CVE-2025-39759 – A race condition in the Linux kernel's Btrfs fi... (How to Fix)

Q: What is the severity of CVE-2025-39759?

CVE-2025-39759 has a CVSS score of 7.0 (HIGH). A race condition in the Linux kernel's Btrfs filesystem allows use-after-free of qgroup records when quota disable and quota rescan operations occur simultaneously. This vulnerability can lead to kernel crashes or potential privilege escalation. It affects Linux systems using Btrfs with quotas enabled.

📋 TL;DR

A race condition in the Linux kernel's Btrfs filesystem allows use-after-free of qgroup records when quota disable and quota rescan operations occur simultaneously. This vulnerability can lead to kernel crashes or potential privilege escalation. It affects Linux systems using Btrfs with quotas enabled.

💻 Affected Systems

Products:

Linux kernel with Btrfs support

Versions: Specific kernel versions containing the vulnerable code (check git commits for exact ranges)

Operating Systems: Linux distributions using affected kernel versions

Default Config Vulnerable: ✅ No

Notes: Only vulnerable when Btrfs filesystem has quotas enabled and both quota disable and quota rescan operations are performed concurrently.

📦 What is this software?

Debian Linux by Debian

View all CVEs affecting Debian Linux →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

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Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

Linux Kernel by Linux

The Linux Kernel is the core component of the Linux operating system, serving as the critical interface between computer hardware and software processes. As the heart of millions of servers, cloud infrastructure, embedded systems, Android devices, and IoT deployments worldwide, the Linux Kernel mana...

Learn more about Linux Kernel →

⚠️ Risk & Real-World Impact

🔴

Worst Case

Kernel panic leading to system crash, or potential privilege escalation to root via memory corruption.

🟠

Likely Case

System instability, kernel crashes, or denial of service affecting Btrfs filesystems.

🟢

If Mitigated

Minimal impact if quotas are disabled or systems don't use Btrfs with quotas.

🌐 Internet-Facing: LOW - Requires local access and specific Btrfs configuration.

🏢 Internal Only: MEDIUM - Local users could potentially crash systems or escalate privileges on vulnerable Btrfs configurations.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: UNKNOWN

Unauthenticated Exploit: ✅ No

Complexity: HIGH - Requires race condition timing and specific Btrfs configuration.

Exploitation requires local access and ability to trigger both quota operations simultaneously.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Kernel versions containing fixes from git commits: 2fd0f5ceb997f90f4332ccbab6c7e907e6b2d0eb, 7cda0fdde5d9890976861421d207870500f9aace, b172535ccba12f0cf7d23b3b840989de47fc104d, c38028ce0d0045ca600b6a8345a0ff92bfb47b66, dd0b28d877b293b1d7f8727a7de08ae36b6b9ef0

Vendor Advisory: https://git.kernel.org/stable/c/2fd0f5ceb997f90f4332ccbab6c7e907e6b2d0eb

Restart Required: Yes

Instructions:

1. Update Linux kernel to patched version. 2. Check distribution-specific security advisories. 3. Reboot system after kernel update.

🔧 Temporary Workarounds

Disable Btrfs quotas

linux

Disable quota functionality on Btrfs filesystems to prevent the race condition.

btrfs quota disable /mount/point

Avoid concurrent quota operations

linux

Ensure quota disable and quota rescan operations are not performed simultaneously.

🧯 If You Can't Patch

Disable Btrfs quota functionality on all affected filesystems
Implement access controls to prevent unauthorized users from performing quota operations

🔍 How to Verify

Check if Vulnerable:

Check kernel version and verify if Btrfs quotas are enabled on any filesystem.

Check Version:

uname -r

Verify Fix Applied:

Verify kernel version contains the fix commits and test quota operations.

📡 Detection & Monitoring

Log Indicators:

Kernel panic messages
Btrfs quota operation errors in system logs
Use-after-free kernel warnings

SIEM Query:

Search for: 'kernel panic', 'btrfs', 'quota', 'use-after-free' in system logs

📊 Metadata

CVE ID: CVE-2025-39759

CVSS v3 Score: 7.0 (HIGH)

CVSS Vector: CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H

CWE: CWE-362

EPSS Score: 0.01% exploit probability (0.7th percentile)

Published: September 11, 2025

Last Updated: January 9, 2026

🔗 References

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🔗 Related Vulnerabilities

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