CVE-2026-49978
DOMPurify IN_PLACE Sanitization Bypass via Attached Shadow Root Inside <template>.content
Description
If the HTML you give it contains a <template> element, and inside that template there's an element with a shadow DOM attached to it, DOMPurify quietly skips over the shadow contents. Whatever the attacker put in there - an image with an onerror handler, a link with a javascript: URL, even a full script - survives untouched. The moment the application uses that template the way templates are meant to be used (cloning it and inserting the result into the page), the malicious payload comes along and runs as if it had never been sanitized. From there an attacker gets everything XSS normally gets them: session cookies, stored tokens, the ability to act as the user, and the ability to leave persistent payloads behind for the next person who visits. [advisory.pdf](https://github.com/user-attachments/files/28275600/advisory.pdf) [poc.html](https://github.com/user-attachments/files/28275708/poc.html)
How to fix CVE-2026-49978
To remediate CVE-2026-49978, upgrade the affected package to a fixed version below.
- —upgrade to 3.4.7 or later
Is CVE-2026-49978 being exploited?
No exploitation signal available. Neither CISA KEV nor a current EPSS score has been published for CVE-2026-49978.
Affected packages (1)
- from 0, < 3.4.7
CVSS scores
| Source | Version | Severity | Vector |
|---|---|---|---|
| osv | CVSS 4.0 | — | CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N |