CVE-2026-33635: iCalendar is a Ruby library for dealing with iCalendar files in the iCalendar format defined by RFC-5545

Routine

MEDIUM 4.3UPGRADE

CVE-2026-33635 is a medium-severity flaw (CVSS 4.3) affecting icalendar. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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 `valu

Why this priority

Public exploit reference

A fixed release or distro patch exists.

Risk facts

CVSS score	4.3 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	Required
EPSS exploit chance	0.3% (top 83%)
CISA exploited list	No
Weakness	CWE-93
Published	2026-03-26
Last modified	2026-06-17

Publisher (CNA): GitHub, Inc. · Vendor: icalendar project

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-03-26 21:17 UTCPublished CVE record published
2026-06-17 10:37 UTCUpdated Record last modified

icalendar project icalendar UPGRADE

Affected: >= 2.0.0 and < 2.12.2

Fixed in: 2.12.2

Safe to downgrade to: a release before 2.0.0

1. Check installed version

apt-cache policy icalendar || rpm -q icalendar || icalendar --version

2. UPGRADE to 2.12.2 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade icalendar

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade icalendar

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

3. If no update is available yet, DOWNGRADE to a release before 2.0.0 (Debian / Ubuntu)

apt-cache madison icalendar sudo apt install icalendar=<VERSION_BEFORE_2.0.0> sudo apt-mark hold icalendar

3. If no update is available yet, DOWNGRADE (RHEL / Amazon Linux / Fedora)

dnf --showduplicates list icalendar sudo dnf downgrade icalendar-<VERSION_BEFORE_2.0.0> sudo dnf versionlock add icalendar

Downgrade risk

Downgrading can break dependent software and may reopen older bugs. Read the distro advisory first and test on a non-production host.

After downgrade

Restart the service, and keep the package held until a fixed release ships.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-33635?

icalendar: upgrade to 2.12.2. Exact commands for each OS are on this page.

Is CVE-2026-33635 being exploited?

CVE-2026-33635 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-33635 affect?

CVE-2026-33635 affects icalendar. Severity is medium (CVSS 4.3). 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

How urgent is CVE-2026-33635?

Priority 24/100. Public exploit reference

Software pages

Ruby vulnerabilities

References

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