CVE-2021-38166 – CVE-2021-38166 is an integer overflow vulnerabi... (How to Fix)

Q: What is the severity of CVE-2021-38166?

CVE-2021-38166 has a CVSS score of 7.8 (HIGH). CVE-2021-38166 is an integer overflow vulnerability in the Linux kernel's BPF hashtab implementation that can lead to out-of-bounds writes. This vulnerability affects Linux kernel versions through 5.13.8 and requires CAP_SYS_ADMIN privileges for practical exploitation, making it primarily a concern for systems where users have elevated privileges.

📋 TL;DR

CVE-2021-38166 is an integer overflow vulnerability in the Linux kernel's BPF hashtab implementation that can lead to out-of-bounds writes. This vulnerability affects Linux kernel versions through 5.13.8 and requires CAP_SYS_ADMIN privileges for practical exploitation, making it primarily a concern for systems where users have elevated privileges.

💻 Affected Systems

Products:

Linux kernel

Versions: Linux kernel through version 5.13.8

Operating Systems: Linux distributions using affected kernel versions

Default Config Vulnerable: ⚠️ Yes

Notes: Exploitation requires CAP_SYS_ADMIN capability, which is typically restricted to root and certain system services.

📦 What is this software?

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⚠️ Risk & Real-World Impact

🔴

Worst Case

Local privilege escalation to kernel-level code execution, potentially leading to full system compromise.

🟠

Likely Case

Kernel panic leading to denial of service (system crash) when exploited by users with CAP_SYS_ADMIN capabilities.

🟢

If Mitigated

Limited impact if proper privilege separation is enforced and users don't have CAP_SYS_ADMIN capabilities.

🌐 Internet-Facing: LOW - Requires local access and specific privileges for exploitation.

🏢 Internal Only: MEDIUM - Could be exploited by malicious insiders or compromised accounts with elevated privileges.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: UNKNOWN

Unauthenticated Exploit: ✅ No

Complexity: MEDIUM

Exploitation requires local access and CAP_SYS_ADMIN privileges. The vulnerability involves triggering specific BPF hashtab operations to cause integer overflow.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Linux kernel 5.13.9 and later

Vendor Advisory: https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git/commit/?id=c4eb1f403243fc7bbb7de644db8587c03de36da6

Restart Required: Yes

Instructions:

1. Update Linux kernel to version 5.13.9 or later. 2. For distributions: Use package manager (apt/yum/dnf) to update kernel package. 3. Reboot system to load new kernel.

🔧 Temporary Workarounds

Restrict CAP_SYS_ADMIN

linux

Limit CAP_SYS_ADMIN capabilities to essential users and processes only

# Review and modify capabilities using setcap or security modules
# Example: Remove CAP_SYS_ADMIN from non-essential users

Disable BPF for non-root users

linux

Restrict BPF system calls to privileged users only

sysctl -w kernel.unprivileged_bpf_disabled=1
# Add to /etc/sysctl.conf: kernel.unprivileged_bpf_disabled=1

🧯 If You Can't Patch

Implement strict privilege separation and limit CAP_SYS_ADMIN to essential services only
Monitor for unusual BPF-related system calls and kernel panic events

🔍 How to Verify

Check if Vulnerable:

Check kernel version: uname -r. If version is 5.13.8 or earlier, system is vulnerable.

Check Version:

uname -r

Verify Fix Applied:

After update, verify kernel version is 5.13.9 or later with: uname -r

📡 Detection & Monitoring

Log Indicators:

Kernel panic logs
OOM killer messages related to BPF
System crashes with hashtab.c references

Network Indicators:

No network indicators - local vulnerability only

SIEM Query:

Search for: kernel panic OR oops AND (bpf OR hashtab) OR system crash events from affected kernel versions

📊 Metadata

CVE ID: CVE-2021-38166

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-190

Published: August 7, 2021

Last Updated: November 21, 2024

🔗 References

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