CVE-2021-36159 – CVE-2021-36159 is an out-of-bounds read vulnera... (How to Fix)

Q: What is the severity of CVE-2021-36159?

CVE-2021-36159 has a CVSS score of 9.1 (CRITICAL). CVE-2021-36159 is an out-of-bounds read vulnerability in libfetch's FTP passive mode implementation that occurs when parsing numeric strings. This vulnerability affects systems using libfetch (such as apk-tools, xbps, and other products) and could allow attackers to read sensitive memory contents. The issue stems from improper boundary checking when using strtol to parse address bytes.

📋 TL;DR

CVE-2021-36159 is an out-of-bounds read vulnerability in libfetch's FTP passive mode implementation that occurs when parsing numeric strings. This vulnerability affects systems using libfetch (such as apk-tools, xbps, and other products) and could allow attackers to read sensitive memory contents. The issue stems from improper boundary checking when using strtol to parse address bytes.

💻 Affected Systems

Products:

apk-tools
xbps
other products using libfetch

Versions: libfetch versions before 2021-07-26

Operating Systems: Alpine Linux, FreeBSD, other systems using affected packages

Default Config Vulnerable: ⚠️ Yes

Notes: Vulnerable when using FTP or HTTP protocols through libfetch library.

📦 What is this software?

Libfetch by Freebsd

View all CVEs affecting Libfetch →

⚠️ Risk & Real-World Impact

🔴

Worst Case

Remote code execution or sensitive information disclosure through memory read operations, potentially leading to full system compromise.

🟠

Likely Case

Information disclosure through memory reads, potentially exposing sensitive data like credentials or cryptographic keys.

🟢

If Mitigated

Limited impact with proper network segmentation and minimal exposure of affected services.

🌐 Internet-Facing: HIGH

🏢 Internal Only: MEDIUM

🎯 Exploit Status

Public PoC: ⚠️ Yes

Weaponized: LIKELY

Unauthenticated Exploit: ⚠️ Yes

Complexity: LOW

Exploitation requires network access to FTP/HTTP services using vulnerable libfetch implementation.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: libfetch 2021-07-26 or later

Vendor Advisory: https://gitlab.alpinelinux.org/alpine/apk-tools/-/issues/10749

Restart Required: Yes

Instructions:

1. Update libfetch to version 2021-07-26 or later. 2. Update dependent packages (apk-tools, xbps). 3. Restart affected services.

🔧 Temporary Workarounds

Disable FTP passive mode

all

Configure FTP clients to not use passive mode where possible

Network segmentation

all

Restrict network access to affected services

🧯 If You Can't Patch

Implement strict network access controls to limit exposure
Monitor for unusual FTP/HTTP traffic patterns and memory access attempts

🔍 How to Verify

Check if Vulnerable:

Check libfetch version: grep -r 'libfetch' /etc/apk/world or check package manager for libfetch version

Check Version:

apk info -a libfetch | grep version

Verify Fix Applied:

Verify libfetch version is 2021-07-26 or later: apk version libfetch or equivalent package manager command

📡 Detection & Monitoring

Log Indicators:

Unusual FTP passive mode connection attempts
Memory access errors in system logs

Network Indicators:

Suspicious FTP PORT/PASV command sequences
Unexpected outbound connections from affected services

SIEM Query:

source="*ftp*" AND ("PASV" OR "PORT") AND dest_ip="[internal_ip]"

📊 Metadata

CVE ID: CVE-2021-36159

CVSS v3 Score: 9.1 (CRITICAL)

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

CWE: CWE-125

Published: August 3, 2021

Last Updated: November 21, 2024

🔗 References

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🔗 Related Vulnerabilities

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