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CVE-2025-21646

5.5 MEDIUM
Denial of Service

📋 TL;DR

A Linux kernel vulnerability in the AFS filesystem where overly long cell names (256+ bytes) cause a kernel warning when creating procfs entries. This affects systems using the AFS filesystem with long cell names, potentially causing system instability.

💻 Affected Systems

Products:
  • Linux kernel AFS filesystem implementation
Versions: Linux kernel versions before fixes in stable releases (specific commits listed in references)
Operating Systems: Linux distributions with AFS support enabled
Default Config Vulnerable: ✅ No
Notes: Only vulnerable if AFS filesystem is configured and used with cell names exceeding 253 bytes

📦 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 warning leading to system instability, denial of service, or potential information disclosure through kernel logs

🟠

Likely Case

Kernel warning messages in logs when using AFS with long cell names, potentially causing AFS mount failures

🟢

If Mitigated

Minor logging noise with no functional impact if cell names are under 253 bytes

🌐 Internet-Facing: LOW - Requires AFS filesystem usage and specific cell name configuration
🏢 Internal Only: MEDIUM - Internal AFS deployments with long cell names could experience service disruption

🎯 Exploit Status

Public PoC: ✅ No
Weaponized: NO
Unauthenticated Exploit: ✅ No
Complexity: LOW

Exploitation requires ability to configure AFS with long cell names, typically requiring administrative access

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Linux kernel stable releases containing commits: 7673030efe0f8ca1056d3849d61784c6caa052af, 7922b1f058fe24a93730511dd0ae2e1630920096, 7cb3e77e9b4e6ffa325a5559393d3283c9af3d01, 8fd56ad6e7c90ac2bddb0741c6b248c8c5d56ac8, 9340385468d056bb700b8f28df236b81fc86a079

Vendor Advisory: https://git.kernel.org/stable/c/7673030efe0f8ca1056d3849d61784c6caa052af

Restart Required: Yes

Instructions:

1. Update Linux kernel to version containing the fix commits. 2. Reboot system to load new kernel. 3. Verify AFS functionality with cell names under 253 bytes.

🔧 Temporary Workarounds

Limit AFS cell name length

linux

Ensure all AFS cell names are 253 bytes or less to avoid triggering the vulnerability

# Check current AFS cell names
cat /proc/fs/afs/cells
# Ensure cell names in configuration are <= 253 characters

🧯 If You Can't Patch

  • Ensure all AFS cell names are limited to 253 characters or less
  • Monitor kernel logs for WARNING messages related to procfs and AFS

🔍 How to Verify

Check if Vulnerable:

Check if AFS is in use and cell names exceed 253 characters: 'cat /proc/fs/afs/cells' and examine cell name lengths

Check Version:

uname -r

Verify Fix Applied:

Check kernel version contains fix commits: 'uname -r' and verify against patched kernel versions

📡 Detection & Monitoring

Log Indicators:

  • Kernel WARNING messages mentioning 'fs/proc/generic.c:405' or AFS cell name issues

Network Indicators:

  • AFS protocol errors when mounting with long cell names

SIEM Query:

source="kernel" AND "WARNING" AND ("proc/generic.c" OR "afs" OR "cell")

📊 Metadata

CVE ID: CVE-2025-21646
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
EPSS Score: 0.06% exploit probability (17.5th percentile)
Published: January 19, 2025
Last Updated: November 3, 2025

🔗 References

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