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URI versions before 5.36 for Perl encode non-NFC host names to non-standard punycode labels via missing normalization in nameprep

Type: advisorySources: OSV

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.

Products
Not specified
Affected versions
Not specified
Fixed versions
Not specified
CVEs
CVE-2026-19953
Confirmed fixed CVEs
Not specified
Updated in this feed
5 Nov 2026, 23:55 UTC · Revision 1

Source evidence

URI versions before 5.36 for Perl encode non-NFC host names to non-standard punycode labels via missing normalization in nameprep

core-osv · Original source

Published 31 Aug 2026, 17:29 UTC

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.

osv:CVE-2026-19953
Complete source record
{
  "id": "c14aa310dd314929c9d67a7cfe4de7fd2814d567016bcd20e1e37e34d121dd06",
  "url": "https://osv.dev/vulnerability/CVE-2026-19953",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "URI versions before 5.36 for Perl encode non-NFC host names to non-standard punycode labels via missing normalization in nameprep",
    "summary": "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.",
    "evidence": "osv:CVE-2026-19953",
    "products": [],
    "severity": "medium",
    "structured": {
      "id": "CVE-2026-19953",
      "aliases": [
        "CVE-2026-19953"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://github.com/libwww-perl/uri",
              "type": "GIT",
              "events": [
                {
                  "introduced": "0"
                },
                {
                  "fixed": "b265e58bb1ead8133eb7614b76bb7891bd96cea0"
                },
                {
                  "fixed": "956619a9e94f86d8d2c529b4e06a3674c54a73e7"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "range": [
                      {
                        "introduced": "0"
                      },
                      {
                        "fixed": "5.36"
                      }
                    ],
                    "source": "AFFECTED_FIELD",
                    "strategy": "StandardRange"
                  },
                  {
                    "range": [
                      {
                        "fixed": "956619a9e94f86d8d2c529b4e06a3674c54a73e7"
                      }
                    ],
                    "source": "REFERENCES_COMMIT"
                  }
                ]
              }
            }
          ],
          "versions": [
            "v5.35",
            "v5.34",
            "v5.33",
            "v5.32",
            "v5.31",
            "v5.30",
            "v5.29",
            "v5.28",
            "v5.27",
            "v5.26",
            "v5.25",
            "v5.24",
            "v5.23",
            "v5.22",
            "v5.21",
            "v5.20",
            "v5.19",
            "v5.18",
            "v5.17",
            "v5.16",
            "v5.15",
            "v5.14",
            "v5.13",
            "v5.12",
            "v5.11",
            "v5.10",
            "v5.09",
            "v5.08",
            "v5.07",
            "v5.06",
            "v5.05",
            "v1.76",
            "v1.75",
            "v1.74",
            "v1.73",
            "v1.72",
            "1.71",
            "1.70_001",
            "1.70",
            "1.69",
            "1.68",
            "1.67",
            "1.66",
            "1.65",
            "1.64",
            "1.63",
            "1.62",
            "1.61",
            "1.60",
            "1.59",
            "1.58",
            "1.57",
            "1.56",
            "1.54",
            "1.53",
            "1.52",
            "1.51",
            "1.50",
            "1.40",
            "1.39",
            "1.38",
            "1.37",
            "R1.36",
            "R1.35",
            "R1.34",
            "R1.33",
            "R1.32",
            "R1.30",
            "R1.29",
            "R1.28",
            "R1.27",
            "R1.26",
            "R1.25",
            "R1.24",
            "R1.23",
            "R1.22",
            "R1.21",
            "R1.20",
            "R1.19",
            "R1.18",
            "R1.17",
            "R1.16",
            "R1.15",
            "R1.14",
            "R1.12",
            "R1.11",
            "R1.10",
            "R1.09",
            "R1.08",
            "R1.07",
            "R1.06",
            "R1.05",
            "R1.03",
            "R1.02",
            "R1.01",
            "R1.00",
            "R0.90.02",
            "R0.90.01",
            "R0.09.02",
            "R0.09",
            "R5.36",
            "R0.01",
            "R5.22",
            "R5.18",
            "R5.16",
            "B13",
            "B11",
            "B7",
            "B6"
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-19953.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:45.416654104Z",
      "provider": "osv",
      "references": [
        "https://cpan.org/modules",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/19xxx/CVE-2026-19953.json",
        "https://metacpan.org/release/OALDERS/URI-5.36/changes",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-19953",
        "https://github.com/libwww-perl/URI/pull/191",
        "https://github.com/libwww-perl/URI/commit/956619a9e94f86d8d2c529b4e06a3674c54a73e7.patch",
        "https://github.com/libwww-perl/URI",
        "https://www.rfc-editor.org/rfc/rfc5891#section-5.2"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-1289"
        ],
        "cna_assigner": "CPANSec",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/19xxx/CVE-2026-19953.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-19953",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 513253,
  "published_at": "2026-08-31T17:29:42.929+00:00"
}