GHSA-97jj-33gv-5xf9: league/commonmark DisallowedRawHtml Bypassed When A Disallowed Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-97jj-33gv-5xf9",
  "modified" : "2026-09-30T15:36:33Z",
  "published" : "2026-09-30T15:36:33Z",
  "aliases" : [ ],
  "summary" : "league/commonmark: DisallowedRawHtml bypassed when a disallowed tag name ends the raw-HTML literal",
  "details" : "## Summary\n\nThe `DisallowedRawHtml` extension does not escape a disallowed tag when the tag name is the last thing in the raw HTML. A Markdown line containing just `<script` is emitted unchanged, and the next block can supply its attributes. With the shipped GFM defaults this allows stored XSS by anyone who can post Markdown.\n\n## Details\n\n`DisallowedRawHtmlRenderer` escapes tags with this regex:\n\n```\n/<(\\/?(?:title|textarea|style|xmp|iframe|noembed|noframes|script|plaintext)[\\s\\/>])/i\n```\n\nThe trailing character class requires one character after the tag name. The block parser does not: `RegexHelper::PARTIAL_HTMLBLOCKOPEN` accepts end of line after a tag name, so `<script` alone opens an HTML block. Because a rendered `HtmlBlock` has no trailing newline, the regex has nothing to match and the `<` passes through.\n\nIn the browser the newline is still present, so the tag name terminates there and whatever follows becomes attributes.\n\nThis is the same filter that GHSA-4v6x-c7xx-hw9f fixed in 2.8.1. That fix widened the character class but still requires one character, so this case was not covered.\n\n## Reproduction\n\nRender this with `GithubFlavoredMarkdownConverter` and default settings:\n\n```\n<div>\n<script\n\n<span src=\"/evil.js\">\n```\n\nOutput:\n\n```html\n<div>\n<script\n<span src=\"/evil.js\">\n```\n\nA browser parses that as `<script src=\"/evil.js\">` with a junk `<span` attribute, and the script runs. `<iframe` with `<span onload=\"...\">` works the same way and does not need a later `</script>` in the page.\n\nControl: `<script src=\"/evil.js\"></script>` is correctly escaped to `&lt;script src=\"/evil.js\">&lt;/script>`.\n\n## Affected versions\n\n1.3.0 (when the extension was added) through the current release. The `</style` and mid-line forms are only affected as continuation lines inside an already-open HTML block.\n\n## Preconditions\n\n- `html_input` is `allow` (the default)\n- The `DisallowedRawHtml` extension is active, which the GFM extension enables automatically\n- Untrusted users can post Markdown\n\nSetting `html_input` to `escape` or `strip` fully mitigates this.\n\n## Suggested fix\n\nAllow end of string after the tag name:\n\n```php\n$regex = \\sprintf('/<(\\/?(?:%s))([\\s\\/>]|$)/i', \\implode('|', \\array_map('preg_quote', $tags)));\n\nreturn \\preg_replace($regex, '&lt;$1$2', $rendered);\n```\n\nThis escapes every bypass shape above and leaves `<div>`, `<scripts>` and `<span class=\"a\">` untouched. The existing unit test only covers tag names followed by another character, so a case for a bare tag name should be added.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "league/commonmark"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.3.0"
      }, {
        "fixed" : "2.10.2"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 2.10.1"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/thephpleague/commonmark/security/advisories/GHSA-97jj-33gv-5xf9"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/thephpleague/commonmark/commit/411afcc2a7402756d96c89af8882c724d12d47ca"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/thephpleague/commonmark"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/thephpleague/commonmark/releases/tag/2.10.2"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-78", "CWE-79" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-30T15:36:33Z",
    "nvd_published_at" : null
  }
}