CVE-2026-33635Public exploitNetwork attack vector

Calendar-data injection via unescaped URI control bytes

Published Mar 26, 2026 · Updated Mar 26, 2026

ICS injection in the icalendar Ruby library 2.0.0 through 2.12.1 allows remote attackers to add arbitrary calendar lines through crafted URI values. Icalendar::Values::Uri retains raw input when URI parsing fails, and value_ical serializes CR and LF bytes without encoding them, letting a value terminate its property line. Exposure requires an application to place partially untrusted metadata in an ICS URI field and a downstream calendar client or importer to process the generated file.

CVSS severity4.3
Medium
EPSS probability0.24%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

ICS injection in the icalendar Ruby library 2.0.0 through 2.12.1 allows remote attackers to add arbitrary calendar lines through crafted URI values. Icalendar::Values::Uri retains raw input when URI parsing fails, and value_ical serializes CR and LF bytes without encoding them, letting a value terminate its property line. Exposure requires an application to place partially untrusted metadata in an ICS URI field and a downstream calendar client or importer to process the generated file.

The record

CVE
CVE-2026-33635
Published
Mar 26, 2026
Updated
Mar 26, 2026
Vendor
Collective
Product
icalendar
Classifications
CWE-93, T1565.002
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 26, 2026CVE publishedPublication date reported by the CVE source.
  2. Mar 26, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=>= 2.0.0, < 2.12.2

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Collective · icalendarversion=>= 2.0.0, < 2.12.2

Published CVSS scores

4.3GitHub, Inc.CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-93Improper Neutralization of CRLF Sequences ('CRLF Injection')
T1565.002Transmitted Data Manipulation

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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