CVE-2026-33979

Description

A vulnerability has been identified in express-xss-sanitizer (<= 2.0.1) where restrictive sanitization configurations are silently ignored.

When a developer explicitly sets:

allowedTags: []
allowedAttributes: {}

the library incorrectly treats these values as "not provided" due to length/emptiness checks, and falls back to sanitize-html's default configuration.

As a result, instead of stripping all HTML tags and attributes, the sanitizer allows a permissive set of tags (e.g., <a>, <p>, <div>, etc.) and attributes (e.g., href on <a>).

This behavior violates the expected API contract and may lead to security issues such as content injection or XSS, depending on how the sanitized output is used.

Proof of Concept

const { sanitize } = require('express-xss-sanitizer');
const sanitizeHtml = require('sanitize-html');

const input = '<a href="http://evil.com">click</a><p>phish</p>';

// Using express-xss-sanitizer (v2.0.1)
sanitize(input, { allowedTags: [], allowedAttributes: {} });
// => '<a href="http://evil.com">click</a><p>phish</p>'

// Expected behavior (sanitize-html directly)
sanitizeHtml(input, { allowedTags: [], allowedAttributes: {} });
// => 'clickphish'

Root Cause

The issue was caused by validation logic that checked for non-empty arrays/objects:

• allowedTags required length > 0
• allowedAttributes required Object.keys(...).length > 0

This caused empty configurations ([]) and ({}) to be ignored, resulting in fallback to default permissive settings.

Developers intending to fully strip HTML content by providing empty allowedTags or allowedAttributes configurations may unknowingly allow a wide range of HTML elements and attributes.

This can result in:

• Injection of unintended HTML content (e.g., <div>, <table>, headings)
• Injection of links via <a href="...">
• Potential XSS vectors depending on downstream usage

The impact depends on how the sanitized output is rendered or consumed, but the root issue is a mismatch between developer intent and actual behavior.

Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.

CVE-2026-33979 has a CVSS score of 8.2 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.0.2); upgrading removes the vulnerable code path.

Kodem intelligence

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.

1. What is CVE-2026-33979? CVE-2026-33979 is a high-severity cross-site scripting (XSS) vulnerability in express-xss-sanitizer (npm), affecting versions < 2.0.2. It is fixed in 2.0.2. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. How severe is CVE-2026-33979? CVE-2026-33979 has a CVSS score of 8.2 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of express-xss-sanitizer are affected by CVE-2026-33979? express-xss-sanitizer (npm) versions < 2.0.2 is affected.
4. Is there a fix for CVE-2026-33979? Yes. CVE-2026-33979 is fixed in 2.0.2. Upgrade to this version or later.
5. Is CVE-2026-33979 exploitable, and should I be worried? Whether CVE-2026-33979 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-33979 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-33979? Upgrade express-xss-sanitizer to 2.0.2 or later.