CVE-2021-20276 – This vulnerability in Privoxy allows an attacke... (How to Fix)

Q: What is the severity of CVE-2021-20276?

CVE-2021-20276 has a CVSS score of 7.5 (HIGH). This vulnerability in Privoxy allows an attacker to cause denial of service by passing invalid patterns to the pcre_compile() function, leading to invalid memory access. It affects Privoxy web proxy servers running versions before 3.0.32. Organizations using vulnerable Privoxy instances as forward or reverse proxies are at risk.

📋 TL;DR

This vulnerability in Privoxy allows an attacker to cause denial of service by passing invalid patterns to the pcre_compile() function, leading to invalid memory access. It affects Privoxy web proxy servers running versions before 3.0.32. Organizations using vulnerable Privoxy instances as forward or reverse proxies are at risk.

💻 Affected Systems

Products:

Privoxy

Versions: All versions before 3.0.32

Operating Systems: Linux, BSD, Unix-like systems

Default Config Vulnerable: ⚠️ Yes

Notes: All Privoxy configurations are vulnerable regardless of settings. The vulnerability is in the core pattern compilation code.

📦 What is this software?

Debian Linux by Debian

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Privoxy by Privoxy

View all CVEs affecting Privoxy →

⚠️ Risk & Real-World Impact

🔴

Worst Case

Complete service disruption of Privoxy proxy, affecting all web traffic routing through it, potentially causing business interruption for dependent services.

🟠

Likely Case

Privoxy process crashes, requiring manual restart and causing temporary service interruption for users relying on the proxy.

🟢

If Mitigated

Minimal impact with proper monitoring and automated restart mechanisms in place.

🌐 Internet-Facing: HIGH - Privoxy is often deployed as an internet-facing proxy, making it directly accessible to attackers.

🏢 Internal Only: MEDIUM - Internal-only deployments reduce attack surface but still vulnerable to internal threats.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: UNKNOWN

Unauthenticated Exploit: ⚠️ Yes

Complexity: LOW

The vulnerability can be triggered by sending specially crafted patterns to Privoxy's pattern matching engine, which doesn't require authentication.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: 3.0.32 and later

Vendor Advisory: https://www.privoxy.org/announce.txt

Restart Required: Yes

Instructions:

1. Download Privoxy 3.0.32 or later from https://www.privoxy.org. 2. Stop the current Privoxy service. 3. Install the new version. 4. Restart the Privoxy service.

🔧 Temporary Workarounds

Restrict pattern input

all

Implement input validation to filter or reject complex patterns before they reach pcre_compile()

# Requires custom configuration or wrapper script around Privoxy

Process monitoring and auto-restart

linux

Set up monitoring to automatically restart Privoxy if it crashes

systemctl enable privoxy
# Configure monitoring with tools like monit or supervisor

🧯 If You Can't Patch

Implement network segmentation to isolate Privoxy instances from untrusted networks
Deploy rate limiting and input filtering at network perimeter to block malicious patterns

🔍 How to Verify

Check if Vulnerable:

Check Privoxy version with: privoxy --version | grep -i version

Check Version:

privoxy --version

Verify Fix Applied:

Verify version is 3.0.32 or higher: privoxy --version

📡 Detection & Monitoring

Log Indicators:

Privoxy process crashes
Segmentation fault errors in system logs
Unexpected service restarts

Network Indicators:

Sudden drop in proxy traffic
Connection timeouts to Privoxy service

SIEM Query:

source="privoxy.log" AND ("segmentation fault" OR "crash" OR "pcre_compile")

📊 Metadata

CVE ID: CVE-2021-20276

CVSS v3 Score: 7.5 (HIGH)

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

CWE: CWE-119

Published: March 9, 2021

Last Updated: November 21, 2024

🔗 References

https://bugzilla.redhat.com/show_bug.cgi?id=1936668 Issue Tracking,Patch,Third Party Advisory
https://lists.debian.org/debian-lts-announce/2021/03/msg00009.html Third Party Advisory
https://security.gentoo.org/glsa/202107-16 Third Party Advisory
https://www.privoxy.org/announce.txt Release Notes,Vendor Advisory
https://bugzilla.redhat.com/show_bug.cgi?id=1936668 Issue Tracking,Patch,Third Party Advisory
https://lists.debian.org/debian-lts-announce/2021/03/msg00009.html Third Party Advisory
https://security.gentoo.org/glsa/202107-16 Third Party Advisory
https://www.privoxy.org/announce.txt Release Notes,Vendor Advisory

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🔗 Related Vulnerabilities

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