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CVE-2021-47634

7.8 HIGH

📋 TL;DR

This is a use-after-free vulnerability in the Linux kernel's UBI (Unsorted Block Images) subsystem caused by a race condition between device control and volume management operations. It allows local attackers to potentially crash the system or execute arbitrary code with kernel privileges. Systems running vulnerable Linux kernel versions with UBI enabled are affected.

💻 Affected Systems

Products:
  • Linux Kernel
Versions: Specific vulnerable versions not explicitly stated, but references indicate stable kernel patches from 2021-2022 timeframe
Operating Systems: Linux distributions using affected kernel versions
Default Config Vulnerable: ✅ No
Notes: Only vulnerable when UBI subsystem is enabled and in use (typically in embedded systems with flash storage). Most desktop/server distributions may not have UBI enabled by default.

📦 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 →

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 denial of service, or potential privilege escalation to root via arbitrary code execution in kernel context.

🟠

Likely Case

System crash or kernel panic causing denial of service, potentially requiring physical access or reboot to recover.

🟢

If Mitigated

No impact if UBI subsystem is not in use or proper kernel hardening prevents local user access.

🌐 Internet-Facing: LOW - This requires local access to the system, not remotely exploitable.
🏢 Internal Only: MEDIUM - Local users or processes could exploit this to crash the system or potentially escalate privileges.

🎯 Exploit Status

Public PoC: ✅ No
Weaponized: UNKNOWN
Unauthenticated Exploit: ✅ No
Complexity: MEDIUM

Requires local access and ability to trigger race conditions between UBI device operations. Exploitation requires precise timing.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Kernel versions with commits: 1a3f1cf87054833242fcd0218de0481cf855f888, 3cbf0e392f173ba0ce425968c8374a6aa3e90f2e, 432b057f8e847ae5a2306515606f8d2defaca178, 5f9e9c223e48c264241d2f34d0bfc29e5fcb5c1b, a8ecee49259f8f78d91ddb329ab2be7e6fd01974

Vendor Advisory: https://git.kernel.org/stable/c/1a3f1cf87054833242fcd0218de0481cf855f888

Restart Required: Yes

Instructions:

1. Update Linux kernel to patched version from your distribution vendor. 2. Reboot system to load new kernel. 3. Verify UBI subsystem is functioning correctly after update.

🔧 Temporary Workarounds

Disable UBI subsystem

Linux

If UBI is not required for your system, disable it in kernel configuration

echo 'blacklist ubi' >> /etc/modprobe.d/blacklist.conf
rmmod ubi

🧯 If You Can't Patch

  • Restrict local user access to systems using UBI
  • Implement strict process isolation and limit UBI device operations to trusted processes only

🔍 How to Verify

Check if Vulnerable:

Check if UBI module is loaded: lsmod | grep ubi && check kernel version against patched versions

Check Version:

uname -r

Verify Fix Applied:

Verify kernel version includes the fix commits and test UBI operations don't cause crashes

📡 Detection & Monitoring

Log Indicators:

  • Kernel panic messages
  • KASAN reports about use-after-free in UBI subsystem
  • System crashes during UBI operations

Network Indicators:

  • None - local vulnerability only

SIEM Query:

kernel: "BUG: KASAN: use-after-free" AND "ubi" OR kernel: "panic" AND "ubi"

📊 Metadata

CVE ID: CVE-2021-47634
CVSS v3 Score: 7.8 (HIGH)
CVSS Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
CWE: CWE-416
EPSS Score: 0.1% exploit probability (27th percentile)
Published: February 26, 2025
Last Updated: March 24, 2025

🔗 References

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