CVE-2025-39888: Linux Kernel Buffer Overflow Vulnerability

CVE-2025-39888 Overview

CVE-2025-39888 is an out-of-bounds write vulnerability in the Filesystem in Userspace (FUSE) subsystem of the Linux kernel. The flaw resides in the fuse_dev_do_write function and was reported by the syzkaller fuzzing framework as a slab-out-of-bounds write. When the number of bytes to be retrieved is truncated to the upper limit by fc->max_pages and an offset is present, the folio access crosses the allocated boundary. A local, authenticated attacker interacting with a FUSE device can trigger memory corruption, leading to kernel instability or potential privilege escalation. The issue is tracked under [CWE-787: Out-of-bounds Write].

Critical Impact
Local attackers with access to a FUSE device can corrupt kernel slab memory, threatening confidentiality, integrity, and availability of the host system.

Affected Products

Linux kernel 6.17-rc1 through 6.17-rc5
Linux kernel branches containing the vulnerable FUSE folio handling logic prior to the fix
Distributions shipping the unpatched mainline FUSE driver

Discovery Timeline

2025-09-23 - CVE-2025-39888 published to the National Vulnerability Database
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-39888

Vulnerability Analysis

The vulnerability affects fuse_dev_do_write, the kernel-side handler that copies data from a userspace FUSE daemon into kernel folios backing a filesystem request. FUSE lets unprivileged processes implement filesystems in userspace, and the kernel driver mediates data transfers between the daemon and the requesting process.

During a write from the FUSE device, the driver iterates over folios that back the request buffer. The length of data to copy is clamped against fc->max_pages, the per-connection page limit. When this clamp truncates the transfer while a non-zero starting offset within the first folio is in effect, the loop can advance past the last valid folio and write into adjacent slab memory.

The corruption occurs in kernel address space, so the impact extends beyond the calling process. Adjacent slab objects, function pointers, or reference counters can be overwritten, opening the door to kernel memory corruption primitives suitable for privilege escalation.

Root Cause

The root cause is a missing loop termination condition when the requested byte count is truncated to fc->max_pages and the transfer begins at a non-zero folio offset. The loop consumes bytes based on the truncated length without checking whether the current folio index still falls within the allocated array, resulting in a slab-out-of-bounds write on the folio-tracking structure.

Attack Vector

Exploitation requires local access and the ability to interact with /dev/fuse or to mount a FUSE filesystem. In many distributions, unprivileged users can mount FUSE filesystems, which lowers the barrier to reaching the vulnerable code path. An attacker crafts a FUSE daemon that issues write replies whose length and offset combination triggers the truncation condition. The malformed reply causes fuse_dev_do_write to write past the folio array in the kernel slab allocator. See the upstream fix in the Linux kernel stable tree for the corrected loop bound.

// No verified public exploit code is available.
// The vulnerability is described in prose above; refer to the
// upstream commits for the exact patched loop condition.

Detection Methods for CVE-2025-39888

Indicators of Compromise

Kernel logs containing KASAN: slab-out-of-bounds reports referencing fuse_dev_do_write or fuse_copy_folio
Unexpected kernel oops, panics, or general protection fault traces originating from the FUSE subsystem
Unprivileged processes repeatedly mounting FUSE filesystems and issuing malformed write replies over /dev/fuse

Detection Strategies

Enable KASAN on test and canary hosts to surface out-of-bounds writes in the FUSE code path during fuzzing or QA
Monitor dmesg and journald for kernel warnings referencing fuse_dev_do_write, max_pages, or folio handling
Audit process execution for non-root users spawning custom FUSE daemons that write to /dev/fuse with unusual reply sizes

Monitoring Recommendations

Ship kernel ring buffer output to a centralized logging platform and alert on new FUSE-related stack traces
Track syscall telemetry for mount with fuse or fuse.* filesystem types initiated by unprivileged UIDs
Correlate crashes in kernel threads with recent FUSE activity to identify exploitation attempts before privilege escalation succeeds

How to Mitigate CVE-2025-39888

Immediate Actions Required

Apply the upstream Linux kernel patch that adds the loop termination condition in fuse_dev_do_write
Update to a distribution kernel that incorporates the fix and reboot affected systems
Restrict FUSE mount privileges to trusted users on multi-tenant hosts until the patch is deployed

Patch Information

The fix is available in the upstream stable tree. Review the primary commit and the related change for the exact code modification. Distribution vendors typically backport such FUSE fixes to their supported long-term kernels; consult vendor advisories for package versions containing the correction.

Workarounds

Disable or blacklist the fuse kernel module on systems that do not require userspace filesystems
Set user_allow_other=no and remove the setuid bit from fusermount to prevent unprivileged FUSE mounts
Use Linux Security Modules such as AppArmor or SELinux to confine processes that can open /dev/fuse

bash

# Prevent unprivileged users from loading the FUSE module
echo 'install fuse /bin/true' | sudo tee /etc/modprobe.d/disable-fuse.conf

# Remove setuid from fusermount to block unprivileged mounts
sudo chmod u-s /usr/bin/fusermount3
sudo chmod u-s /usr/bin/fusermount

# Verify the running kernel version after patching
uname -r