Silent Bypass of XSS Sanitization in Express XSS Sanitizer

Publication date: 2026-03-27

Last updated on: 2026-03-31

Assigner: GitHub, Inc.

Description

Express XSS Sanitizer is Express 4.x and 5.x middleware which sanitizes user input data (in req.body, req.query, req.headers and req.params) to prevent Cross Site Scripting (XSS) attack. A vulnerability has been identified in versions prior to 2.0.2 where restrictive sanitization configurations are silently ignored. In version 2.0.2, the validation logic has been updated to respect explicitly provided empty configurations. Now, if allowedTags or allowedAttributes are provided (even if empty), they are passed directly to sanitize-html without being overridden.

CVSS Scores

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Meta Information

Published

2026-03-27

Last Modified

2026-03-31

Generated

2026-07-06

AI Q&A

2026-03-28

EPSS Evaluated

2026-07-05

NVD

CVE-2026-33979

Affected Vendors & Products

Vendor	Product	Version / Range
express_xss_sanitizer_project	express_xss_sanitizer	to 2.0.2 (exc)

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-183	The product implements a protection mechanism that relies on a list of inputs (or properties of inputs) that are explicitly allowed by policy because the inputs are assumed to be safe, but the list is too permissive - that is, it allows an input that is unsafe, leading to resultant weaknesses.
CWE-79	The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.

Attack-Flow Graph

Executive Summary

The vulnerability exists in Express XSS Sanitizer versions prior to 2.0.2. This middleware is designed to sanitize user input data to prevent Cross Site Scripting (XSS) attacks. However, in affected versions, restrictive sanitization configurations are silently ignored, meaning that if a user explicitly sets allowedTags or allowedAttributes (even as empty), these settings are not respected and are overridden. This flaw allows potentially unsafe input to bypass the intended sanitization.

Impact Analysis

This vulnerability can lead to Cross Site Scripting (XSS) attacks because the sanitization configurations meant to restrict harmful input are ignored. As a result, malicious scripts could be injected and executed in the context of a user's browser, potentially leading to data theft, session hijacking, or other malicious activities.

Mitigation Strategies

To mitigate this vulnerability, update the Express XSS Sanitizer middleware to version 2.0.2 or later. This version includes updated validation logic that respects explicitly provided empty configurations, ensuring that restrictive sanitization settings are not silently ignored.

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