CVE-2026-23768
📋 TL;DR
This vulnerability in lucy-xss-filter allows attackers to trigger server-side HEAD requests to arbitrary URLs when specific security listeners are enabled and embed/object tags lack file extensions. It affects applications using vulnerable versions of lucy-xss-filter with ObjectSecurityListener or EmbedSecurityListener enabled.
💻 Affected Systems
- lucy-xss-filter
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Server-side request forgery (SSRF) leading to internal network reconnaissance, credential theft from metadata services, or denial of service through resource exhaustion.
Likely Case
Information disclosure through internal service responses or limited denial of service via resource consumption.
If Mitigated
Minimal impact if proper network segmentation and request filtering are implemented.
🎯 Exploit Status
Exploitation requires user input containing malformed embed/object tags, which is common in web applications.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Commit 7c1de6d or later
Vendor Advisory: https://github.com/naver/lucy-xss-filter/pull/31
Restart Required: Yes
Instructions:
1. Update lucy-xss-filter to version including commit 7c1de6d. 2. Restart affected applications. 3. Verify the fix by testing with malformed embed/object tags.
🔧 Temporary Workarounds
Disable vulnerable listeners
allTemporarily disable ObjectSecurityListener and EmbedSecurityListener if not essential.
Modify application configuration to set ObjectSecurityListener=false and EmbedSecurityListener=false
Input validation filter
allAdd pre-filtering to reject embed/object tags without file extensions in src attributes.
Implement custom filter before lucy-xss-filter: if (input.contains('<embed') || input.contains('<object')) { validate src attribute has file extension }
🧯 If You Can't Patch
- Implement network egress filtering to block unexpected outbound HTTP requests from application servers.
- Deploy WAF rules to detect and block malformed embed/object tags in user input.
🔍 How to Verify
Check if Vulnerable:
Check if lucy-xss-filter version predates commit 7c1de6d and if ObjectSecurityListener/EmbedSecurityListener are enabled in configuration.Check Version:
Check project dependencies or build files for lucy-xss-filter version; git log for commit 7c1de6d presence.Verify Fix Applied:
Test with payload containing <embed src="http://internal-service/"> (no extension) - should be blocked or sanitized.📡 Detection & Monitoring
Log Indicators:
- Unexpected HEAD requests from application server to internal/external URLs
- Errors from lucy-xss-filter processing embed/object tags
Network Indicators:
- HTTP HEAD requests from application servers to unusual destinations
- Increased outbound traffic from web servers
SIEM Query:
source="web-server" AND (http_method="HEAD" AND NOT destination_ip IN [allowed_ips])