CVE-2021-20272
📋 TL;DR
This vulnerability in Privoxy allows remote attackers to crash the proxy server by sending a specially crafted CGI request that triggers an assertion failure. It affects all Privoxy installations before version 3.0.32. The server crash leads to denial of service for all users relying on the proxy.
💻 Affected Systems
- Privoxy
📦 What is this software?
Privoxy by Privoxy
⚠️ Risk & Real-World Impact
Worst Case
Complete denial of service for all users relying on the proxy, potentially disrupting internet access for entire organizations if Privoxy is deployed as a gateway proxy.
Likely Case
Temporary service disruption until the server is restarted, affecting users who depend on the proxy for web access.
If Mitigated
Minimal impact if the proxy is behind a firewall with restricted CGI access or if request filtering is implemented.
🎯 Exploit Status
The exploit requires sending a crafted HTTP request to the CGI interface, which is straightforward to implement.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 3.0.32
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 following platform-specific instructions. 4. Restart the Privoxy service.
🔧 Temporary Workarounds
Disable CGI Interface
linuxDisable CGI functionality in Privoxy configuration to prevent exploitation.
Edit privoxy config file and set 'enable-remote-toggle 0' and 'enable-edit-actions 0'
Restart privoxy: systemctl restart privoxy
Restrict CGI Access
linuxUse firewall rules to restrict access to Privoxy CGI port (typically 8118) to trusted IPs only.
iptables -A INPUT -p tcp --dport 8118 -s TRUSTED_IP -j ACCEPT
iptables -A INPUT -p tcp --dport 8118 -j DROP
🧯 If You Can't Patch
- Implement network segmentation to isolate Privoxy servers from untrusted networks
- Deploy a web application firewall (WAF) in front of Privoxy to filter malicious CGI requests
🔍 How to Verify
Check if Vulnerable:
Check Privoxy version: privoxy --version. If version is below 3.0.32, the system is vulnerable.Check Version:
privoxy --versionVerify Fix Applied:
After patching, verify version is 3.0.32 or higher with: privoxy --version📡 Detection & Monitoring
Log Indicators:
- Privoxy crash logs
- Assertion failure messages in system logs
- Unexpected service restarts
Network Indicators:
- HTTP requests to CGI endpoints with unusual patterns
- Multiple connection attempts to Privoxy port 8118
SIEM Query:
source="privoxy.log" AND ("assertion failure" OR "crash" OR "aborted")🔗 References
- https://bugzilla.redhat.com/show_bug.cgi?id=1936651
- https://lists.debian.org/debian-lts-announce/2021/03/msg00009.html
- https://security.gentoo.org/glsa/202107-16
- https://www.privoxy.org/announce.txt
- https://bugzilla.redhat.com/show_bug.cgi?id=1936651
- https://lists.debian.org/debian-lts-announce/2021/03/msg00009.html
- https://security.gentoo.org/glsa/202107-16
- https://www.privoxy.org/announce.txt
