# CVE-2026-33635: iCalendar has ICS injection via unsanitized URI property values

> Attackers can inject arbitrary calendar lines into generated ICS files via unsanitized URI properties, potentially manipulating calendar data in downstream clients.

- **Severity:** Medium 4.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 26 Mar 2026
- **Updated:** 30 Mar 2026
- **Also known as:** DEBIAN-CVE-2026-33635, GHSA-pv9c-9mfh-hvxq, UBUNTU-CVE-2026-33635

## What is CVE-2026-33635?

CVE-2026-33635 is a medium-severity vulnerability in icalendar, published on 26 Mar 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

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.

Weakness: CWE-93 (Improper Neutralization of CRLF Sequences)

## Which versions are affected by CVE-2026-33635?

2 open source packages ship an affected version, including icalendar. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `icalendar` | RubyGems | From 2.0.0 | 2.12.2 |
| `icalendar` | RubyGems | From 2.0.0 | 2.12.2 |

Products: Icalendar Project Icalendar.

## How do I fix CVE-2026-33635?

Upgrade icalendar to 2.12.2 or later.

## Is CVE-2026-33635 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33635 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-33635 and fix it if it is affected.

What it is: CVE-2026-33635 is a medium-severity vulnerability in icalendar, published on 26 Mar 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33635 exists.

Affected packages and fixed versions:
- icalendar (RubyGems): affected from 2.0.0; fixed in 2.12.2
- icalendar (RubyGems): affected from 2.0.0; fixed in 2.12.2

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-33635/ (Markdown: https://insights.sbomify.com/CVE-2026-33635/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-33635
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-33635
- OSV: https://osv.dev/vulnerability/CVE-2026-33635

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-33635/
Summaries are generated and may contain mistakes; the official description above is the record.