CVE-2026-104477
Received Received - Intake

XSS in Showdown Markdown Parser via Unescaped Double Quotes

Publication date: 2026-10-03

Last updated on: 2026-10-03

Assigner: VulnCheck

Description
Showdown through 2.1.0 contains a cross-site scripting vulnerability in the makehtml link and image subparsers, which fail to escape double quotes in destination URLs placed into href and src attributes. Attackers can craft markdown links or images containing a double quote followed by onerror or onmouseover handlers to execute script when victims view rendered HTML.

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Meta Information

Published
2026-10-03
Last Modified
2026-10-03
Generated
2026-10-03
AI Q&A
2026-10-03
EPSS Evaluated
N/A
NVD
CVE-2026-104477
EUVD
EUVD-2026-91853

Affected Vendors & Products

Showing 1 associated CPE
Vendor Product Version / Range
showdownjs showdown to 2.1.0 (inc)

Helpful Resources

Exploitability

CWE
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KEV
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CWE ID Description
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.

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Executive Summary

This vulnerability is a cross-site scripting (XSS) flaw in Showdown through version 2.1.0. It occurs because the makehtml link and image subparsers fail to escape double quotes in destination URLs placed into href and src attributes. Attackers can craft markdown links or images with a double quote followed by event handlers like onerror or onmouseover to execute malicious scripts when victims view the rendered HTML.

Detection Guidance

Check if your Showdown library version is 2.1.0 or earlier. Use commands like npm list showdown or grep -r "showdown" package.json to inspect versions. Review HTML output for unescaped double quotes in href or src attributes from user-provided Markdown input.

Impact Analysis

If you use Showdown to process untrusted markdown content, attackers could inject scripts that run in your users' browsers. This could lead to session hijacking, data theft, or defacement of your web pages. The impact depends on how Showdown is used in your application.

Compliance Impact

This XSS vulnerability could lead to unauthorized access to user data, violating GDPR's data protection principles and HIPAA's security requirements for protected health information. Organizations may face compliance violations if user data is compromised due to this flaw.

Mitigation Strategies

Upgrade Showdown to the latest version beyond 2.1.0. Enable safeMode in Showdown options. Use a sanitizer like DOMPurify on rendered HTML to remove malicious scripts. Avoid processing untrusted Markdown without sanitization.

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