CVE-2021-41732
📋 TL;DR
CVE-2021-41732 is an HTTP request splitting vulnerability in Zeek 4.1.0 that allows attackers to manipulate HTTP traffic analysis by injecting malicious content into HTTP requests. This affects organizations using Zeek for network security monitoring and HTTP-based security analysis. The vendor considers this intended behavior rather than a vulnerability.
💻 Affected Systems
- Zeek (formerly Bro)
📦 What is this software?
Zeek by Zeek
⚠️ Risk & Real-World Impact
Worst Case
Attackers could bypass Zeek's HTTP security analysis entirely, allowing malicious traffic to go undetected while appearing legitimate in security logs.
Likely Case
HTTP-based security alerts and analysis become unreliable, potentially missing actual threats or generating false positives.
If Mitigated
With proper network segmentation and additional security controls, the impact is limited to potential gaps in HTTP traffic analysis.
🎯 Exploit Status
Exploitation requires ability to send crafted HTTP requests to monitored network segments. No public exploit code available.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: N/A
Vendor Advisory: https://github.com/zeek/zeek/issues/1798
Restart Required: No
Instructions:
No official patch as vendor considers this intended behavior. Consider upgrading to newer versions for other improvements.
🔧 Temporary Workarounds
Disable HTTP protocol analysis
allRemove or disable Zeek's HTTP protocol analyzer to prevent the vulnerability from being triggered
Edit local.zeek to remove @load policy/protocols/http
Use alternative HTTP monitoring
allSupplement Zeek with alternative HTTP traffic analysis tools
🧯 If You Can't Patch
- Implement network segmentation to limit exposure of Zeek monitoring interfaces
- Deploy additional HTTP security monitoring solutions to compensate for potential Zeek analysis gaps
🔍 How to Verify
Check if Vulnerable:
Check Zeek version: zeek --version. If version is exactly 4.1.0 and HTTP analysis is enabled, system is affected.Check Version:
zeek --versionVerify Fix Applied:
Verify HTTP analysis is disabled or alternative monitoring is in place. No official fix exists.📡 Detection & Monitoring
Log Indicators:
- Unusual HTTP request patterns, malformed HTTP headers in Zeek logs
- Gaps in HTTP traffic analysis logs
Network Indicators:
- HTTP requests with crafted headers designed to split requests
- Inconsistent HTTP traffic patterns
SIEM Query:
source="zeek" http.* AND (header_manipulation OR request_splitting)