CVE-2020-20341
📋 TL;DR
YzmCMS v5.5 contains a server-side request forgery (SSRF) vulnerability in the grab_image() function that allows attackers to make arbitrary HTTP requests from the vulnerable server. This can lead to internal network scanning, service enumeration, and potential data exfiltration. All users running YzmCMS v5.5 are affected.
💻 Affected Systems
- YzmCMS
📦 What is this software?
Yzmcms by Yzmcms
⚠️ Risk & Real-World Impact
Worst Case
Attackers could pivot to internal systems, access sensitive internal services, perform port scanning, or exploit other vulnerabilities on internal network resources.
Likely Case
Internal network reconnaissance, accessing metadata services (like AWS/Azure instance metadata), or interacting with internal APIs to gather information.
If Mitigated
Limited impact if network segmentation restricts outbound connections and internal services require authentication.
🎯 Exploit Status
Exploitation requires access to the vulnerable function, which typically requires some level of authentication or specific user role in YzmCMS.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: v5.6 or later
Vendor Advisory: https://github.com/yzmcms/yzmcms/issues/44
Restart Required: No
Instructions:
1. Upgrade YzmCMS to version 5.6 or later. 2. Download the latest version from the official repository. 3. Replace the affected files, particularly those containing the grab_image() function. 4. Verify the fix by testing the SSRF vulnerability.
🔧 Temporary Workarounds
Input Validation and Filtering
allImplement strict input validation on URLs passed to grab_image() function to block internal IP addresses and restricted protocols.
Network Segmentation
allRestrict outbound network connections from the YzmCMS server to only necessary external services.
🧯 If You Can't Patch
- Disable or restrict access to the grab_image() function if not required
- Implement web application firewall (WAF) rules to detect and block SSRF attempts
🔍 How to Verify
Check if Vulnerable:
Check if YzmCMS version is 5.5 by examining the version file or admin panel. Test the grab_image() function with internal URLs to see if it makes requests.Check Version:
Check the version.php file or admin dashboard for YzmCMS version information.Verify Fix Applied:
After patching, attempt to exploit the SSRF vulnerability with internal URLs. The system should reject or sanitize the input.📡 Detection & Monitoring
Log Indicators:
- Unusual outbound HTTP requests from the YzmCMS server to internal IP addresses
- Multiple failed attempts to access internal services
Network Indicators:
- HTTP requests from YzmCMS server to internal network ranges
- Port scanning patterns originating from the YzmCMS server
SIEM Query:
source_ip=[YzmCMS_Server_IP] AND (dest_ip=10.* OR dest_ip=172.16.* OR dest_ip=192.168.*) AND http_method=GET