CVE-2025-39730: Linux Kernel NFS Bounds Check Vulnerability

CVE-2025-39730 Overview

CVE-2025-39730 is a Linux kernel vulnerability in the Network File System (NFS) subsystem. The flaw resides in the nfs_fh_to_dentry() function, which failed to validate the minimal filehandle length before accessing the embedded filehandle. A local attacker with low privileges can trigger out-of-bounds memory access, leading to compromise of confidentiality, integrity, and availability. The issue affects multiple stable branches of the Linux kernel and Debian Linux 11. Kernel maintainers resolved the vulnerability by adding proper bounds checking prior to filehandle dereference.

Critical Impact
A local, low-privileged attacker can exploit the missing bounds check in nfs_fh_to_dentry() to trigger out-of-bounds reads in kernel memory, potentially resulting in denial of service or kernel information disclosure.

Affected Products

Linux Kernel (multiple stable branches, per upstream kernel.org commits)
Debian Linux 11 (Bullseye)
Systems exporting or consuming NFS filesystems using the affected kernel builds

Discovery Timeline

2025-09-07 - CVE-2025-39730 published to the National Vulnerability Database (NVD)
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-39730

Vulnerability Analysis

The vulnerability exists in the Linux kernel NFS client code path, specifically in the nfs_fh_to_dentry() function. This function is invoked when the kernel needs to convert an NFS filehandle into a directory entry (dentry), typically during file lookup operations initiated through the export or reconstruction interfaces.

The function accessed the embedded filehandle structure without first verifying that the incoming buffer met the minimum required size. When a shorter-than-expected filehandle was supplied, the code read beyond the allocated buffer boundary. This constitutes an out-of-bounds read [CWE-125-class behavior], with downstream effects on data integrity and availability tracked by the CVSS impact metrics.

The attack requires local access and low privileges. Exploitation vectors include supplying crafted filehandles through interfaces that call into the NFS export operations, such as open_by_handle_at() on a system with NFS mounts or servers.

Root Cause

The root cause is missing input validation. The nfs_fh_to_dentry() handler dereferenced fields inside the filehandle structure before confirming that the caller-supplied length covered those fields. Without the length check, malformed or truncated filehandles caused the kernel to read outside the intended buffer, corrupting subsequent processing logic and potentially exposing adjacent memory contents.

Attack Vector

A local attacker with the ability to invoke filehandle-processing syscalls (for example, open_by_handle_at(), which requires CAP_DAC_READ_SEARCH) can craft undersized NFS filehandles. When the kernel processes these handles through nfs_fh_to_dentry(), the missing bounds check triggers an out-of-bounds read. Depending on kernel memory layout and hardening options, this may crash the kernel, leak sensitive memory, or corrupt data structures used in file resolution.

No public proof-of-concept exploit is currently associated with this CVE, and it is not listed in the CISA Known Exploited Vulnerabilities catalog.

Detection Methods for CVE-2025-39730

Indicators of Compromise

Unexpected kernel oops or panic messages referencing nfs_fh_to_dentry, exportfs_decode_fh, or NFS filesystem symbols in dmesg or /var/log/kern.log
KASAN (Kernel Address Sanitizer) reports flagging out-of-bounds reads originating from the NFS module on debug-enabled kernels
Unexpected user-space invocations of open_by_handle_at() or name_to_handle_at() from non-administrative processes

Detection Strategies

Audit installed kernel packages against the fixed versions referenced in the upstream commits (12ad3def, 2ad40b79, 3570ef5c, 763810bb, 7dd36f74, 7f8eca87, b7f78669, cb09afa0, ef93a685) and Debian LTS advisories
Enable auditd rules for the open_by_handle_at and name_to_handle_at syscalls on systems with NFS mounts to identify anomalous filehandle usage
Monitor for repeated kernel crashes or Machine Check Exceptions on NFS clients and servers, which may indicate exploitation attempts

Monitoring Recommendations

Centralize kernel logs from NFS-serving and NFS-consuming hosts into a SIEM or data lake for correlation of oops signatures
Track process lineage for binaries invoking filehandle syscalls, particularly from unprivileged users escalated through capability grants
Alert on new local processes acquiring CAP_DAC_READ_SEARCH outside of expected administrative workflows

How to Mitigate CVE-2025-39730

Immediate Actions Required

Apply the Linux kernel security update from your distribution vendor as soon as it is available for your kernel branch
Debian 11 users should install the kernel package updates referenced in Debian LTS announcements DLA msg00007 and DLA msg00008
Reboot affected systems after applying the kernel update to ensure the patched image is loaded
Inventory all NFS clients and servers to confirm patch coverage across the environment

Patch Information

The fix was committed to multiple stable kernel branches. Reference the upstream commits at Kernel Commit 12ad3def, Kernel Commit 2ad40b79, Kernel Commit 3570ef5c, Kernel Commit 763810bb, Kernel Commit 7dd36f74, Kernel Commit 7f8eca87, Kernel Commit b7f78669, Kernel Commit cb09afa0, and Kernel Commit ef93a685. Each commit adds the minimum filehandle length check prior to dereferencing the embedded filehandle in nfs_fh_to_dentry().

Workarounds

Restrict CAP_DAC_READ_SEARCH grants so that only trusted administrative processes can invoke open_by_handle_at()
Unload the NFS client module (nfs) on systems that do not require NFS functionality using modprobe -r nfs
Constrain local user access on NFS-enabled hosts until the kernel update can be scheduled

bash

# Verify the running kernel version and check for the NFS module
uname -r
lsmod | grep -E '^nfs '

# Debian: apply the kernel security update
sudo apt update && sudo apt install --only-upgrade linux-image-$(uname -r | sed 's/.*-//')
sudo reboot

# Optional: unload NFS client if the service is not required
sudo modprobe -r nfs