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CVE-2021-26930

7.8 HIGH
Denial of Service

📋 TL;DR

This vulnerability in the Linux kernel's Xen block back-end driver (xen-blkback) can cause memory mapping errors when processing grant references from virtual machine frontends. When exploited, it leads to use of unmapped memory space or prevents safe error recovery, potentially causing crashes or privilege escalation. Affects Linux kernel versions 3.11 through 5.10.16 when using Xen virtualization.

💻 Affected Systems

Products:
  • Linux kernel
  • Xen virtualization
Versions: Linux kernel 3.11 through 5.10.16
Operating Systems: Linux distributions using affected kernel versions with Xen support
Default Config Vulnerable: ⚠️ Yes
Notes: Only affects systems using Xen virtualization with the xen-blkback driver enabled. Not vulnerable on bare metal or other hypervisors.

📦 What is this software?

Debian Linux by Debian

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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 →

⚠️ Risk & Real-World Impact

🔴

Worst Case

Privilege escalation to kernel-level access, host system compromise, or denial of service through kernel panic/crash

🟠

Likely Case

System instability, kernel crashes, or denial of service affecting virtual machines using Xen block devices

🟢

If Mitigated

Limited to denial of service in isolated virtualization environments with proper access controls

🌐 Internet-Facing: LOW - Requires access to Xen virtualization infrastructure and specific block device operations
🏢 Internal Only: MEDIUM - Virtualization hosts in data centers or cloud environments are at risk if running vulnerable kernels

🎯 Exploit Status

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

Exploitation requires access to Xen virtual machine infrastructure and ability to trigger specific error conditions in grant reference mapping

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Linux kernel 5.10.17 and later, or backported patches for earlier versions

Vendor Advisory: http://xenbits.xen.org/xsa/advisory-365.html

Restart Required: Yes

Instructions:

1. Update Linux kernel to version 5.10.17 or later
2. Apply kernel patches from git commit 871997bc9e423f05c7da7c9178e62dde5df2a7f8
3. Reboot system to load patched kernel
4. Verify xen-blkback driver is using patched code

🔧 Temporary Workarounds

Disable Xen block back-end

linux

Temporarily disable the vulnerable xen-blkback driver if not required

modprobe -r xen_blkback
echo 'blacklist xen_blkback' > /etc/modprobe.d/disable-xen-blkback.conf

Use alternative storage back-end

linux

Switch to alternative storage solutions (like file-based or network storage) instead of Xen block devices

🧯 If You Can't Patch

  • Isolate Xen virtualization hosts from untrusted networks
  • Implement strict access controls to Xen management interfaces
  • Monitor for kernel crashes or unusual Xen-related errors

🔍 How to Verify

Check if Vulnerable:

Check kernel version: uname -r and verify if between 3.11 and 5.10.16, and check if xen_blkback module is loaded: lsmod | grep xen_blkback

Check Version:

uname -r

Verify Fix Applied:

Verify kernel version is 5.10.17+ or check if patch commit 871997bc9e423f05c7da7c9178e62dde5df2a7f8 is applied in kernel source

📡 Detection & Monitoring

Log Indicators:

  • Kernel panic messages
  • Xen-related error messages in dmesg
  • Unexpected virtual machine crashes

Network Indicators:

  • Unusual Xen management traffic patterns

SIEM Query:

source="kernel" AND ("panic" OR "Oops" OR "xen-blkback")

📊 Metadata

CVE ID: CVE-2021-26930
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
Published: February 17, 2021
Last Updated: November 21, 2024

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