CVE-2021-20217 – This vulnerability in Privoxy allows attackers ... (How to Fix)

Q: What is the severity of CVE-2021-20217?

CVE-2021-20217 has a CVSS score of 7.5 (HIGH). This vulnerability in Privoxy allows attackers to trigger an assertion failure via a crafted CGI request, causing a denial of service. Systems running Privoxy versions before 3.0.31 are affected. The primary impact is service disruption.

📋 TL;DR

This vulnerability in Privoxy allows attackers to trigger an assertion failure via a crafted CGI request, causing a denial of service. Systems running Privoxy versions before 3.0.31 are affected. The primary impact is service disruption.

💻 Affected Systems

Products:

Privoxy

Versions: All versions before 3.0.31

Operating Systems: Linux, Unix-like systems, Windows

Default Config Vulnerable: ⚠️ Yes

Notes: Any Privoxy installation with CGI functionality enabled is vulnerable. The vulnerability is in the CGI request handling code.

📦 What is this software?

Privoxy by Privoxy

View all CVEs affecting Privoxy →

⚠️ Risk & Real-World Impact

🔴

Worst Case

Privoxy crashes completely, blocking all web proxy functionality and potentially requiring manual restart.

🟠

Likely Case

Targeted DoS attack against Privoxy instances, causing temporary service interruption.

🟢

If Mitigated

Minimal impact if patched or if workarounds are implemented to filter malicious requests.

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

🏢 Internal Only: MEDIUM - Internal attackers or compromised internal systems could exploit this vulnerability.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: LIKELY

Unauthenticated Exploit: ⚠️ Yes

Complexity: LOW

Crafting the malicious CGI request requires understanding of Privoxy's CGI interface but is straightforward for attackers.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: 3.0.31 and later

Vendor Advisory: https://security.gentoo.org/glsa/202107-16

Restart Required: Yes

Instructions:

1. Download Privoxy 3.0.31 or later from official sources. 2. Stop the Privoxy service. 3. Install the updated version. 4. Restart the Privoxy service.

🔧 Temporary Workarounds

Disable CGI functionality

all

Temporarily disable CGI support in Privoxy configuration to prevent exploitation.

Edit privoxy config file and comment out or remove CGI-related directives

Network filtering

linux

Use firewall rules to restrict access to Privoxy's CGI endpoints.

iptables -A INPUT -p tcp --dport 8118 -m string --string "cgi" --algo bm -j DROP

🧯 If You Can't Patch

Implement network segmentation to isolate Privoxy instances from untrusted networks
Monitor for abnormal traffic patterns or repeated connection attempts to CGI endpoints

🔍 How to Verify

Check if Vulnerable:

Check Privoxy version: privoxy --version. If version is below 3.0.31, system is vulnerable.

Check Version:

privoxy --version

Verify Fix Applied:

After patching, verify version is 3.0.31 or higher and test CGI functionality if needed.

📡 Detection & Monitoring

Log Indicators:

Privoxy crash logs
Assertion failure messages in system logs
Repeated CGI request failures

Network Indicators:

Unusual traffic patterns to Privoxy CGI endpoints
Multiple failed CGI requests from single sources

SIEM Query:

source="privoxy.log" AND ("assertion" OR "crash" OR "CGI failure")

📊 Metadata

CVE ID: CVE-2021-20217

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-617

Published: March 25, 2021

Last Updated: November 21, 2024

🔗 References

https://bugzilla.redhat.com/show_bug.cgi?id=1923252 Issue Tracking,Third Party Advisory
https://security.gentoo.org/glsa/202107-16 Third Party Advisory
https://bugzilla.redhat.com/show_bug.cgi?id=1923252 Issue Tracking,Third Party Advisory
https://security.gentoo.org/glsa/202107-16 Third Party Advisory

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🔗 Related Vulnerabilities

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