CVE-2021-33511
📋 TL;DR
CVE-2021-33511 is a Server-Side Request Forgery (SSRF) vulnerability in Plone CMS that allows attackers to make unauthorized requests from the server to internal or external systems. It affects Plone installations using Diazo themes, Dexterity TTW schemas, or modeleditors via the lxml parser. This vulnerability enables attackers to potentially access internal services, perform port scanning, or interact with cloud metadata services.
💻 Affected Systems
- Plone CMS
📦 What is this software?
Plone by Plone
⚠️ Risk & Real-World Impact
Worst Case
Attackers could access internal services, cloud metadata (potentially obtaining credentials), perform port scanning, or interact with internal APIs leading to data exfiltration or further compromise.
Likely Case
Unauthorized access to internal services, port scanning of internal networks, and potential interaction with cloud metadata services if deployed in cloud environments.
If Mitigated
Limited to port scanning and information disclosure about internal services, with no direct code execution or data access if proper network segmentation exists.
🎯 Exploit Status
Exploitation requires authenticated access to Plone with permissions to edit themes, schemas, or models. SSRF payloads can be injected via XML parsing.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Plone 5.2.5 and later
Vendor Advisory: https://plone.org/security/hotfix/20210518/server-side-request-forgery-via-lxml-parser
Restart Required: Yes
Instructions:
1. Update Plone to version 5.2.5 or later. 2. Apply the security hotfix if available. 3. Restart the Plone instance. 4. Verify the fix by checking the version and testing SSRF attempts.
🔧 Temporary Workarounds
Restrict XML External Entity Processing
allConfigure lxml parser to disable external entity resolution and restrict URL schemes
Modify Plone configuration to set lxml parser options: resolve_entities=False, no_network=True
Network Segmentation
allRestrict outbound network access from Plone servers to only necessary services
Configure firewall rules to block outbound HTTP/HTTPS from Plone to internal networks except required services
🧯 If You Can't Patch
- Disable Diazo themes, Dexterity TTW schemas, and modeleditors if not required
- Implement strict network egress filtering to limit Plone server outbound connections
🔍 How to Verify
Check if Vulnerable:
Check Plone version: if ≤5.2.4 and using affected features (Diazo, Dexterity TTW, modeleditors), system is vulnerable. Test with controlled SSRF payload to internal service.Check Version:
Check Plone control panel or run: bin/instance debug then print(app.__version__)Verify Fix Applied:
Verify Plone version is ≥5.2.5. Test SSRF attempts should fail or be blocked. Check that lxml parser settings restrict external entities.📡 Detection & Monitoring
Log Indicators:
- Unusual outbound HTTP requests from Plone server
- XML parsing errors with external entity references
- Requests to internal IP addresses or cloud metadata endpoints
Network Indicators:
- HTTP requests from Plone server to unexpected internal services
- Port scanning patterns from Plone server IP
SIEM Query:
source="plone-server" AND (dest_ip=~"10.*" OR dest_ip=~"192.168.*" OR dest_ip=~"172.16.*" OR dest_ip="169.254.169.254")