iCalendar up to 2.12.1 Icalendar::Values crlf injection

Summaryinfo

A vulnerability has been found in iCalendar up to 2.12.1 and classified as problematic. The affected element is the function Icalendar::Values. This manipulation causes crlf injection. This vulnerability is registered as CVE-2026-33635. Remote exploitation of the attack is possible. No exploit is available. The affected component should be upgraded.

Detailsinfo

A vulnerability classified as problematic was found in iCalendar up to 2.12.1. Affected by this vulnerability is the function Icalendar::Values. The manipulation with an unknown input leads to a crlf injection vulnerability. The CWE definition for the vulnerability is CWE-93. The product uses CRLF (carriage return line feeds) as a special element, e.g. to separate lines or records, but it does not neutralize or incorrectly neutralizes CRLF sequences from inputs. As an impact it is known to affect integrity. The summary by CVE is:

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.

It is possible to read the advisory at github.com. This vulnerability is known as CVE-2026-33635 since 03/23/2026. The exploitation appears to be easy. The attack can be launched remotely. The exploitation doesn't need any form of authentication. It demands that the victim is doing some kind of user interaction. Technical details of the vulnerability are known, but there is no available exploit.

The vulnerability scanner Nessus provides a plugin with the ID 304171 (Linux Distros Unpatched Vulnerability : CVE-2026-33635), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 2.12.2 eliminates this vulnerability. Applying the patch b8d23b490363ee5fffaec1d269a8618a912ca265 is able to eliminate this problem. The bugfix is ready for download at github.com. The best possible mitigation is suggested to be upgrading to the latest version.

The vulnerability is also documented in the vulnerability database at Tenable (304171). Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.

Productinfo

Type

• Calendar Software

Name

• iCalendar

Version

• 2.12.0
• 2.12.1

License

• open-source

Website

• Product: https://github.com/icalendar/icalendar/

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