CVE-2026-23769 in lucy-xss-filter
Summary
by MITRE • 01/16/2026
lucy-xss-filter before commit e5826c0 allows an attacker to execute malicious JavaScript due to improper sanitization caused by misconfigured default superset rule files.
If you want to get best quality of vulnerability data, you may have to visit VulDB.
Analysis
by VulDB Data Team • 01/24/2026
The vulnerability identified as CVE-2026-23769 affects the lucy-xss-filter library prior to commit e5826c0, representing a critical security flaw that enables cross-site scripting attacks through inadequate input sanitization. This issue stems from the improper handling of user-supplied data within the filter's default superset rule configuration, creating a pathway for malicious javascript execution in vulnerable applications. The flaw specifically manifests when the library processes input through its default security rules that fail to properly neutralize potentially dangerous script content.
The technical root cause of this vulnerability lies in the misconfiguration of default superset rule files within the lucy-xss-filter implementation. These rule files are responsible for defining which characters, patterns, and constructs should be filtered out or escaped to prevent XSS attacks. When the default configuration fails to adequately sanitize input, attackers can craft malicious payloads that bypass the intended security measures. The vulnerability operates under CWE-79 which classifies improper neutralization of input during web page generation, specifically targeting the failure to properly sanitize user-controllable data that gets rendered in web browsers.
The operational impact of this vulnerability extends beyond simple script execution, as it fundamentally undermines the security posture of applications relying on the lucy-xss-filter library. Attackers can leverage this flaw to inject malicious javascript code that executes in the context of a victim's browser session, potentially leading to session hijacking, data theft, or redirection to malicious sites. The vulnerability affects applications where user input is processed through the filter without additional validation layers, making it particularly dangerous in web applications that handle untrusted input from multiple sources. The default nature of the affected rule files means that any application using the library without explicit configuration overrides becomes vulnerable by default.
Security professionals should prioritize immediate mitigation efforts by updating to the fixed version after commit e5826c0 or implementing compensating controls such as explicit rule configuration overrides. Organizations should conduct comprehensive code reviews to identify all applications using this library and ensure proper input validation is implemented at multiple layers. The ATT&CK framework categorizes this vulnerability under T1203 - Exploitation for Client Execution, as it enables attackers to execute malicious code within the victim's browser environment. Additionally, this weakness aligns with T1584 - Compromise of Third-Party Applications, highlighting how vulnerable dependencies can serve as entry points for broader security breaches. Organizations should also consider implementing Content Security Policy headers as an additional defensive measure, though these should complement rather than replace proper input sanitization mechanisms. The vulnerability demonstrates the critical importance of proper default security configurations in open source libraries and the potential for widespread impact when such defaults are compromised.