CVE-2026-46644
Summary
by MITRE • 07/14/2026
Symfony Polyfill backports PHP features and provides compatibility layers for extensions and functions. From 1.17.1 until 1.38.1, symfony/polyfill-intl-idn accepts xn-- labels whose Punycode payload is empty or decodes to ASCII-only code points because Idn::process() does not enforce the UTS #46 revision 33 requirement that decoded ACE labels contain at least one non-ASCII code point. Originally unequal domain names can be regarded as equal, which can lead to blacklist bypassing, inconsistent URL parsing, and server-side request forgery in applications using the polyfill to canonicalise or compare hostnames. This issue is fixed in version 1.38.1.
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Analysis
by VulDB Data Team • 07/15/2026
The Symfony Polyfill project serves as a critical compatibility layer that backports modern PHP features and provides extensions for older PHP versions, enabling developers to leverage contemporary functionality across diverse environments. The specific vulnerability identified affects the symfony/polyfill-intl-idn component which handles internationalized domain name processing through the Idn::process() function. This flaw exists in versions ranging from 1.17.1 through 1.38.1 and represents a significant security weakness in how Punycode labels are validated during internationalized domain name processing.
The technical root cause of this vulnerability stems from the polyfill's failure to properly enforce UTS #46 revision 33 compliance standards for Internationalized Domain Name processing. Specifically, the Idn::process() method accepts xn-- labels where the Punycode payload is either empty or decodes to ASCII-only code points, violating fundamental requirements that decoded ACE labels must contain at least one non-ASCII code point. This relaxation of validation criteria creates a condition where domain names that should be distinct can be treated as equivalent due to the improper handling of empty or ASCII-only Punycode sequences.
The operational impact of this vulnerability extends beyond simple domain name comparison issues and presents serious security implications for applications relying on hostname canonicalization and comparison functions. Attackers can exploit this weakness to bypass security blacklists by creating domain names that appear distinct but are processed as equivalent, potentially allowing malicious domains to evade detection systems. Additionally, the vulnerability enables inconsistent URL parsing behavior that can lead to unexpected application states, while also creating opportunities for server-side request forgery attacks where applications incorrectly process hostnames due to faulty canonicalization logic.
Applications using the Symfony Polyfill for hostname comparison, security filtering, or URL processing are particularly at risk from this vulnerability. The issue creates a scenario where malicious actors can craft domain names that exploit the lax validation rules to bypass security controls designed to prevent access to harmful resources. This vulnerability directly relates to CWE-284 Access Control Issues and aligns with ATT&CK techniques involving privilege escalation through input manipulation. The fix implemented in version 1.38.1 addresses the core validation problem by enforcing proper UTS #46 revision 33 compliance, ensuring that all decoded ACE labels contain at least one non-ASCII code point as required by international standards for proper domain name processing and security enforcement. Organizations should immediately upgrade to version 1.38.1 or later to mitigate this vulnerability and restore proper security controls around hostname validation and comparison operations.