CVE-2025-21962 – An integer overflow vulnerability in the Linux ... (How to Fix)

Q: What is the severity of CVE-2025-21962?

CVE-2025-21962 has a CVSS score of 5.5 (MEDIUM). An integer overflow vulnerability in the Linux kernel's CIFS filesystem driver allows attackers to cause denial of service or potentially execute arbitrary code by providing a malicious closetimeo mount option value. This affects Linux systems using CIFS/SMB filesystem mounts. Attackers need local access or ability to control mount parameters.

📋 TL;DR

An integer overflow vulnerability in the Linux kernel's CIFS filesystem driver allows attackers to cause denial of service or potentially execute arbitrary code by providing a malicious closetimeo mount option value. This affects Linux systems using CIFS/SMB filesystem mounts. Attackers need local access or ability to control mount parameters.

💻 Affected Systems

Products:

Linux kernel

Versions: Linux kernel versions before the fix commits (specific versions vary by distribution)

Operating Systems: Linux distributions using vulnerable kernel versions

Default Config Vulnerable: ✅ No

Notes: Only affects systems using CIFS/SMB filesystem mounts with closetimeo parameter. Not vulnerable if CIFS is not used or closetimeo parameter is not specified.

📦 What is this software?

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 →

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 →

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 kernel-level code execution if combined with other vulnerabilities.

🟠

Likely Case

Denial of service through kernel crash or system instability when processing malicious mount parameters.

🟢

If Mitigated

Limited impact with proper access controls preventing unauthorized users from mounting CIFS shares.

🌐 Internet-Facing: LOW - Requires local access or ability to control mount parameters, not directly exploitable over network.

🏢 Internal Only: MEDIUM - Internal users with ability to mount CIFS shares could cause system instability or crashes.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: UNKNOWN

Unauthenticated Exploit: ✅ No

Complexity: MEDIUM

Requires ability to control mount parameters, typically needing local access or administrative privileges to mount filesystems.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Linux kernel with commits 1c46673be93dd2954f44fe370fb4f2b8e6214224, 513f6cf2e906a504b7ab0b62b2eea993a6f64558, 6c13fcb7cf59ae65940da1dfea80144e42921e53, 9968fcf02cf6b0f78fbacf3f63e782162603855a, or b24edd5c191c2689c59d0509f0903f9487eb6317

Vendor Advisory: https://git.kernel.org/stable/c/1c46673be93dd2954f44fe370fb4f2b8e6214224

Restart Required: Yes

Instructions:

1. Update Linux kernel to version containing the fix commits. 2. Check distribution-specific security advisories for patched kernel packages. 3. Reboot system after kernel update.

🔧 Temporary Workarounds

Disable CIFS mounts

linux

Prevent use of CIFS filesystem mounts if not required

modprobe -r cifs
echo 'blacklist cifs' > /etc/modprobe.d/blacklist-cifs.conf

Restrict mount privileges

linux

Limit which users can mount filesystems using sudoers or other access controls

# In /etc/sudoers: restrict mount commands
# Example: %admin ALL=(ALL) /bin/mount, /bin/umount

🧯 If You Can't Patch

Restrict CIFS mount usage to trusted administrators only
Implement strict input validation for mount parameters in automation scripts

🔍 How to Verify

Check if Vulnerable:

Check kernel version and if CIFS module is loaded: 'uname -r' and 'lsmod | grep cifs'

Check Version:

uname -r

Verify Fix Applied:

Verify kernel version is updated and check git commit history contains the fix commits

📡 Detection & Monitoring

Log Indicators:

Kernel panic logs
CIFS mount failures with unusual parameters
System crash/reboot events

Network Indicators:

Unusual CIFS mount attempts from unauthorized systems

SIEM Query:

source="kernel" AND ("panic" OR "Oops") AND "CIFS"

📊 Metadata

CVE ID: CVE-2025-21962

CVSS v3 Score: 5.5 (MEDIUM)

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

CWE: CWE-190

EPSS Score: 0.09% exploit probability (25.8th percentile)

Published: April 1, 2025

Last Updated: November 3, 2025

🔗 References

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🔗 Related Vulnerabilities

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