CVE-2021-33580
📋 TL;DR
This CVE describes a Regular Expression Denial of Service (ReDoS) vulnerability in Apache Roller where user-controlled inputs (Referer header, request URL, and query string) are used to build and execute regex patterns. Attackers can send specially crafted requests to cause catastrophic backtracking, leading to server resource exhaustion and denial of service. This affects Apache Roller installations prior to version 6.0.2.
💻 Affected Systems
- Apache Roller
📦 What is this software?
Roller by Apache
⚠️ Risk & Real-World Impact
Worst Case
Complete server unavailability due to CPU exhaustion from regex processing, potentially affecting all users and requiring server restart.
Likely Case
Degraded server performance or temporary service disruption for legitimate users while the regex engine processes malicious input.
If Mitigated
Minimal impact with proper input validation and regex optimization in place.
🎯 Exploit Status
Exploitation requires sending specially crafted HTTP requests with malicious Referer headers or URLs to trigger regex backtracking.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 6.0.2
Vendor Advisory: https://lists.apache.org/thread.html/r9d967d80af941717573e531db2c7353a90bfd0886e9b5d5d79f75506%40%3Cuser.roller.apache.org%3E
Restart Required: Yes
Instructions:
1. Download Apache Roller 6.0.2 or later from the official Apache website. 2. Backup your current Roller installation and database. 3. Replace the vulnerable Roller files with the patched version. 4. Restart your web application server.
🔧 Temporary Workarounds
Input Validation Filter
allImplement a web application filter to sanitize or reject malicious Referer headers and URLs before they reach the vulnerable regex processing.
Rate Limiting
allImplement rate limiting on HTTP requests to prevent attackers from sending multiple malicious requests that could trigger the ReDoS.
🧯 If You Can't Patch
- Implement a WAF (Web Application Firewall) with rules to block requests containing suspicious Referer headers or URL patterns.
- Monitor server CPU usage and set up alerts for abnormal regex processing spikes to detect potential exploitation attempts.
🔍 How to Verify
Check if Vulnerable:
Check your Apache Roller version. If it's earlier than 6.0.2, you are vulnerable.Check Version:
Check the Roller web application interface or configuration files for version information.Verify Fix Applied:
After patching, verify the version is 6.0.2 or later and test that the application still processes legitimate Referer headers and URLs correctly.📡 Detection & Monitoring
Log Indicators:
- Unusually long processing times for HTTP requests
- High CPU usage spikes correlated with specific request patterns
- Requests with malformed or extremely long Referer headers
Network Indicators:
- Multiple HTTP requests with crafted Referer headers from single IPs
- Abnormal request patterns targeting the vulnerable endpoints
SIEM Query:
source="web_server_logs" AND (referer="*[malicious_pattern]*" OR url="*[malicious_pattern]*") AND response_time>5000ms🔗 References
- http://www.openwall.com/lists/oss-security/2021/08/18/1
- https://lists.apache.org/thread.html/r9d967d80af941717573e531db2c7353a90bfd0886e9b5d5d79f75506%40%3Cuser.roller.apache.org%3E
- http://www.openwall.com/lists/oss-security/2021/08/18/1
- https://lists.apache.org/thread.html/r9d967d80af941717573e531db2c7353a90bfd0886e9b5d5d79f75506%40%3Cuser.roller.apache.org%3E
