CVE-2026-33635 is a medium-severity (CVSS 4.3) CWE-93 vulnerability in icalendar. A fix is available for icalendar — see the affected versions and patch details below.
iCalendar has ICS injection via unsanitized URI property values
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-33635.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
icalendarReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects RubyGems packages — download data is not available via public APIs for these ecosystems.
Description
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:[email protected]")
puts v.to_ical(Icalendar::Values::Text)
output:
;VALUE=URI:https://a.example/ok
ATTENDEE:mailto:[email protected]
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 💎RubyGems | icalendar | ≥ 2.0.0&&< 2.12.2 | 2.12.2bundle update icalendar --conservative |
Affected Products
icalendaricalendar_projectDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for icalendar, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update icalendar to 2.12.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-33635 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
Frequently Asked Questions
Is CVE-2026-33635 in your dependencies?
Find it across RubyGems, including transitive dependencies.