symfony/polyfill-intl-idn: xn-- labels with ASCII-only Punycode payloads are treated as equivalent to their decoded formGHSA-2xf4-cg6j-vhgq
Fix: symfony/polyfill@1be936eGHSA-2xf4-cg6j-vhgq is a CWE-1289 vulnerability in symfony/polyfill. A fix is available for symfony/polyfill — see the affected versions and patch details below.
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for GHSA-2xf4-cg6j-vhgq.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
Real-World Exposure
symfony/polyfill🐘symfony/polyfill-intl-idnReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Packagist packages — download data is not available via public APIs for these ecosystems.
Description
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 Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐘Packagist | symfony/polyfill | ≥ 1.17.1&&< 1.38.1 | 1.38.1composer require symfony/polyfill:^1.38.1 |
| 🐘Packagist | symfony/polyfill-intl-idn | ≥ 1.17.1&&< 1.38.1 | 1.38.1composer require symfony/polyfill-intl-idn:^1.38.1 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for symfony/polyfill, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update symfony/polyfill to 1.38.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-2xf4-cg6j-vhgq is resolved across your whole dependency graph.
Workarounds
Restrict outbound requests from the affected component to an allowlist of hosts, block access to link-local and internal address ranges at the network layer, and require authentication on internal services so a forged request cannot reach them unauthenticated.
Frequently Asked Questions
Is GHSA-2xf4-cg6j-vhgq in your dependencies?
Find it across Packagist, including transitive dependencies.