symfony/polyfill-intl-idn: xn-- labels with ASCII-only Punycode payloads are treated as equivalent to their decoded form
Description
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.
As 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).
Example with IDNA_USE_STD3_RULES | IDNA_CHECK_BIDI | IDNA_CHECK_CONTEXTJ | IDNA_NONTRANSITIONAL_TO_ASCII:
| Input | Polyfill output | Native ext-intl output |
|---|---|---|
poc.xn--kc1zs4-.com |
poc.kc1zs4.com |
false (errors=1024) |
poc.kc1zs4.xn-- |
poc.kc1zs4. |
false (errors=1024) |
Applications using the polyfill to canonicalise or compare hostnames inherit the inconsistency.
Resolution
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.
The patch for this issue is available here for branch 1.x.
Credits
Symfony would like to thank Nazy Mad for reporting the issue and Nicolas Grekas for providing the fix.
Affected software
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| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Composer package | symfony/polyfill | >= 1.17.1, < 1.38.1 | 1.38.1 |
| Composer package | symfony/polyfill-intl-idn | >= 1.17.1, < 1.38.1 | 1.38.1 |
References
- https://github.com/symfony/polyfill/security/advisories/GHSA-2xf4-cg6j-vhgq
- https://github.com/symfony/polyfill/commit/1be936e2491ccebe152bd736dfc91eb1422c8bec
- https://github.com/FriendsOfPHP/security-advisories/blob/master/symfony/polyfill-intl-idn/CVE-2026-46644.yaml
- https://github.com/FriendsOfPHP/security-advisories/blob/master/symfony/polyfill/CVE-2026-46644.yaml
- https://github.com/symfony/polyfill
- https://symfony.com/cve-2026-46644
- CVSS vector
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- Weakness (CWE)
CWE-1289- Platform
- Composer
- Package
symfony/polyfill, symfony/polyfill-intl-idn- Disclosed
- 28.05.2026
- Updated
- 11.10.2026 10:22
Sources and licenses
OSV / GitHub Advisory Database - GHSA-2xf4-cg6j-vhgq - CC BY 4.0
Vulnerability history: symfony/polyfill-intl-idn
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This is the only known vulnerability of this package.
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