CVE-2026-33635
MediumCVSS 4.3Summary
The iCalendar Ruby library (versions 2.0.0 to before 2.12.2) does not properly sanitize URI property values during .ics serialization, enabling ICS injection through attacker-controlled input and adding arbitrary calendar lines to the output. Icalendar::Values::Uri falls back to the raw input string when URI.parse fails and serializes it without removing CRLF characters, allowing termination of the original property and creation of a new one. Applications generating .ics files from partially untrusted metadata are impacted, and calendar clients may process injected data as legitimate events.
Risk Assessment
An attacker can inject malicious data into .ics calendar files, potentially leading to added fake attendees, modified URLs, alarms, or other calendar fields processed by calendar clients. This could result in phishing or unauthorized actions in users' calendars.
Recommendation
Update the iCalendar library to version 2.12.2 or later, which contains the patch. Additionally, validate and sanitize input data before generating .ics files.
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Original NVD description (English source)
iCalendar is a Ruby library for dealing with iCalendar files in the iCalendar format defined by RFC-5545. Starting in version 2.0.0 and prior to version 2.12.2, .ics serialization does not properly sanitize URI property values, enabling ICS injection through attacker-controlled input, adding arbitrary calendar lines to the output. `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). 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. Version 2.12.2 contains a patch for the issue.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS