iCalendar has ICS injection via unsanitized URI property values

Summary

.ics serialization does not properly sanitize URI property values, enabling ICS injection through attacker-controlled input, adding arbitrary calendar lines to the output.

Details

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)

Relevant code:

• lib/icalendar/values/uri.rb:16

PoC

Run the following with the library loaded:

require "icalendar/value"
require "icalendar/values/uri"

v = Icalendar::Values::Uri.new("https://a.example/ok\r\nATTENDEE:mailto:[email protected]")
puts v.to_ical(Icalendar::Values::Text)

output:

;VALUE=URI:https://a.example/ok
ATTENDEE:mailto:[email protected]

Impact

Applications 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.

Fix

Reject raw CR and LF characters in URI-typed values before serialization, or escape/encode them so they cannot terminate the current ICS content line.