CVE-2026-49210: symfony/ux-live-component Cross-Site Scripting Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-38x5-rcv4-xf7x",
  "modified" : "2026-06-19T19:34:49Z",
  "published" : "2026-06-19T19:34:49Z",
  "aliases" : [ "CVE-2026-49210" ],
  "summary" : "symfony/ux-live-component: XSS via attacker-controlled child component tag",
  "details" : "### Description\n\n`Symfony\\UX\\LiveComponent\\Util\\ChildComponentPartialRenderer::createHtml()` interpolates the `$childTag` argument directly into the HTML output as a tag name, without escaping or validation. The value originates from client-controlled JSON (`children[id].tag`) parsed by `LiveComponentSubscriber` and propagated through `InterceptChildComponentRenderSubscriber`, so an attacker who can reach the Live Component endpoint can inject arbitrary HTML, including `<script>` tags, on any re-render of a Live Component that contains at least one child component.\n\nIn the default configuration, the Live Component endpoint is gated by an `Accept: application/vnd.live-component+html` request-header check that cannot be set cross-origin without a CORS preflight, so the issue is primarily a defense-in-depth gap. It becomes directly exploitable on applications that have relaxed CORS to allow this header from untrusted origins, or that have been pivoted from another same-origin XSS.\n\n### Resolution\n\n`ChildComponentPartialRenderer` now validates `$childTag` against a strict HTML tag-name regex before interpolating it, and rejects any value that doesn't match. Anything that wouldn't be a valid HTML tag is dropped before reaching the response.\n\nThe patch for this issue is available [here](https://github.com/symfony/ux/commit/fbc5e9a1bda7e4556be21bb1d970f382760ed9a9) for branch 2.x (and forward-ported to 3.x).\n\n### Credits\n\nSymfony would like to thank Pascal Cescon for reporting the issue and Hugo Alliaume for providing the fix.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "symfony/ux-live-component"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "2.8.0"
      }, {
        "fixed" : "2.36.0"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "symfony/ux-live-component"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "3.0.0"
      }, {
        "fixed" : "3.1.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/symfony/ux/security/advisories/GHSA-38x5-rcv4-xf7x"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/symfony/ux/commit/fbc5e9a1bda7e4556be21bb1d970f382760ed9a9"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/FriendsOfPHP/security-advisories/blob/master/symfony/ux-live-component/CVE-2026-49210.yaml"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/symfony/ux"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-79" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-06-19T19:34:49Z",
    "nvd_published_at" : null
  }
}