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CVE-2026-63670: ApostropheCMS: Mutation-XSS / allowedTags bypass via literal `</textarea/>` solidus close

• Severity: medium (CVSS 6.1 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-08-17 · updated 2026-10-23
• Weakness: CWE-79

Description

ApostropheCMS is an open-source Node.js content management system. Prior to 2.17.6, sanitizeHtml() can pass disallowed executable markup through packages/sanitize-html/index.js when textarea or xmp is included in allowedTags because a literal solidus after the raw-text end-tag name is treated as text by htmlparser2 and the ontext handler emits that content without escaping, while a browser parses the following img onerror markup as active HTML. This issue is fixed in version 2.17.6.

Affected products

• apostrophecms apostrophe: before 2.17.6

References

• https://github.com/apostrophecms/apostrophe/commit/eae1fb2b72ec5d1c27d0977509c6482a0408f725
• https://github.com/apostrophecms/apostrophe/pull/5501
• https://github.com/apostrophecms/apostrophe/security/advisories/GHSA-jxwj-j7wr-gfrw
• https://github.com/apostrophecms/apostrophe/security/advisories/GHSA-jxwj-j7wr-gfrw

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