CVE-2026-23768: lucy-xss-filter SSRF Vulnerability

CVE-2026-23768 Overview

CVE-2026-23768 is a Server-Side Request Forgery (SSRF) vulnerability discovered in the lucy-xss-filter library, a popular XSS sanitization component. The vulnerability allows attackers to induce server-side HEAD requests to arbitrary URLs when the ObjectSecurityListener or EmbedSecurityListener option is enabled and embed or object tags are used with a src attribute missing a file extension.

Critical Impact
Attackers can leverage this SSRF vulnerability to probe internal network resources, bypass firewall restrictions, and potentially access sensitive internal services that should not be externally reachable.

Affected Products

lucy-xss-filter versions before commit 7c1de6d
Applications using ObjectSecurityListener configuration
Applications using EmbedSecurityListener configuration

Discovery Timeline

2026-01-16 - CVE-2026-23768 published to NVD
2026-01-16 - Last updated in NVD database

Technical Details for CVE-2026-23768

Vulnerability Analysis

This vulnerability is classified under CWE-918 (Server-Side Request Forgery), which occurs when a web application fetches a remote resource without sufficiently validating the user-supplied URL. In the case of lucy-xss-filter, the security listeners (ObjectSecurityListener and EmbedSecurityListener) are designed to validate content embedded via object and embed HTML tags. However, when the src attribute lacks a file extension, the filter initiates a server-side HEAD request to verify the resource type, inadvertently allowing attackers to direct requests to arbitrary URLs.

The vulnerability requires user interaction as the malicious payload must be processed through the XSS filter. The changed scope indicates that the vulnerable component (lucy-xss-filter) can affect resources beyond its security scope, enabling attackers to reach internal network services or cloud metadata endpoints.

Root Cause

The root cause lies in the URL validation logic within the security listeners. When processing embed or object tags with src attributes that do not contain a file extension, the filter attempts to determine the content type by making a HEAD request to the specified URL. This behavior was intended to enhance security by validating resource types, but the implementation failed to restrict the target URLs to safe, external-only destinations. The filter does not properly validate or whitelist allowed URL schemes and domains before initiating the server-side request.

Attack Vector

An attacker can exploit this vulnerability by crafting malicious HTML content containing embed or object tags with specially crafted src attributes pointing to internal network resources. When this content is processed by an application using lucy-xss-filter with the vulnerable security listeners enabled, the server will make HEAD requests to the attacker-specified URLs.

The attack is network-based and requires user interaction to trigger the vulnerable code path. Attackers can leverage this to:

Scan internal network infrastructure
Access cloud metadata services (e.g., http://169.254.169.254/)
Probe internal APIs and services
Bypass firewall and network segmentation controls

For technical details on the vulnerability mechanism, refer to the GitHub Pull Request that addresses this issue.

Detection Methods for CVE-2026-23768

Indicators of Compromise

Unusual outbound HEAD requests from web application servers to internal IP ranges
HTTP requests to cloud metadata endpoints (169.254.169.254) from application servers
Unexpected network traffic patterns from servers running lucy-xss-filter to internal services
Log entries showing HEAD requests to suspicious or internal URLs originating from the filter processing pipeline

Detection Strategies

Monitor web application logs for processing of embed or object tags with unusual src attributes lacking file extensions
Implement network monitoring rules to detect server-initiated requests to internal RFC1918 address ranges (10.x.x.x, 172.16.x.x-172.31.x.x, 192.168.x.x)
Deploy egress filtering with alerting on requests to cloud provider metadata services
Use application-level logging to track URL validation events in lucy-xss-filter security listeners

Monitoring Recommendations

Enable verbose logging for lucy-xss-filter security listener operations to capture URL validation attempts
Set up alerts for HEAD requests from application servers to non-whitelisted internal destinations
Monitor for attempts to access commonly targeted SSRF endpoints such as localhost, internal service names, or cloud metadata URLs
Implement network-level detection for unusual request patterns from web application tier to internal networks

How to Mitigate CVE-2026-23768

Immediate Actions Required

Update lucy-xss-filter to commit 7c1de6d or later, which contains the fix for this vulnerability
If immediate patching is not possible, consider temporarily disabling ObjectSecurityListener and EmbedSecurityListener options
Implement network-level egress controls to restrict server-initiated requests to trusted external destinations only
Review application configurations to identify all instances using the vulnerable security listener options

Patch Information

The vulnerability has been addressed in the lucy-xss-filter repository. The fix is available in commit 7c1de6d and can be applied by updating to this version or later. For detailed information about the patch, refer to the GitHub Pull Request.

Workarounds

Disable ObjectSecurityListener and EmbedSecurityListener options in lucy-xss-filter configuration if these features are not critical to your application
Implement an application-level URL whitelist that restricts the destinations for any server-side requests
Deploy network segmentation to limit the impact of potential SSRF attacks by restricting application server access to internal resources
Use a forward proxy for all outbound server requests with strict URL filtering and logging capabilities

bash

# Network-level mitigation: Block outbound requests to internal ranges from application servers
# Example iptables rules (adjust interface and source IPs as needed)
iptables -A OUTPUT -d 169.254.169.254 -j DROP
iptables -A OUTPUT -d 10.0.0.0/8 -j DROP
iptables -A OUTPUT -d 172.16.0.0/12 -j DROP
iptables -A OUTPUT -d 192.168.0.0/16 -j DROP