CVE-2026-46637: Twig: HTML-output filters in twig/* extras incorrectly declared `is_safe => ['all']`

Description

Several filters in the twig/ extras packages are registered with issafe => ['all'], which tells Twig's autoescaper to treat their output as safe in every context (html, js, css, url, ...). The output of these filters is plain text or HTML markup, neither of which is safe in every escaping context.

Affected filters:

- htmltomarkdown (twig/markdown-extra) emits plain Markdown text. league/html-to-markdown decodes HTML entities when producing code spans and fenced blocks, so an attacker-controlled <code>&lt;img src=x onerror=alert(1)&gt;</code> becomes <img src=x onerror=alert(1)> , which renders live when interpolated into an HTML page. - markdowntohtml (twig/markdown-extra) emits HTML. Safe in an HTML context but not in JS, CSS or URL contexts (e.g. when interpolated into an inline <script> block). - inlinecss (twig/cssinliner-extra) emits HTML with inlined styles. Same constraint as markdowntohtml.

In all three cases, issafe => ['all'] causes the autoescaper to emit the output verbatim in any context, even when the developer never wrote |raw. In a context such as a JS string or a URL parameter, this produces unescaped HTML and is exploitable as XSS.

Resolution

- htmltomarkdown no longer claims to be safe in any escaping context; its plain-text output is now autoescaped for the surrounding context. - markdowntohtml and inlinecss are now declared issafe => ['html'], asserting only what they actually guarantee.

Credits

Twig would like to thank Claude Mythos Preview (via Project Glasswing) for reporting the issue and providing the fix for htmltomarkdown and markdowntohtml in twig/markdown-extra, and Christophe Coevoet for extending the audit to inlinecss in twig/cssinliner-extra.