CVE-2020-25212 – This CVE describes a Time-of-Check Time-of-Use ... (How to Fix)

Q: What is the severity of CVE-2020-25212?

CVE-2020-25212 has a CVSS score of 7.0 (HIGH). This CVE describes a Time-of-Check Time-of-Use (TOCTOU) vulnerability in the Linux kernel's NFS client code that allows local attackers to corrupt kernel memory. The vulnerability exists because a size check is incorrectly placed in fs/nfs/nfs4proc.c instead of fs/nfs/nfs4xdr.c, creating a race condition window. Systems running Linux kernels before version 5.8.3 with NFS client functionality are affected.

📋 TL;DR

This CVE describes a Time-of-Check Time-of-Use (TOCTOU) vulnerability in the Linux kernel's NFS client code that allows local attackers to corrupt kernel memory. The vulnerability exists because a size check is incorrectly placed in fs/nfs/nfs4proc.c instead of fs/nfs/nfs4xdr.c, creating a race condition window. Systems running Linux kernels before version 5.8.3 with NFS client functionality are affected.

💻 Affected Systems

Products:

Linux kernel

Versions: All versions before 5.8.3

Operating Systems: Linux distributions using affected kernel versions

Default Config Vulnerable: ⚠️ Yes

Notes: Only affects systems with NFS client functionality enabled/used. Most Linux distributions include NFS client support 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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Ubuntu Linux by Canonical

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Ubuntu Linux by Canonical

View all CVEs affecting Ubuntu Linux →

Ubuntu Linux by Canonical

View all CVEs affecting Ubuntu Linux →

Ubuntu Linux by Canonical

View all CVEs affecting Ubuntu Linux →

⚠️ Risk & Real-World Impact

🔴

Worst Case

Local privilege escalation to root, complete system compromise, or kernel panic causing denial of service.

🟠

Likely Case

Kernel memory corruption leading to system instability, crashes, or limited privilege escalation depending on exploitation success.

🟢

If Mitigated

Minimal impact if proper access controls restrict local user accounts and NFS client usage is limited.

🌐 Internet-Facing: LOW - This is a local attack vector requiring local system access.

🏢 Internal Only: MEDIUM - Internal users with local shell access could exploit this vulnerability.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: UNKNOWN

Unauthenticated Exploit: ✅ No

Complexity: MEDIUM

Exploitation requires local access and knowledge of TOCTOU race conditions. No public exploits have been documented.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Linux kernel 5.8.3 and later

Vendor Advisory: https://cdn.kernel.org/pub/linux/kernel/v5.x/ChangeLog-5.8.3

Restart Required: Yes

Instructions:

1. Update kernel to version 5.8.3 or later. 2. For distributions: Use package manager (apt/yum/dnf) to update kernel package. 3. Reboot system to load new kernel.

🔧 Temporary Workarounds

Disable NFS client module

linux

Prevent loading of NFS client kernel module to eliminate attack surface

echo 'blacklist nfs' >> /etc/modprobe.d/blacklist-nfs.conf
rmmod nfs

Restrict local user access

all

Limit shell access to trusted users only

🧯 If You Can't Patch

Implement strict access controls to limit local user accounts
Monitor for unusual NFS client activity and system crashes

🔍 How to Verify

Check if Vulnerable:

Check kernel version: uname -r. If version is earlier than 5.8.3, system is vulnerable if NFS client is used.

Check Version:

uname -r

Verify Fix Applied:

After update, verify kernel version is 5.8.3 or later with: uname -r

📡 Detection & Monitoring

Log Indicators:

Kernel panic logs
OOM killer activity related to NFS
System crashes after NFS operations

Network Indicators:

Unusual NFS mount requests from local users

SIEM Query:

source="kernel" AND ("panic" OR "Oops") AND "nfs"

📊 Metadata

CVE ID: CVE-2020-25212

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-367

Published: September 9, 2020

Last Updated: November 21, 2024

🔗 References

http://lists.opensuse.org/opensuse-security-announce/2020-10/msg00021.html Mailing List,Third Party Advisory
http://lists.opensuse.org/opensuse-security-announce/2020-10/msg00035.html Mailing List,Third Party Advisory
http://lists.opensuse.org/opensuse-security-announce/2020-10/msg00042.html Mailing List,Third Party Advisory
https://cdn.kernel.org/pub/linux/kernel/v5.x/ChangeLog-5.8.3 Mailing List,Vendor Advisory
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=b4487b93545214a9db8cbf32e86411677b0cca21 Mailing List,Patch,Vendor Advisory
https://lists.debian.org/debian-lts-announce/2020/09/msg00025.html Mailing List,Third Party Advisory
https://lists.debian.org/debian-lts-announce/2020/10/msg00032.html Mailing List,Third Party Advisory
https://lists.debian.org/debian-lts-announce/2020/10/msg00034.html Mailing List,Third Party Advisory
https://twitter.com/grsecurity/status/1303370421958578179 Third Party Advisory
https://usn.ubuntu.com/4525-1/ Third Party Advisory
https://usn.ubuntu.com/4527-1/ Third Party Advisory
https://usn.ubuntu.com/4578-1/ Third Party Advisory
http://lists.opensuse.org/opensuse-security-announce/2020-10/msg00021.html Mailing List,Third Party Advisory
http://lists.opensuse.org/opensuse-security-announce/2020-10/msg00035.html Mailing List,Third Party Advisory
http://lists.opensuse.org/opensuse-security-announce/2020-10/msg00042.html Mailing List,Third Party Advisory
https://cdn.kernel.org/pub/linux/kernel/v5.x/ChangeLog-5.8.3 Mailing List,Vendor Advisory
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=b4487b93545214a9db8cbf32e86411677b0cca21 Mailing List,Patch,Vendor Advisory
https://lists.debian.org/debian-lts-announce/2020/09/msg00025.html Mailing List,Third Party Advisory
https://lists.debian.org/debian-lts-announce/2020/10/msg00032.html Mailing List,Third Party Advisory
https://lists.debian.org/debian-lts-announce/2020/10/msg00034.html Mailing List,Third Party Advisory
https://twitter.com/grsecurity/status/1303370421958578179 Third Party Advisory
https://usn.ubuntu.com/4525-1/ Third Party Advisory
https://usn.ubuntu.com/4527-1/ Third Party Advisory
https://usn.ubuntu.com/4578-1/ Third Party Advisory

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💻 Affected Systems

📦 What is this software?

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⚠️ Risk & Real-World Impact

Worst Case

Likely Case

If Mitigated

🎯 Exploit Status

🛠️ Fix & Mitigation

✅ Official Fix

Instructions:

🔧 Temporary Workarounds

Disable NFS client module

Restrict local user access

🧯 If You Can't Patch

🔍 How to Verify

Check if Vulnerable:

Check Version:

Verify Fix Applied:

📡 Detection & Monitoring

Log Indicators:

Network Indicators:

SIEM Query:

📊 Metadata

🔗 References

📤 Share & Export

🔗 Related Vulnerabilities