Cross-Site Scripting Vulnerability in Showdown by ShowdownJS
CVE-2026-59711

5.3MEDIUM

Key Information:

Vendor

Showdown

Status
Showdown
Vendor
CVE Published:
6 July 2026

What is CVE-2026-59711?

A cross-site scripting vulnerability exists in the Showdown product related to the handling of metadata titles. When the completeHTMLDocument option is enabled, unescaped less-than and greater-than characters in Markdown frontmatter metadata are directly inserted into HTML title tags. This flaw allows attackers to break out of the title context, resulting in the execution of arbitrary HTML and JavaScript on the rendered page, potentially compromising user security.

Affected Version(s)

showdown 0 <= 2.1.0

References

https://github.com/showdownjs/showdown/issues/1047
issue-tracking
https://github.com/showdownjs/showdown
product
https://www.vulncheck.com/advisories/showdown-cross-site-...
third-party-advisory

CVSS V4

Score:
5.3
Severity:
MEDIUM
Confidentiality:
Low
Integrity:
Low
Availability:
None
Attack Vector:
Network
Attack Complexity:
Low
Attack Required:
None
Privileges Required:
Undefined
User Interaction:
Unknown

Timeline

  • Vulnerability published

  • Vulnerability Reserved

Credit

vc-smoore
.