CVE-2026-46637: Twig HTML-output Filters Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-jv8m-2544-3pg3",
  "modified" : "2026-05-21T21:27:20Z",
  "published" : "2026-05-21T21:27:20Z",
  "aliases" : [ "CVE-2026-46637" ],
  "summary" : "Twig: HTML-output filters in twig/* extras incorrectly declared `is_safe => ['all']`",
  "details" : "### Description\n\nSeveral filters in the `twig/*` extras packages are registered with `is_safe => ['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.\n\nAffected filters:\n\n- `html_to_markdown` (`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.\n- `markdown_to_html` (`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).\n- `inline_css` (`twig/cssinliner-extra`) emits HTML with inlined styles. Same constraint as `markdown_to_html`.\n\nIn all three cases, `is_safe => ['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.\n\n### Resolution\n\n- `html_to_markdown` no longer claims to be safe in any escaping context; its plain-text output is now autoescaped for the surrounding context.\n- `markdown_to_html` and `inline_css` are now declared `is_safe => ['html']`, asserting only what they actually guarantee.\n\n### Credits\n\nTwig would like to thank Claude Mythos Preview (via Project Glasswing) for reporting the issue and providing the fix for `html_to_markdown` and `markdown_to_html` in `twig/markdown-extra`, and Christophe Coevoet for extending the audit to `inline_css` in `twig/cssinliner-extra`.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N/E:U"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "twig/markdown-extra"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "3.26.0"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "twig/cssinliner-extra"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "3.26.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/twigphp/Twig/security/advisories/GHSA-jv8m-2544-3pg3"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/FriendsOfPHP/security-advisories/blob/master/twig/cssinliner-extra/CVE-2026-46637.yaml"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/FriendsOfPHP/security-advisories/blob/master/twig/markdown-extra/CVE-2026-46637.yaml"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/twigphp/Twig"
  }, {
    "type" : "WEB",
    "url" : "https://symfony.com/cve-2026-46637"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-116" ],
    "severity" : "LOW",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-05-21T21:27:20Z",
    "nvd_published_at" : null
  }
}