CVE-2026-46644: symfony/polyfill-intl-idn Xn-- Labels With ASCII-only Punycode Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-2xf4-cg6j-vhgq",
  "modified" : "2026-05-28T19:52:36Z",
  "published" : "2026-05-28T19:52:36Z",
  "aliases" : [ "CVE-2026-46644" ],
  "summary" : "symfony/polyfill-intl-idn: xn-- labels with ASCII-only Punycode payloads are treated as equivalent to their decoded form",
  "details" : "### Description\n\n`symfony/polyfill-intl-idn` provides a userland implementation of `idn_to_utf8()` and `idn_to_ascii()` for runtimes that lack the `intl` extension. Its `Idn::process()` method decodes labels prefixed with `xn--` using Punycode but never enforces the validity criterion added in UTS #46 revision 33 Section 4 step 4.1.2: after a successful Punycode decode, the result must contain at least one non-ASCII code point.\n\nAs a consequence, `xn--` labels whose Punycode payload is empty (`xn--`) or decodes to a string made of only ASCII code points (e.g. `xn--kc1zs4-`) are accepted by the polyfill while PHP's native `ext-intl` rejects them with `IDNA_ERROR_INVALID_ACE_LABEL`. Originally unequal domain names are therefore regarded as equal, which can lead to blacklist bypassing, inconsistent URL parsing and server-side request forgery (similar to CVE-2024-12224).\n\nExample with `IDNA_USE_STD3_RULES | IDNA_CHECK_BIDI | IDNA_CHECK_CONTEXTJ | IDNA_NONTRANSITIONAL_TO_ASCII`:\n\n| Input | Polyfill output | Native `ext-intl` output |\n| --- | --- | --- |\n| `poc.xn--kc1zs4-.com` | `poc.kc1zs4.com` | `false` (`errors=1024`) |\n| `poc.kc1zs4.xn--` | `poc.kc1zs4.` | `false` (`errors=1024`) |\n\nApplications using the polyfill to canonicalise or compare hostnames inherit the inconsistency.\n\n### Resolution\n\n`Idn::process()` now records `IDNA_ERROR_INVALID_ACE_LABEL` when a Punycode payload decodes to an empty string or to a string containing only ASCII code points, matching the native `ext-intl` behaviour and UTS #46 revision 33.\n\nThe patch for this issue is available [here](https://github.com/symfony/polyfill/commit/1be936e2491ccebe152bd736dfc91eb1422c8bec) for branch 1.x.\n\n### Credits\n\nSymfony would like to thank Nazy Mad for reporting the issue and Nicolas Grekas for providing the fix.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:L/SA:N/E:U"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "symfony/polyfill"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.17.1"
      }, {
        "fixed" : "1.38.1"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "symfony/polyfill-intl-idn"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.17.1"
      }, {
        "fixed" : "1.38.1"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/symfony/polyfill/security/advisories/GHSA-2xf4-cg6j-vhgq"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/symfony/polyfill/commit/1be936e2491ccebe152bd736dfc91eb1422c8bec"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/FriendsOfPHP/security-advisories/blob/master/symfony/polyfill-intl-idn/CVE-2026-46644.yaml"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/FriendsOfPHP/security-advisories/blob/master/symfony/polyfill/CVE-2026-46644.yaml"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/symfony/polyfill"
  }, {
    "type" : "WEB",
    "url" : "https://symfony.com/cve-2026-46644"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-1289" ],
    "severity" : "LOW",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-05-28T19:52:36Z",
    "nvd_published_at" : null
  }
}