Vulnerability

[ Description ]Official description

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.

[ Fixes ]Fixes and patches

No fix or interim patch is linked from our sources yet. Check the vendor references below and limit exposure of affected systems until a fix is available.

[ Advisories ]Government CERT advisories

• BSI CERT-Bund18 Oct 2026WID-SEC-2026-4024

  Red Hat Quay and Enterprise Linux (pypdf, urllib3, URI): Multiple vulnerabilities Translated from German

  Risk mediumCVE-2026-102998CVE-2026-97687CVE-2026-19953+2 more CVEs

[ References ]References

• github.com/libwww-perl/URI/commit/956619a9e94f86d8d2c529b4e06a3674c54a73e7.patchpatch
• github.com/libwww-perl/URI/pull/191issue-tracking
• metacpan.org/release/OALDERS/URI-5.36/changesrelease-notes
• www.rfc-editor.org/rfc/rfc5891technical-description