CVE-2021-4069 – CVE-2021-4069 is a use-after-free vulnerability... (How to Fix)

Q: What is the severity of CVE-2021-4069?

CVE-2021-4069 has a CVSS score of 7.8 (HIGH). CVE-2021-4069 is a use-after-free vulnerability in Vim that could allow an attacker to execute arbitrary code by tricking a user into opening a specially crafted file. This affects users who open untrusted files with vulnerable Vim versions. The vulnerability occurs due to improper memory handling in Vim's text processing.

📋 TL;DR

CVE-2021-4069 is a use-after-free vulnerability in Vim that could allow an attacker to execute arbitrary code by tricking a user into opening a specially crafted file. This affects users who open untrusted files with vulnerable Vim versions. The vulnerability occurs due to improper memory handling in Vim's text processing.

💻 Affected Systems

Products:

Vim
Neovim (potentially affected)
Applications embedding Vim libraries

Versions: Vim versions before 8.2.4069

Operating Systems: Linux, Unix-like systems, Windows, macOS

Default Config Vulnerable: ⚠️ Yes

Notes: All default configurations are vulnerable. The vulnerability is triggered when processing specially crafted text files.

📦 What is this software?

⚠️ Risk & Real-World Impact

🔴

Worst Case

Remote code execution with the privileges of the user running Vim, potentially leading to full system compromise.

🟠

Likely Case

Application crash (denial of service) or limited code execution within Vim's context.

🟢

If Mitigated

No impact if users don't open untrusted files or if proper file integrity controls are in place.

🌐 Internet-Facing: LOW - Vim is typically not directly internet-facing, though could be exploited via file uploads to web applications.

🏢 Internal Only: MEDIUM - Internal users could be targeted via phishing or shared malicious files.

🎯 Exploit Status

Public PoC: ⚠️ Yes

Weaponized: LIKELY

Unauthenticated Exploit: ⚠️ Yes

Complexity: MEDIUM

Exploitation requires user interaction (opening a malicious file). Proof-of-concept code is available in public repositories.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: Vim 8.2.4069 and later

Vendor Advisory: https://github.com/vim/vim/commit/e031fe90cf2e375ce861ff5e5e281e4ad229ebb9

Restart Required: No

Instructions:

1. Update Vim using your system's package manager. 2. For Debian/Ubuntu: sudo apt update && sudo apt upgrade vim. 3. For RHEL/CentOS: sudo yum update vim-enhanced. 4. For macOS: brew upgrade vim. 5. For Windows: Download latest version from vim.org.

🔧 Temporary Workarounds

Disable modeline processing

all

Prevents Vim from executing commands embedded in files

Add 'set nomodeline' to ~/.vimrc

Use secure mode

all

Restricts potentially dangerous operations

Run Vim with: vim -Z (restricted mode)

🧯 If You Can't Patch

Restrict Vim usage to trusted users only
Implement application whitelisting to prevent execution of vulnerable Vim versions

🔍 How to Verify

Check if Vulnerable:

Check Vim version with: vim --version | head -1

Check Version:

vim --version | head -1

Verify Fix Applied:

Verify version is 8.2.4069 or higher: vim --version | grep 'Included patches'

📡 Detection & Monitoring

Log Indicators:

Vim process crashes with segmentation faults
Unexpected child processes spawned from Vim

Network Indicators:

Unusual outbound connections from systems where users run Vim

SIEM Query:

Process creation where parent process contains 'vim' AND (command line contains suspicious file paths OR child process is unusual)

📊 Metadata

CVE ID: CVE-2021-4069

CVSS v3 Score: 7.8 (HIGH)

CVSS Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

CWE: CWE-416

Published: December 6, 2021

Last Updated: November 21, 2024

🔗 References

http://www.openwall.com/lists/oss-security/2022/01/15/1 Mailing List,Third Party Advisory
https://github.com/vim/vim/commit/e031fe90cf2e375ce861ff5e5e281e4ad229ebb9 Patch,Third Party Advisory
https://huntr.dev/bounties/0efd6d23-2259-4081-9ff1-3ade26907d74 Exploit,Patch,Third Party Advisory
https://lists.debian.org/debian-lts-announce/2022/03/msg00018.html Mailing List,Third Party Advisory
https://lists.debian.org/debian-lts-announce/2022/11/msg00009.html Mailing List,Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/FNXY7T5OORA7UJIMGSJBGHFMU6UZWS6P/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/WYB2LLNUFJUKJJ5HYCZ6MV3Z6YX3U5BN/
https://security.gentoo.org/glsa/202208-32 Third Party Advisory
http://www.openwall.com/lists/oss-security/2022/01/15/1 Mailing List,Third Party Advisory
https://github.com/vim/vim/commit/e031fe90cf2e375ce861ff5e5e281e4ad229ebb9 Patch,Third Party Advisory
https://huntr.dev/bounties/0efd6d23-2259-4081-9ff1-3ade26907d74 Exploit,Patch,Third Party Advisory
https://lists.debian.org/debian-lts-announce/2022/03/msg00018.html Mailing List,Third Party Advisory
https://lists.debian.org/debian-lts-announce/2022/11/msg00009.html Mailing List,Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/FNXY7T5OORA7UJIMGSJBGHFMU6UZWS6P/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/WYB2LLNUFJUKJJ5HYCZ6MV3Z6YX3U5BN/
https://security.gentoo.org/glsa/202208-32 Third Party Advisory

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