CVE-2026-33635
ADVISORY - githubSummary
Summary
.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:evil@example.com")
puts v.to_ical(Icalendar::Values::Text)
output:
;VALUE=URI:https://a.example/ok
ATTENDEE:mailto:evil@example.com
Impact
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.
Fix
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.
Common Weakness Enumeration (CWE)
Improper Neutralization of CRLF Sequences ('CRLF Injection')
Improper Neutralization of CRLF Sequences ('CRLF Injection')
GitHub
2.8
CVSS SCORE
4.3medium| Package | Type | OS Name | OS Version | Affected Ranges | Fix Versions |
|---|---|---|---|---|---|
| icalendar | gem | - | - | >=2.0.0,<2.12.2 | 2.12.2 |
CVSS:3 Severity and metrics
The CVSS metrics represent different qualitative aspects of a vulnerability that impact the overall score, as defined by the CVSS Specification.
The vulnerable component is bound to the network stack, but the attack is limited at the protocol level to a logically adjacent topology. This can mean an attack must be launched from the same shared physical (e.g., Bluetooth or IEEE 802.11) or logical (e.g., local IP subnet) network, or from within a secure or otherwise limited administrative domain (e.g., MPLS, secure VPN to an administrative network zone). One example of an Adjacent attack would be an ARP (IPv4) or neighbor discovery (IPv6) flood leading to a denial of service on the local LAN segment (e.g., CVE-2013-6014).
Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success when attacking the vulnerable component.
The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
Successful exploitation of this vulnerability requires a user to take some action before the vulnerability can be exploited. For example, a successful exploit may only be possible during the installation of an application by a system administrator.
An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
There is no loss of confidentiality.
Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is limited. The data modification does not have a direct, serious impact on the impacted component.
There is no impact to availability within the impacted component.
NIST
2.8
CVSS SCORE
4.3mediumDebian
-
Ubuntu
2.8