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CVE-2026-59711 showdown CVE debrief

The showdown library contains a cross-site scripting vulnerability in its metadata title handling. When the 'completeHTMLDocument' option is enabled, unescaped less-than and greater-than characters in markdown frontmatter metadata are inserted directly into HTML title tags. This allows attackers to break out of the title context and execute malicious scripts in the rendered page. The vulnerability has a CVSS score of 5.3 and a severity of MEDIUM. Developers and users of the showdown library should be aware of this vulnerability, especially if they enable the 'completeHTMLDocument' option.

Vendor
showdown
Product
Unknown
CVSS
MEDIUM 5.3
CISA KEV
Not listed in stored evidence
Original CVE published
2026-07-06
Original CVE updated
2026-07-07
Advisory published
2026-07-06
Advisory updated
2026-07-07

Who should care

Developers and users of the showdown library should be aware of this vulnerability, especially if they enable the 'completeHTMLDocument' option. This vulnerability has a CVSS score of 5.3 and a severity of MEDIUM. Users should review the library's documentation and consider implementing additional security measures, such as Content Security Policy (CSP).

Technical summary

The vulnerability exists in the showdown library's handling of metadata titles. Specifically, when the 'completeHTMLDocument' option is enabled, the library fails to properly escape less-than and greater-than characters in markdown frontmatter metadata. This allows attackers to inject arbitrary HTML and JavaScript code, which can be executed in the rendered page. The vulnerability has a CVSS score of 5.3 and a severity of MEDIUM.

Defensive priority

MEDIUM

Recommended defensive actions

  • Review and update showdown library to the latest version
  • Disable the 'completeHTMLDocument' option if possible
  • Properly escape user-input data in metadata titles
  • Implement additional security measures, such as Content Security Policy (CSP)
  • Review compensating controls for exposed systems while remediation is scheduled and verified
  • Check relevant monitoring, detection, and logs for exposed assets that need extra review
  • Track exceptions, retest remediated assets, and close the item only after evidence is documented

Evidence notes

The CVE record was published on 2026-07-06T21:16:58.660Z and last modified on 2026-07-07T15:16:49.570Z. The NVD entry is currently Deferred. There is limited information available about the vulnerability, and defenders should verify the affected scope and severity with the vendor. The showdown library's handling of metadata titles is a critical aspect of this vulnerability, and users should review the library's documentation for more information.

Sources and references

Verified primary and authoritative sources

  • CVE-2026-59711 CVE Program record

    Publisher, destination, and source semantics verified

    URL: https://www.cve.org/CVERecord?id=CVE-2026-59711

    CVE Program - Official CVE Program record with source-provided CVE metadata.

  • CVE-2026-59711 NVD vulnerability detail

    Publisher, destination, and source semantics verified

    URL: https://nvd.nist.gov/vuln/detail/CVE-2026-59711

    NIST National Vulnerability Database - Official NIST NVD detail page and source-specific vulnerability assessment.

Supplemental references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.