CVE-2021-20215
📋 TL;DR
This vulnerability in Privoxy versions before 3.0.29 involves memory leaks in the show-status CGI handler when memory allocations fail, which can lead to a denial of service (system crash). It affects systems running vulnerable Privoxy versions with the CGI handler enabled. The flaw allows attackers to crash the Privoxy service, disrupting proxy functionality.
💻 Affected Systems
- Privoxy
📦 What is this software?
Privoxy by Privoxy
⚠️ Risk & Real-World Impact
Worst Case
An attacker could repeatedly trigger the memory leak to cause a complete system crash, resulting in prolonged denial of service for all users relying on Privoxy for web filtering or privacy.
Likely Case
Targeted attacks causing service disruption through denial of service, potentially affecting web access for users behind the proxy.
If Mitigated
With proper network segmentation and access controls, impact is limited to internal service disruption without broader system compromise.
🎯 Exploit Status
Exploitation requires sending requests to trigger memory allocation failures in the CGI handler. No authentication is needed if the handler is exposed.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 3.0.29
Vendor Advisory: https://www.privoxy.org/3.0.29/user-manual/whatsnew.html
Restart Required: Yes
Instructions:
1. Download Privoxy 3.0.29 or later from the official website. 2. Stop the current Privoxy service. 3. Install the updated version. 4. Restart the Privoxy service.
🔧 Temporary Workarounds
Disable show-status CGI handler
allPrevent access to the vulnerable CGI handler by disabling it in the configuration.
Edit privoxy config file (e.g., /etc/privoxy/config) and comment out or remove lines enabling show-status, such as 'enable-remote-toggle' or 'enable-edit-actions'.
🧯 If You Can't Patch
- Restrict network access to Privoxy to trusted internal networks only.
- Implement rate limiting or firewall rules to block excessive requests to the CGI handler.
🔍 How to Verify
Check if Vulnerable:
Check the Privoxy version; if it is earlier than 3.0.29, the system is vulnerable.Check Version:
privoxy --versionVerify Fix Applied:
Verify the installed version is 3.0.29 or later and test the show-status handler for stability under load.📡 Detection & Monitoring
Log Indicators:
- Unusual number of requests to the show-status CGI endpoint
- Privoxy service crash logs or restarts
Network Indicators:
- High volume of HTTP requests to Privoxy's CGI paths from single or multiple sources
SIEM Query:
source="privoxy" AND (uri_path="/show-status" OR event="crash")🔗 References
- https://bugzilla.redhat.com/show_bug.cgi?id=1928746
- https://security.gentoo.org/glsa/202107-16
- https://www.privoxy.org/3.0.29/user-manual/whatsnew.html
- https://bugzilla.redhat.com/show_bug.cgi?id=1928746
- https://security.gentoo.org/glsa/202107-16
- https://www.privoxy.org/3.0.29/user-manual/whatsnew.html
