GHSA-3843-rr4g-m8jq
HIGHExpress XSS Sanitizer: allowedTags/allowedAttributes bypass leads to permissive sanitization (XSS risk)
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Blast Radius
express-xss-sanitizerReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects npm packages — download data is not available via public APIs for these ecosystems.
Description
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.
Impact
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.
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.
Fix
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | express-xss-sanitizer | all versions | 2.0.2 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for express-xss-sanitizer. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update express-xss-sanitizer to 2.0.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-3843-rr4g-m8jq is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether GHSA-3843-rr4g-m8jq is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-3843-rr4g-m8jq. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-3843-rr4g-m8jq in your dependencies?
O3 detects GHSA-3843-rr4g-m8jq across npm dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.