CVE-2026-59711: showdown - Cross-Site Scripting via Unescaped Metadata Title in completeHTMLDocument

Severity: medium (CVSS 6.1 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-07-06 · updated 2026-10-25
Weakness: CWE-79

Description

showdown contains a cross-site scripting vulnerability in metadata title handling that allows attackers to inject arbitrary HTML and JavaScript. When completeHTMLDocument option is enabled, unescaped less-than and greater-than characters in markdown frontmatter metadata are inserted directly into HTML title tags, enabling attackers to break out of the title context and execute malicious scripts in the rendered page.

Affected products

showdown: through 2.1.0

References

https://github.com/showdownjs/showdown
https://github.com/showdownjs/showdown/issues/1047
https://www.vulncheck.com/advisories/showdown-cross-site-scripting-via-unescaped-metadata-title-in-completehtmldocument
https://github.com/showdownjs/showdown/issues/1047

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