CVE-2021-20268 – This CVE describes an out-of-bounds access vuln... (How to Fix)

Q: What is the severity of CVE-2021-20268?

CVE-2021-20268 has a CVSS score of 7.8 (HIGH). This CVE describes an out-of-bounds access vulnerability in the Linux kernel's eBPF code verifier. A local attacker can exploit this flaw to crash the system or potentially escalate privileges, affecting confidentiality, integrity, and availability. Only Linux systems with eBPF functionality enabled are affected.

📋 TL;DR

This CVE describes an out-of-bounds access vulnerability in the Linux kernel's eBPF code verifier. A local attacker can exploit this flaw to crash the system or potentially escalate privileges, affecting confidentiality, integrity, and availability. Only Linux systems with eBPF functionality enabled are affected.

💻 Affected Systems

Products:

Linux kernel

Versions: Kernel versions before 5.10.19, 5.11.2, and other distributions' backported fixes

Operating Systems: Linux distributions using affected kernel versions

Default Config Vulnerable: ⚠️ Yes

Notes: Requires CONFIG_BPF_SYSCALL enabled (common in modern distributions). Non-root users may need CAP_BPF capability.

📦 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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⚠️ Risk & Real-World Impact

🔴

Worst Case

Local privilege escalation to root, complete system compromise, or persistent denial of service.

🟠

Likely Case

Local denial of service (system crash/kernel panic) or limited privilege escalation.

🟢

If Mitigated

Minimal impact if proper access controls restrict local user accounts and eBPF is disabled.

🌐 Internet-Facing: LOW - Requires local access to exploit, not directly reachable from internet.

🏢 Internal Only: HIGH - Any local user (including compromised accounts) can potentially exploit this vulnerability.

🎯 Exploit Status

Public PoC: ⚠️ Yes

Weaponized: LIKELY

Unauthenticated Exploit: ✅ No

Complexity: MEDIUM

Exploit requires local access and knowledge of eBPF programming. Proof-of-concept code exists in security advisories.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Linux kernel 5.10.19, 5.11.2, and distribution-specific backports

Vendor Advisory: https://bugzilla.redhat.com/show_bug.cgi?id=1923816

Restart Required: Yes

Instructions:

1. Update kernel package using distribution's package manager. 2. For RHEL/CentOS: 'yum update kernel'. 3. For Ubuntu/Debian: 'apt update && apt upgrade linux-image-*'. 4. Reboot system to load new kernel.

🔧 Temporary Workarounds

Disable eBPF functionality

linux

Prevents exploitation by disabling the vulnerable eBPF subsystem

echo 0 > /proc/sys/kernel/unprivileged_bpf_disabled
sysctl -w kernel.unprivileged_bpf_disabled=1

Restrict eBPF capabilities

linux

Limit which users can load eBPF programs using Linux capabilities

setcap -r /path/to/binary
capsh --drop=cap_bpf -- -c 'your_command'

🧯 If You Can't Patch

Restrict local user access and implement least privilege principles
Monitor for unusual eBPF program loading or kernel panic events

🔍 How to Verify

Check if Vulnerable:

Check kernel version: 'uname -r'. Compare against affected versions. Check if eBPF is enabled: 'grep CONFIG_BPF_SYSCALL /boot/config-$(uname -r)'

Check Version:

uname -r

Verify Fix Applied:

Verify kernel version is patched: 'uname -r' should show 5.10.19+, 5.11.2+, or distribution-provided fixed version

📡 Detection & Monitoring

Log Indicators:

Kernel panic logs
eBPF verifier error messages in dmesg
Failed eBPF program loads

Network Indicators:

None - local exploitation only

SIEM Query:

source="kernel" AND ("eBPF" OR "verifier" OR "kernel panic")

📊 Metadata

CVE ID: CVE-2021-20268

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-20

Published: March 9, 2021

Last Updated: November 21, 2024

🔗 References

https://bugzilla.redhat.com/show_bug.cgi?id=1923816 Issue Tracking,Patch,Third Party Advisory
https://lore.kernel.org/bpf/CACAyw99bEYWJCSGqfLiJ9Jp5YE1ZsZSiJxb4RFUTwbofipf0dA%40mail.gmail.com/T/#m8929643e99bea9c18ed490a7bc2591145eac6444
https://security.netapp.com/advisory/ntap-20210409-0006/ Third Party Advisory
https://bugzilla.redhat.com/show_bug.cgi?id=1923816 Issue Tracking,Patch,Third Party Advisory
https://lore.kernel.org/bpf/CACAyw99bEYWJCSGqfLiJ9Jp5YE1ZsZSiJxb4RFUTwbofipf0dA%40mail.gmail.com/T/#m8929643e99bea9c18ed490a7bc2591145eac6444
https://security.netapp.com/advisory/ntap-20210409-0006/ Third Party Advisory

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🔗 Related Vulnerabilities

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