CVE-2026-59711 in showdown
Summary
by MITRE • 07/07/2026
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.
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Analysis
by VulDB Data Team • 07/07/2026
This vulnerability exists within the showdown markdown parsing library where improper handling of metadata title elements creates a cross-site scripting vector through the completeHTMLDocument option. The flaw specifically manifests when markdown documents contain frontmatter metadata with unescaped special characters in title fields, allowing attackers to inject malicious code directly into HTML document title tags. The technical implementation involves the library's failure to properly sanitize or escape less-than and greater-than characters that appear within metadata title values, particularly when the completeHTMLDocument flag is enabled during processing.
The operational impact of this vulnerability extends beyond simple script execution as it enables attackers to manipulate the entire document context through the title element injection. When the completeHTMLDocument option is active, showdown renders full HTML documents including head sections where the title tag resides, making the injected malicious code capable of executing within the victim's browser context. This creates a persistent threat vector that can be exploited across multiple pages or applications using the vulnerable library version. The vulnerability aligns with CWE-79 which categorizes cross-site scripting flaws, specifically addressing improper neutralization of input during web page generation.
Attackers can leverage this weakness by crafting markdown documents containing malicious payloads within title metadata fields such as <script>alert(document.cookie)</script> or more sophisticated payload delivery mechanisms that exploit the title tag injection point. The vulnerability is particularly dangerous in environments where user-generated content is processed through showdown, as it allows for arbitrary code execution without requiring additional attack vectors beyond the initial markdown document submission. This represents a critical security risk in web applications that process untrusted markdown input, especially those with privileged user access or sensitive data handling capabilities.
The recommended mitigations include implementing proper input sanitization and HTML escaping of all metadata values before rendering them into HTML title tags, particularly when processing user-supplied content. Organizations should disable the completeHTMLDocument option unless absolutely required for functionality, and implement comprehensive content validation mechanisms that filter out potentially dangerous characters from metadata fields. Additionally, regular library updates and security audits are essential to prevent exploitation of known vulnerabilities in third-party components. The ATT&CK framework categorizes this as a code injection technique under T1203, specifically targeting web application input validation weaknesses where the vulnerability can be exploited through markdown processing workflows that lack proper sanitization controls.