamental part of the kernel that enables users and applications to read, write, and query information on these network-attached storage devices.

What does use-after-free mean for this vulnerability?

This is a memory management weakness where the kernel attempts to access data after it has been deleted. In the context of CVE-2025-40320, a specific object identifier, or 'cfid,' is incorrectly handled during a retry operation. Because the pointer to this object is not reset after its initial release, the system may mistakenly try to use that stale memory address again, which causes instability.

How is this Linux kernel vulnerability triggered?

The flaw occurs during internal error recovery within the CIFS module, specifically when a file system query operation fails and must be retried. It is not triggered by a typical network request alone. The bug requires the system to be actively performing file information queries, making it a byproduct of local system-level operations rather than a simple incoming network packet.

Why does Halo Surface Signal suggest this is unlikely to be exposed?

Halo Surface Signal indicates that because this bug is embedded deep within the kernel's local filesystem logic, it cannot be reached by external network services. Since the vulnerability is triggered by local operations, such as system calls initiated by a process already running on the machine, standard internet-facing services do not provide a direct path for an attacker to reach this specific code path.

What should I do if I run systems using the CIFS module?

Begin by inventorying your Linux systems to identify those actively utilizing the CIFS/SMB client for file sharing. Prioritize these systems based on their business function, even though external reachability is unlikely. The primary path for resolution will be applying official kernel updates provided by your Linux distribution maintainers once they release patches addressing this specific memory management error.

References

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.