CVE-2026-116804

DOMPurify from 3.4.4 before 3.4.17 contains a SAFE_FOR_TEMPLATES bypass that lets attackers preserve template expressions in attached shadow roots when using RETURN_DOM, RETURN_DOM_FRAGMENT, or IN_PLACE modes. Attackers can split expressions across text nodes separated by disallowed elements, which merge after removal and are evaluated by downstream template engines for cross-site scripting.

Summary

When DOMPurify is configured with SAFE_FOR_TEMPLATES: true together with a DOM-returning mode (RETURN_DOM: true, RETURN_DOM_FRAGMENT: true, or IN_PLACE: true), template expressions such as {{...}}, ${...}, and <%...%> survive sanitization inside the text content of an attached shadow root. The post-walk safety net _scrubTemplateExpressions (src/purify.ts:1717) was extended to recurse into <template>.content, but it never recurses into attached shadow roots. An attacker who can supply a DOM node carrying a shadow root (the IN_PLACE and shadow-aware RETURN_DOM surfaces) can split an expression across text nodes separated by disallowed elements; after those elements are removed the fragments become adjacent and serialize into a complete expression that a downstream template engine will evaluate.

Root Cause

SAFE_FOR_TEMPLATES neutralizes template expressions through two mechanisms: a per-text-node strip during the main walk (_stripTemplateExpressions, applied in _sanitizeElements at src/purify.ts:2246), and a final post-walk pass (_scrubTemplateExpressions, src/purify.ts:1717) that normalizes adjacent text nodes and re-strips expressions that only form after merging. The per-node strip cannot catch an expression that is split into individual fragments, because each fragment fails the delimiter regexes. The post-walk pass is the only mechanism that would catch the merged form, but it is structurally unable to reach shadow root content.

_scrubTemplateExpressions (src/purify.ts:1717-1751):
  - node.normalize() at line 1718 does not merge text nodes inside shadow roots (DOM spec).
  - NodeIterator at lines 1723-1731 does not descend into shadow trees (DOM spec).
  - Explicit recursion at lines 1743-1750 enters <template>.content only, via
    querySelectorAll('template'); there is no equivalent recursion into attached
    shadow roots.
Shadow roots are sanitized earlier by _sanitizeShadowDOM (src/purify.ts:2710), which
applies the per-node strip but never performs the post-walk normalize-and-rescrub.

Shadow root text nodes are therefore sanitized only per-node and never re-examined as a merged unit.

Impact

An attacker who controls DOM-node input containing an attached shadow root can inject template expressions that survive sanitization and are later evaluated by the application's template engine. In client-side contexts this yields cross-site scripting; in a server-side template context it can escalate to template injection. Exploitation requires non-default application configuration: the application must enable SAFE_FOR_TEMPLATES and a DOM-returning mode, pass a DOM node that already has an attached shadow root, and feed the sanitized shadow root content into a template evaluator. The shadow root does not have to be built programmatically with attachShadow: in engines that support declarative shadow DOM, attacker-supplied markup containing <template shadowrootmode="open"> parsed through setHTMLUnsafe or Document.parseHTMLUnsafe produces the same attached shadow root, so a plain HTML string reaches the gap once the application is configured this way. Demonstrated capability is injection of a complete, attacker-chosen expression into post-sanitization shadow root content; the resulting script execution and any confidentiality or integrity consequence depend on the downstream engine's evaluation context.

Proof of Concept

Preconditions:
  - DOMPurify built from the affected source (dist/purify.js or an ESM/CJS bundle).
  - A browser (or DOM implementation) with shadow DOM support.
  - Application constructs a DOM node with an attached shadow root before sanitizing,
    and later evaluates the shadow root's text content as a template.

Build a host element whose shadow root holds a template expression split across text nodes separated by disallowed elements, then sanitize in place with SAFE_FOR_TEMPLATES:

const host =…

DOMPurify from 3.4.4 before 3.4.17 contains a SAFE_FOR_TEMPLATES bypass that lets attackers preserve template expressions in attached shadow roots when using RETURN_DOM, RETURN_DOM_FRAGMENT, or IN_PLACE modes. Attackers can split expressions across text nodes separated by disallowed elements, which merge after removal and are evaluated by downstream template engines for cross-site scripting.

TuxCare rebuilt dompurify to address 3 CVEs. Fixed in 3.4.5-tuxcare.5.

TuxCare rebuilt dompurify to address 3 CVEs. Fixed in 3.4.8-tuxcare.7.

TuxCare rebuilt dompurify to address 3 CVEs. Fixed in 3.4.13-tuxcare.3.

TuxCare rebuilt dompurify to address 3 CVEs. Fixed in 3.4.11-tuxcare.5.

TuxCare rebuilt dompurify to address 3 CVEs. Fixed in 3.4.6-tuxcare.5.

TuxCare rebuilt dompurify to address 3 CVEs. Fixed in 3.4.7-tuxcare.5.

TuxCare rebuilt dompurify to address 3 CVEs. Fixed in 3.4.14-tuxcare.4.

TuxCare rebuilt dompurify to address 3 CVEs. Fixed in 3.4.9-tuxcare.5.

TuxCare rebuilt dompurify to address 3 CVEs. Fixed in 3.4.10-tuxcare.5.

TuxCare rebuilt dompurify to address 4 CVEs. Fixed in 3.4.12-tuxcare.3.

TuxCare rebuilt dompurify to address 3 CVEs. Fixed in 3.4.15-tuxcare.3.

TuxCare rebuilt dompurify to address 2 CVEs. Fixed in 3.4.16-tuxcare.1.

TuxCare rebuilt dompurify to address 1 CVE. Fixed in 3.4.8-tuxcare.8.

TuxCare rebuilt dompurify to address 1 CVE. Fixed in 3.4.9-tuxcare.6.

TuxCare rebuilt dompurify to address 1 CVE. Fixed in 3.4.10-tuxcare.6.

TuxCare rebuilt dompurify to address 1 CVE. Fixed in 3.4.5-tuxcare.6.

TuxCare rebuilt dompurify to address 1 CVE. Fixed in 3.4.12-tuxcare.4.

TuxCare rebuilt dompurify to address 1 CVE. Fixed in 3.4.14-tuxcare.5.

TuxCare rebuilt dompurify to address 1 CVE. Fixed in 3.4.11-tuxcare.6.

TuxCare rebuilt dompurify to address 1 CVE. Fixed in 3.4.15-tuxcare.4.

Root has patched GHSA-cg2g-xh64-v4q7 in the dompurify package for Root:npm. Multiple fixed versions available.