CVE-2026-33635

.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]

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.

CVE-2026-33635 has a CVSS score of 4.3 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.12.2); upgrading removes the vulnerable code path.

Kodem intelligence

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.

1. What is CVE-2026-33635? CVE-2026-33635 is a medium-severity security vulnerability in icalendar (rubygems), affecting versions >= 2.0.0, < 2.12.2. It is fixed in 2.12.2.
2. How severe is CVE-2026-33635? CVE-2026-33635 has a CVSS score of 4.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of icalendar are affected by CVE-2026-33635? icalendar (rubygems) versions >= 2.0.0, < 2.12.2 is affected.
4. Is there a fix for CVE-2026-33635? Yes. CVE-2026-33635 is fixed in 2.12.2. Upgrade to this version or later.
5. Is CVE-2026-33635 exploitable, and should I be worried? Whether CVE-2026-33635 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-33635 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-33635? Upgrade icalendar to 2.12.2 or later.