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VulnerabilityGHSA-cg2g-xh64-v4q7

DOMPurify SAFE_FOR_TEMPLATES bypass: split template expressions survive in attached shadow root content on DOM output modes

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.

Published Updated

At a glance

Exploited
Not known
Severity
Not scored yet
Chance of exploitation
Not estimated
Sources
1 source

What to do

GitHub lists first patched versions: dompurify: 3.4.17. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-cg2g-xh64-v4q7 (opens a new tab)

What happened

## 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 ` .

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

Continue reading

`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.

```text _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 .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 ` ` 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

```text 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`:

```javascript const host = document.createElement('div'); const root = host.attachShadow({ mode: 'open' });

// Split "${alert(document.domain)}" so no single fragment matches a regex. // is not in the default ALLOWED_TAGS and is removed during sanitization, // leaving the surrounding text-node siblings adjacent. root.appendChild(document.createTextNode('$')); root.appendChild(document.createElement('object')); root.appendChild(document.createTextNode('{alert(document.domain)}'));

DOMPurify.sanitize(host, { IN_PLACE: true, SAFE_FOR_TEMPLATES: true });

// After the disallowed element is removed, the fragments are adjacent and survive. host.shadowRoot.normalize(); console.log(host.shadowRoot.textContent); ```

```text ${alert(document.domain)} ```

A downstream template evaluator reading `shadowRoot.textContent` (for example `new Function('return `' + content + '`')()` or any `${...}`/`{{...}}` engine) then evaluates the fully formed expression.

## Attack Chain

1. **Exposure:** The application calls `DOMPurify.sanitize(node, cfg)` (`src/purify.ts:2854`) with `SAFE_FOR_TEMPLATES: true` and `IN_PLACE: true` (or a shadow-aware `RETURN_DOM`), passing a DOM node that already carries an attached shadow root. 2. **Control:** The attacker controls the shadow root's text nodes and the disallowed separator elements between them; each fragment (`$`, `{alert(document.

domain)}`) is a valid, individually benign text node. 3. **Path:** `_sanitizeAttachedShadowRoots` (`src/purify.ts:2977`) feeds the shadow root into `_sanitizeShadowDOM` (`src/purify.ts:2710`), which strips expressions per text node (`src/purify.ts:2246`) and removes the disallowed separator elements, leaving the fragments adjacent. 4. **Guard:** The post-walk safety net `_scrubTemplateExpressions` (`src/purify.

ts:3131-3132`) is the only pass that would catch the merged expression, but it never recurses into shadow roots (`src/purify.ts:1717-1751` recurses into ` .content` only). 5. **Primitive:** The adjacent fragments serialize as a single concatenated string on `shadowRoot.textContent` / `shadowRoot.innerHTML`,

Affected package ranges reported by GitHub: dompurify: >= 3.4.4, <= 3.4.16

Related technology

Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-cg2g-xh64-v4q7 (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-cg2g-xh64-v4q7, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  3. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-cg2g-xh64-v4q7, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  4. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-cg2g-xh64-v4q7, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  5. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-cg2g-xh64-v4q7, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  6. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-cg2g-xh64-v4q7, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  7. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-cg2g-xh64-v4q7, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  8. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-cg2g-xh64-v4q7, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  9. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-cg2g-xh64-v4q7, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  10. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-cg2g-xh64-v4q7, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  11. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-cg2g-xh64-v4q7, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 22). DOMPurify SAFE_FOR_TEMPLATES bypass: split template expressions survive in attached shadow root content on DOM output modes. https://kevwire.com/record/GHSA-cg2g-xh64-v4q7

MLA

"DOMPurify SAFE_FOR_TEMPLATES bypass: split template expressions survive in attached shadow root content on DOM output modes." KEVWire, 22 Oct. 2026, kevwire.com/record/GHSA-cg2g-xh64-v4q7.

Chicago

KEVWire. "DOMPurify SAFE_FOR_TEMPLATES bypass: split template expressions survive in attached shadow root content on DOM output modes." Last modified October 22, 2026. https://kevwire.com/record/GHSA-cg2g-xh64-v4q7.

Evidence summary (plain text)

DOMPurify SAFE_FOR_TEMPLATES bypass: split template expressions survive in attached shadow root content on DOM output modes (Oct 22, 2026).
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 ` .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.

`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.
Sources:
GitHub-reviewed advisory GHSA-cg2g-xh64-v4q7: https://github.com/advisories/GHSA-cg2g-xh64-v4q7
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo1MTozMFrOAAf0Mg%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0OToxMlrOAAf0LQ%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0NzoxN1rOAAf0Kg%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0NTozNlrOAAf0Jw%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0NTowNVrOAAf0JQ%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0Mzo0NVrOAAf0Iw%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QyMDo0MzozMVrOAAf0Ig%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QxODozNTowMVrOAAZRcw%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QxODozMDoyN1rOAAY_5Q%3D%3D
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yM1QxODoyOTowNVrOAAfz8g%3D%3D
Record ID: GHSA-cg2g-xh64-v4q7.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 26, 2026, 20:55 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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