iCalendar has ICS injection via unsanitized URI property values

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

4.3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N

EPSS

• CVE-2026-33635: 0.25% estimated exploitation activity in the next 30 days; percentile 16.99 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

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.tos 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: output: 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…

Published by the source

24 Mar 2026, 19:13 UTC

Last modified by the source

27 Mar 2026, 22:00 UTC

Source last checked

26 Oct 2026, 15:51 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-pv9c-9mfh-hvxq

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# Patch Next advisory brief: CVE-2026-33635

Advisory: iCalendar has ICS injection via unsanitized URI property values
IDs: CVE-2026-33635, GHSA-pv9c-9mfh-hvxq
Record: https://patchnext.com/advisories/cve-2026-33635 (revision 1, source checked 26 Oct 2026, 15:51 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 4.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: icalendar 2.12.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- RubyGems icalendar: affected >= 2.0.0, < 2.12.2; fixed in 2.12.2

## Suggested inspection (read-only)
- RubyGems icalendar: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on icalendar.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-pv9c-9mfh-hvxq (GitHub-reviewed advisory GHSA-pv9c-9mfh-hvxq lists affected ranges: RubyGems icalendar >=2.0.0 <2.12.2)

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