icalendar: Algorithmic Complexity in Equality

{
  "id": "3e201f128e0547c01e687c93941cd64b5e98967b67b05dc2824db83b7fdc24e9",
  "url": "https://osv.dev/vulnerability/CVE-2026-55099",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "icalendar: Algorithmic Complexity in Equality",
    "summary": "icalendar is an RFC 5545 compatible parser and generator of iCalendar files for Python. From 7.1.0 until 7.1.3, the Component equality method in src/icalendar/cal/component.py compares nested subcomponents with two membership loops, and each membership test invokes the same method on child components, causing O(2^n) work relative to nesting depth. Component.from_ical accepts arbitrarily nested BEGIN:VEVENT blocks without a depth limit, so an attacker can submit a sub-kilobyte .ics file containing equal nested subtrees and trigger the cost when an application performs equality, inequality, membership, deduplication, test-assertion, round-trip, or normalization comparisons. Parsing alone does not trigger the issue, and comparisons that differ early short-circuit, but a few hundred bytes can pin a CPU core for minutes or indefinitely, causing denial of service in calendar sync or import endpoints, invite processing, and other comparison paths. This issue is fixed in version 7.1.3.",
    "evidence": "osv:CVE-2026-55099",
    "products": [],
    "severity": "high",
    "structured": {
      "id": "CVE-2026-55099",
      "aliases": [
        "CVE-2026-55099",
        "GHSA-cv84-9p8j-fj68",
        "PYSEC-2026-3851"
      ],
      "metrics": [
        {
          "type": "CVSS_V3",
          "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://github.com/collective/icalendar",
              "type": "GIT",
              "events": [
                {
                  "introduced": "1300b76fb3fe40d987cc3715735c026b3c1a2191"
                },
                {
                  "fixed": "f7240bc661548f9806c930f2a95ee2f003cfa8ca"
                },
                {
                  "fixed": "b6b2608ae3af6de40695b4e40f71847485aa0b49"
                },
                {
                  "fixed": "cad40cd112c93fd142ec12cc5b37445a849b8a79"
                }
              ],
              "database_specific": {
                "extracted_events": [
                  {
                    "range": [
                      {
                        "introduced": "7.1.0"
                      },
                      {
                        "fixed": "7.1.3"
                      }
                    ],
                    "source": "AFFECTED_FIELD",
                    "strategy": "StringRangeExpression"
                  },
                  {
                    "range": [
                      {
                        "fixed": "b6b2608ae3af6de40695b4e40f71847485aa0b49"
                      }
                    ],
                    "source": "REFERENCES_COMMIT"
                  },
                  {
                    "range": [
                      {
                        "fixed": "cad40cd112c93fd142ec12cc5b37445a849b8a79"
                      }
                    ],
                    "source": "REFERENCES_COMMIT"
                  },
                  {
                    "range": [
                      {
                        "fixed": "f7240bc661548f9806c930f2a95ee2f003cfa8ca"
                      }
                    ],
                    "source": "REFERENCES_TAG",
                    "original_tag": "v7.1.3"
                  }
                ]
              }
            }
          ],
          "versions": [
            "v7.1.2",
            "v7.1.1",
            "v7.1.0"
          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-55099.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:29.905255991Z",
      "provider": "osv",
      "references": [
        "https://github.com/collective/icalendar/releases/tag/v7.1.3",
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/55xxx/CVE-2026-55099.json",
        "https://github.com/collective/icalendar/security/advisories/GHSA-cv84-9p8j-fj68",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-55099",
        "https://github.com/collective/icalendar/commit/b6b2608ae3af6de40695b4e40f71847485aa0b49",
        "https://github.com/collective/icalendar/commit/cad40cd112c93fd142ec12cc5b37445a849b8a79"
      ],
      "provider_metadata": {
        "cwe_ids": [
          "CWE-400",
          "CWE-407"
        ],
        "cna_assigner": "GitHub_M",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/55xxx/CVE-2026-55099.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-55099",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 496698,
  "published_at": "2026-08-25T19:27:02.236+00:00"
}