CVE-2020-26174
📋 TL;DR
This vulnerability allows attackers to bypass client-side file upload restrictions in tangro Business Workflow, enabling them to upload any file type as attachments to workitems. Organizations using affected versions of tangro Business Workflow are at risk, particularly those allowing user file uploads.
💻 Affected Systems
- tangro Business Workflow
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Attackers could upload malicious files (webshells, malware) leading to remote code execution, data exfiltration, or complete system compromise.
Likely Case
Attackers upload malicious files that could be executed or used for phishing, data theft, or lateral movement within the network.
If Mitigated
With proper server-side validation and file type restrictions, impact is limited to potential storage abuse or minor data integrity issues.
🎯 Exploit Status
Exploitation requires bypassing client-side JavaScript validation, which can be done with browser developer tools or automated scripts.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 1.18.1
Vendor Advisory: https://www.tangro.de/
Restart Required: Yes
Instructions:
1. Backup current installation and data. 2. Download version 1.18.1 or later from tangro. 3. Follow vendor upgrade instructions. 4. Restart the application service. 5. Verify the fix by testing file upload restrictions.
🔧 Temporary Workarounds
Implement server-side file validation
allAdd server-side validation to check file types and extensions before accepting uploads
Restrict file upload permissions
allLimit which users can upload files and implement strict access controls
🧯 If You Can't Patch
- Implement web application firewall (WAF) rules to block suspicious file uploads
- Disable file upload functionality entirely if not required
🔍 How to Verify
Check if Vulnerable:
Attempt to upload a file type not in the allowed list by modifying the request (e.g., using Burp Suite or browser dev tools). If upload succeeds, system is vulnerable.Check Version:
Check application version in admin interface or configuration filesVerify Fix Applied:
Attempt the same bypass technique after patching; uploads of disallowed file types should be rejected with server-side validation.📡 Detection & Monitoring
Log Indicators:
- Unusual file uploads, especially with unexpected extensions
- Failed server-side validation attempts
- Large number of upload requests
Network Indicators:
- HTTP POST requests to upload endpoints with modified content-type or file extensions
- Uploads of executable files or scripts
SIEM Query:
source="web_server" AND (method="POST" AND uri CONTAINS "upload" OR "attachment") AND (file_extension IN ("exe", "php", "jsp", "asp", "sh", "bat"))