Graby has stored XSS via iframe srcdoc Attribute in htmLawed Sanitization Config

Summary

## Summary

Graby's `cleanupXss()` function configures htmLawed with conflicting settings: `safe=1` (which removes `<iframe>`) combined with `'elements' => '*+iframe-meta'` (which re-enables `<iframe>`). htmLawed does not sanitize the `srcdoc` attribute, allowing injection of arbitrary JavaScript that executes when the content is rendered via `|raw` in templates.

## Root Cause

**`src/Graby.php` lines 1038-1048:** ```php htmLawed($html, [ 'safe' => 1, // removes <iframe> 'elements' => '*+iframe-meta', // re-adds <iframe>, overrides safe=1 'deny_attribute' => 'style', // srcdoc is NOT denied ]); ```

The `safe=1` and `+iframe` combination is a conflict: `safe` mode is designed to strip dangerous elements, but the elements override re-enables `<iframe>` without also blocking the dangerous `srcdoc` attribute.

## Proof of Concept

Input to `cleanupXss()`: ```html <iframe srcdoc="&lt;script&gt;alert(document.domain)&lt;/script&gt;"></iframe> ```

Output (unchanged — htmLawed passes it through): ```html <iframe srcdoc="&lt;script&gt;alert(document.domain)&lt;/script&gt;"></iframe> ```

When rendered via `{{ content|raw }}` in a template, `srcdoc` executes in an `about:srcdoc` frame with the same origin as the page. **Confirmed via Puppeteer/Chromium headless: `alert(document.domain)` fires.**

Validated on Wallabag (which uses Graby) via Docker: entry created via API with iframe-only content body triggers Readability failure → falls through to `cleanupXss()` path.

## Impact

- Stored XSS in any application rendering Graby-sanitized content via `|raw` - In Wallabag: affects both authenticated views and public share pages (unauthenticated) - No CSP headers in default Wallabag config — no secondary mitigation

## Suggested Fix

Either remove `+iframe` from the elements config to keep iframes blocked: ```php 'elements' => '*-iframe-meta', ```

Or explicitly deny the `srcdoc` attribute: ```php 'deny_attribute' => 'style srcdoc', ```

## Credit

Discovered by @tikket1, 2026-03-25. Redirected from wallabag/wallabag advisory by @j0k3r.