{"id":"CVE-2026-46644","aliases":["GHSA-2xf4-cg6j-vhgq"],"url":"https://o3.security/vulnerability/CVE-2026-46644","summary":"symfony/polyfill-intl-idn accepts xn-- labels whose Punycode payload decodes to ASCII-only: insecure equivalence","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.","published":"2026-07-14T20:48:31.771Z","modified":"2026-08-12T03:51:18.650967266Z","cvss":null,"epss":{"score":0.00394,"percentile":0.32694,"asOf":"2026-08-16"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"Packagist","name":"symfony/polyfill","fixedVersion":"1.38.1"},{"ecosystem":"Packagist","name":"symfony/polyfill-intl-idn","fixedVersion":"1.38.1"}],"fix":{"url":"https://github.com/symfony/polyfill/commit/1be936e2491ccebe152bd736dfc91eb1422c8bec","label":"symfony/polyfill@1be936e"},"references":[{"type":"WEB","url":"https://github.com/symfony/polyfill/releases/tag/v1.38.1"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/46xxx/CVE-2026-46644.json"},{"type":"ADVISORY","url":"https://github.com/symfony/polyfill/security/advisories/GHSA-2xf4-cg6j-vhgq"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-46644"},{"type":"FIX","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"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-08-12T03:51:18.650967266Z"}}