{"id":"CVE-2026-33635","aliases":["GHSA-pv9c-9mfh-hvxq"],"url":"https://o3.security/vulnerability/CVE-2026-33635","summary":"iCalendar has ICS injection via unsanitized URI property values","details":"### Summary\n.ics serialization does not properly sanitize URI property values, enabling ICS injection through attacker-controlled input, adding arbitrary calendar lines to the output.\n\n### Details\n`Icalendar::Values::Uri` falls back to the raw input string when `URI.parse` fails and later serializes it with `value.to_s` without removing or escaping `\\r` or `\\n` characters. That value is embedded directly into the final ICS line by the normal serializer, so a payload containing CRLF can terminate the original property and create a new ICS property or component. (It looks like you can inject via url, source, image, organizer, attach, attendee, conference, tzurl because of this)\n\nRelevant code:\n- `lib/icalendar/values/uri.rb:16`\n\n### PoC\nRun the following with the library loaded:\n\n```ruby\nrequire \"icalendar/value\"\nrequire \"icalendar/values/uri\"\n\nv = Icalendar::Values::Uri.new(\"https://a.example/ok\\r\\nATTENDEE:mailto:evil@example.com\")\nputs v.to_ical(Icalendar::Values::Text)\n```\n\noutput:\n\n```text\n;VALUE=URI:https://a.example/ok\nATTENDEE:mailto:evil@example.com\n```\n\n### Impact\nApplications that generate `.ics` files from partially untrusted metadata are impacted. As a result, downstream calendar clients or importers may process attacker-supplied content as if it were legitimate event data, such as added attendees, modified URLs, alarms, or other calendar fields.\n\n## Fix\nReject raw CR and LF characters in `URI`-typed values before serialization, or escape/encode them so they cannot terminate the current ICS content line.","published":"2026-03-26T20:30:43.696Z","modified":"2026-10-28T02:31:14.089791112Z","cvss":{"score":4.3,"severity":"MEDIUM","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N"},"epss":{"score":0.00244,"percentile":0.15567,"asOf":"2026-09-06"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"RubyGems","name":"icalendar","fixedVersion":"2.12.2"}],"fix":{"url":"https://github.com/icalendar/icalendar/commit/b8d23b490363ee5fffaec1d269a8618a912ca265","label":"icalendar/icalendar@b8d23b4"},"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/33xxx/CVE-2026-33635.json"},{"type":"ADVISORY","url":"https://github.com/icalendar/icalendar/security/advisories/GHSA-pv9c-9mfh-hvxq"},{"type":"ADVISORY","url":"https://github.com/rubysec/ruby-advisory-db/blob/master/gems/icalendar/CVE-2026-33635.yml"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-33635"},{"type":"FIX","url":"https://github.com/icalendar/icalendar/commit/b8d23b490363ee5fffaec1d269a8618a912ca265"},{"type":"PACKAGE","url":"https://github.com/icalendar/icalendar"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-28T02:31:14.089791112Z"}}