CVE-2026-33979 is a high-severity (CVSS 8.2) Cross-site Scripting (XSS) vulnerability in express-xss-sanitizer. A fix is available for express-xss-sanitizer — see the affected versions and patch details below.
Express XSS Sanitizer: allowedTags/allowedAttributes bypass leads to permissive sanitization (XSS risk)
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-33979.
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.
How urgent is this, really
CVE-2026-33979 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 377,636 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
express-xss-sanitizernpmDescription
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 | 2.0.2npm install express-xss-sanitizer@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, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
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 CVE-2026-33979 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 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-33979 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-33979. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-33979 in your dependencies?
O3 Security finds CVE-2026-33979 across npm dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.