CVE-2021-26708 – CVE-2021-26708 is a local privilege escalation ... (How to Fix)

Q: What is the severity of CVE-2021-26708?

CVE-2021-26708 has a CVSS score of 7.0 (HIGH). CVE-2021-26708 is a local privilege escalation vulnerability in the Linux kernel's AF_VSOCK implementation caused by race conditions due to incorrect locking. It allows a local attacker to escalate privileges from an unprivileged user to root. Systems running Linux kernel versions before 5.10.13 with VSOCK functionality are affected.

📋 TL;DR

CVE-2021-26708 is a local privilege escalation vulnerability in the Linux kernel's AF_VSOCK implementation caused by race conditions due to incorrect locking. It allows a local attacker to escalate privileges from an unprivileged user to root. Systems running Linux kernel versions before 5.10.13 with VSOCK functionality are affected.

💻 Affected Systems

Products:

Linux kernel

Versions: Versions before 5.10.13

Operating Systems: Linux distributions using affected kernel versions

Default Config Vulnerable: ⚠️ Yes

Notes: Requires VSOCK functionality which is commonly enabled in virtualization environments and containers.

📦 What is this software?

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⚠️ Risk & Real-World Impact

🔴

Worst Case

Complete system compromise with root access, enabling installation of persistent backdoors, data theft, or lateral movement across the network.

🟠

Likely Case

Local privilege escalation allowing attackers to bypass security controls, access sensitive data, or modify system configurations.

🟢

If Mitigated

Limited impact if proper access controls restrict local user accounts and kernel modules are properly sandboxed.

🌐 Internet-Facing: LOW - This is a local privilege escalation requiring existing local access, not remotely exploitable.

🏢 Internal Only: MEDIUM - Significant risk from malicious insiders or attackers who have gained initial foothold through other means.

🎯 Exploit Status

Public PoC: ⚠️ Yes

Weaponized: LIKELY

Unauthenticated Exploit: ✅ No

Complexity: MEDIUM

Exploitation requires local access and involves race condition timing, making it moderately complex but feasible.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: 5.10.13 and later

Vendor Advisory: https://cdn.kernel.org/pub/linux/kernel/v5.x/ChangeLog-5.10.13

Restart Required: Yes

Instructions:

1. Update kernel to version 5.10.13 or later using your distribution's package manager. 2. For Ubuntu/Debian: sudo apt update && sudo apt upgrade linux-image-$(uname -r). 3. For RHEL/CentOS: sudo yum update kernel. 4. Reboot the system to load the new kernel.

🔧 Temporary Workarounds

Disable VSOCK module

linux

Prevent loading of the vulnerable VSOCK kernel module

echo 'blacklist vmw_vsock_virtio_transport' | sudo tee /etc/modprobe.d/disable-vsock.conf
sudo rmmod vmw_vsock_virtio_transport

🧯 If You Can't Patch

Restrict local user access and implement strict privilege separation
Monitor for suspicious privilege escalation attempts and kernel module loading

🔍 How to Verify

Check if Vulnerable:

Check kernel version with uname -r and compare to 5.10.13. If version is earlier than 5.10.13, system is vulnerable.

Check Version:

uname -r

Verify Fix Applied:

After patching, verify kernel version is 5.10.13 or later with uname -r and check that the system has been rebooted.

📡 Detection & Monitoring

Log Indicators:

Unexpected privilege escalation events in audit logs
Suspicious kernel module loading related to VSOCK

Network Indicators:

Unusual VSOCK socket activity from non-privileged users

SIEM Query:

source="audit.log" AND (event_type="SYSCALL" AND syscall="execve" AND success="yes" AND uid_change="0")

📊 Metadata

CVE ID: CVE-2021-26708

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-667

Published: February 5, 2021

Last Updated: November 21, 2024

🔗 References

http://www.openwall.com/lists/oss-security/2021/02/05/6 Mailing List,Patch,Third Party Advisory
http://www.openwall.com/lists/oss-security/2021/04/09/2 Mailing List
http://www.openwall.com/lists/oss-security/2022/01/25/14 Mailing List
https://cdn.kernel.org/pub/linux/kernel/v5.x/ChangeLog-5.10.13 Release Notes,Vendor Advisory
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c518adafa39f37858697ac9309c6cf1805581446 Patch,Vendor Advisory
https://security.netapp.com/advisory/ntap-20210312-0008/ Patch,Third Party Advisory
https://www.openwall.com/lists/oss-security/2021/02/04/5 Mailing List,Patch,Third Party Advisory
http://www.openwall.com/lists/oss-security/2021/02/05/6 Mailing List,Patch,Third Party Advisory
http://www.openwall.com/lists/oss-security/2021/04/09/2 Mailing List
http://www.openwall.com/lists/oss-security/2022/01/25/14 Mailing List
https://cdn.kernel.org/pub/linux/kernel/v5.x/ChangeLog-5.10.13 Release Notes,Vendor Advisory
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c518adafa39f37858697ac9309c6cf1805581446 Patch,Vendor Advisory
https://security.netapp.com/advisory/ntap-20210312-0008/ Patch,Third Party Advisory
https://www.openwall.com/lists/oss-security/2021/02/04/5 Mailing List,Patch,Third Party Advisory

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