Compliance Impact

The vulnerability allows an attacker to inject arbitrary calendar lines into .ics files by exploiting unsanitized URI property values. This can lead to the modification or addition of calendar data such as attendees, URLs, or alarms, which downstream clients may process as legitimate.

While the CVE description and resources do not explicitly mention compliance with standards like GDPR or HIPAA, the integrity impact of this vulnerability is low, meaning it can alter calendar data but does not directly compromise confidentiality or availability.

However, because attacker-controlled data can be injected and processed as legitimate, organizations subject to regulations requiring data integrity and protection against unauthorized data modification (such as GDPR's data integrity principles or HIPAA's data integrity and audit requirements) could be indirectly affected if calendar data is used in regulated workflows.

No direct references to compliance impact or regulatory violations are provided in the available information.

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Executive Summary

CVE-2026-33635 is a vulnerability in the Ruby library "icalendar" that deals with iCalendar files. The issue occurs in versions from 2.0.0 up to but not including 2.12.2, where the library does not properly sanitize URI property values during .ics file serialization.

Specifically, when the library tries to parse URIs using `URI.parse` and fails, it falls back to using the raw input string. This raw string is then serialized without removing or escaping carriage return (\r) or line feed (\n) characters. Because these characters are embedded directly into the ICS output, an attacker can inject CRLF sequences to terminate the current property line and create new arbitrary ICS properties or components.

This means an attacker can add malicious calendar lines such as additional attendees, modified URLs, alarms, or other calendar fields by injecting specially crafted input into URI-related properties like url, source, image, organizer, attach, attendee, conference, or tzurl.

The vulnerability is fixed in version 2.12.2 by encoding control characters in URIs to prevent injection.

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Impact Analysis

This vulnerability can impact you if your application generates .ics calendar files from partially untrusted metadata that includes URI properties.

An attacker can exploit this flaw to inject arbitrary calendar lines into the .ics file, which downstream calendar clients or importers may process as legitimate event data.

• Injection of additional attendees to calendar events.
• Modification of URLs or other URI-related properties.
• Insertion of alarms or other calendar fields that were not intended by the original event creator.

Such unauthorized modifications can lead to misinformation, confusion, or manipulation of calendar data, potentially disrupting scheduling or causing users to act on maliciously altered information.

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Detection Guidance

This vulnerability involves injection of CRLF characters into URI property values in .ics files generated by the icalendar Ruby library. Detection involves inspecting .ics files generated by your applications for unexpected or maliciously crafted URI fields containing carriage return (\r) or line feed (\n) characters that could lead to ICS injection.

You can detect suspicious .ics files by searching for CR or LF characters in URI-related properties such as url, source, image, organizer, attach, attendee, conference, or tzurl.

Example commands to detect such injection attempts in .ics files or network captures include:

• Using grep or similar tools to find CR or LF characters in URI fields within .ics files: grep -P '\r|\n' suspicious_file.ics
• Searching for suspicious URI property lines in .ics files: grep -E '(url|source|image|organizer|attach|attendee|conference|tzurl):' suspicious_file.ics | grep -P '\r|\n'
• Using network packet capture tools (e.g., tcpdump or Wireshark) to capture .ics file downloads or calendar data and inspecting for injected CRLF sequences in URI fields.

Since the vulnerability arises from unsanitized input in URI serialization, monitoring application logs or input sources for unexpected CR or LF characters in URI metadata can also help detect attempts.

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Mitigation Strategies

The primary mitigation is to upgrade the icalendar Ruby gem to version 2.12.2 or later, where the vulnerability is patched by properly percent-encoding ASCII control characters (including CR and LF) in URI property values during serialization.

If upgrading immediately is not possible, you should sanitize or validate all input metadata used to generate .ics files, ensuring that URI fields do not contain raw carriage return or line feed characters.

Additional mitigation steps include:

• Reject or escape CR (\r) and LF (\n) characters in any URI-related input before passing it to the icalendar library.
• Implement input validation and sanitization on all untrusted metadata sources that contribute to calendar file generation.
• Monitor calendar files and downstream clients for unexpected or unauthorized calendar entries that could indicate exploitation.

These steps reduce the risk of ICS injection attacks that could modify calendar data or add malicious entries.

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