CVE-2021-27850
📋 TL;DR
CVE-2021-27850 is a critical unauthenticated remote code execution vulnerability in Apache Tapestry that allows attackers to bypass previous security fixes and download sensitive class files containing HMAC secret keys. This affects all recent versions of Apache Tapestry including 5.4.5, 5.5.0, 5.6.2 and 5.7.0. Attackers can use the stolen secret key to sign malicious Java gadget chains and achieve remote code execution.
💻 Affected Systems
- Apache Tapestry
📦 What is this software?
Tapestry by Apache
Tapestry by Apache
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise via remote code execution, allowing attackers to execute arbitrary commands, install malware, steal data, and pivot to other systems.
Likely Case
Data exfiltration and server compromise through remote code execution, potentially leading to credential theft, data breaches, and backdoor installation.
If Mitigated
Limited impact with proper network segmentation and access controls, though sensitive information disclosure may still occur.
🎯 Exploit Status
Exploitation requires knowledge of Java gadget chains (e.g., CommonsBeanUtils1 from ysoserial) but the vulnerability itself is simple to trigger.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 5.6.2 for versions 5.4.0-5.6.1; 5.7.1 for version 5.7.0
Vendor Advisory: https://lists.apache.org/thread.html/r237ff7f286bda31682c254550c1ebf92b0ec61329b32fbeb2d1c8751%40%3Cusers.tapestry.apache.org%3E
Restart Required: Yes
Instructions:
1. Identify current Apache Tapestry version. 2. If version is 5.4.0-5.6.1, upgrade to 5.6.2 or later. 3. If version is 5.7.0, upgrade to 5.7.1 or later. 4. Restart the application server.
🔧 Temporary Workarounds
Web Application Firewall Rule
allBlock requests containing .class/ patterns in URLs
Add WAF rule to block URLs containing ".class/" or patterns matching asset file exploitation
Network Access Control
allRestrict access to Tapestry applications to trusted networks only
Configure firewall rules to limit access to Tapestry application ports
🧯 If You Can't Patch
- Implement strict network segmentation to isolate vulnerable Tapestry applications from sensitive systems
- Deploy web application firewall with rules to block exploitation patterns and monitor for attack attempts
🔍 How to Verify
Check if Vulnerable:
Check if application responds to crafted URL: http://[host]:[port]/assets/anything/services/AppModule.class/Check Version:
Check application configuration files or deployment descriptors for Tapestry versionVerify Fix Applied:
After patching, verify the crafted URL no longer returns the AppModule.class file📡 Detection & Monitoring
Log Indicators:
- HTTP requests containing ".class/" in URL paths
- Unusual file access patterns to class files from web requests
- Multiple failed attempts to access protected resources
Network Indicators:
- HTTP GET requests with .class/ patterns in asset paths
- Unusual traffic to /assets/ endpoints with crafted parameters
SIEM Query:
source="web_server_logs" AND (url="*.class/*" OR url="*/services/*.class/*")🔗 References
- http://www.openwall.com/lists/oss-security/2021/04/15/1
- https://lists.apache.org/thread.html/r237ff7f286bda31682c254550c1ebf92b0ec61329b32fbeb2d1c8751%40%3Cusers.tapestry.apache.org%3E
- https://security.netapp.com/advisory/ntap-20210528-0002/
- http://www.openwall.com/lists/oss-security/2021/04/15/1
- https://lists.apache.org/thread.html/r237ff7f286bda31682c254550c1ebf92b0ec61329b32fbeb2d1c8751%40%3Cusers.tapestry.apache.org%3E
- https://security.netapp.com/advisory/ntap-20210528-0002/
