CVE-2026-33635 iCalendar has ICS injection via unsanitized URI property values
Attackers can inject arbitrary calendar lines into generated ICS files via unsanitized URI properties, potentially manipulating calendar data in downstream clients.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-33635?
CVE-2026-33635 is a medium-severity vulnerability in icalendar, published on 26 Mar 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official description
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.
Weakness: CWE-93
Which versions are affected by CVE-2026-33635?
2 open source packages ship an affected version, including icalendar. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
icalendar RubyGems | From 2.0.0 | 2.12.2 |
icalendar RubyGems | From 2.0.0 | 2.12.2 |
Products include Icalendar Project Icalendar.
How do I fix CVE-2026-33635?
Upgrade icalendar to 2.12.2 or later.
Is CVE-2026-33635 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33635 exists.
Sources
Also known as DEBIAN-CVE-2026-33635, GHSA-pv9c-9mfh-hvxq, UBUNTU-CVE-2026-33635.