CVE-2026-19953

URI versions before 5.36 for Perl encode non-NFC host names to non-standard punycode labels via missing normalization in nameprep. nameprep lowercases each host label but performs no Unicode normalization. IDNA requires a label to be normalized to Form C before it is encoded (RFC 5891), so a label that is not already in NFC is encoded to a different A-label than its normalized form. A label built from the precomposed Devanagari sequence U+0958 U+093E encodes to xn--72b5c without normalization but to xn--11b2fg after NFC normalization, and xn--72b5c does not round-trip back to the original label. Any caller that reads host() from a URI built from untrusted input and uses it for a security decision (an allow or deny list, an SSRF filter, deduplication, a cache key) sees the non-standard label, while a client that fetches the same URL resolves the NFC form, so the check and the fetch can disagree about the host.

• CWE-1289

• https://github.com/libwww-perl/URI/commit/956619a9e94f86d8d2c529b4e06a3674c54a73e7.patch
• https://github.com/libwww-perl/URI/pull/191
• https://metacpan.org/release/OALDERS/URI-5.36/changes
• https://www.rfc-editor.org/rfc/rfc5891#section-5.2
• https://www.openwall.com/lists/oss-security/2026/08/31/14

Published

Aug 31, 2026

Last Modified

Sep 3, 2026

Status

Deferred

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