CVE-2026-55099
icalendar: Algorithmic Complexity in Equality
Description
icalendar is an RFC 5545 compatible parser and generator of iCalendar files for Python. From 7.1.0 until 7.1.3, the Component equality method in src/icalendar/cal/component.py compares nested subcomponents with two membership loops, and each membership test invokes the same method on child components, causing O(2^n) work relative to nesting depth. Component.from_ical accepts arbitrarily nested BEGIN:VEVENT blocks without a depth limit, so an attacker can submit a sub-kilobyte .ics file containing equal nested subtrees and trigger the cost when an application performs equality, inequality, membership, deduplication, test-assertion, round-trip, or normalization comparisons. Parsing alone does not trigger the issue, and comparisons that differ early short-circuit, but a few hundred bytes can pin a CPU core for minutes or indefinitely, causing denial of service in calendar sync or import endpoints, invite processing, and other comparison paths. This issue is fixed in version 7.1.3.
INFO
Published Date :
Aug. 25, 2026, 8:16 p.m.
Last Modified :
Aug. 26, 2026, 4:16 p.m.
Remotely Exploit :
Yes !
Source :
[email protected]
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS | 134c704f-9b21-4f2e-91b3-4a467353bcc0 | |||||
| CVSS 3.1 | HIGH | [email protected] | ||||
| CVSS 3.1 | HIGH | MITRE-CVE |
Solution
- Update icalendar to version 7.1.3 or later.
- Apply vendor patches if available.
- Monitor application performance after update.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-55099.
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-55099 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-55099
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-55099 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-55099 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
CVE Modified by [email protected]
Aug. 26, 2026
Action Type Old Value New Value Added Reference https://github.com/collective/icalendar/commit/b6b2608ae3af6de40695b4e40f71847485aa0b49 Added Reference https://github.com/collective/icalendar/commit/cad40cd112c93fd142ec12cc5b37445a849b8a79 Added Reference https://github.com/collective/icalendar/releases/tag/v7.1.3 Added Reference https://github.com/collective/icalendar/security/advisories/GHSA-cv84-9p8j-fj68 Removed Reference https://github.com/collective/icalendar/commit/b6b2608ae3af6de40695b4e40f71847485aa0b49 Removed Reference https://github.com/collective/icalendar/commit/cad40cd112c93fd142ec12cc5b37445a849b8a79 Removed Reference https://github.com/collective/icalendar/releases/tag/v7.1.3 Removed Reference https://github.com/collective/icalendar/security/advisories/GHSA-cv84-9p8j-fj68 -
CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0
Aug. 26, 2026
Action Type Old Value New Value Added Reference https://github.com/collective/icalendar/security/advisories/GHSA-cv84-9p8j-fj68 Added SSVC {'id': 'CVE-2026-55099', 'role': 'CISA Coordinator', 'options': [{'exploitation': 'poc'}, {'automatable': 'yes'}, {'technicalImpact': 'partial'}], 'version': '2.0.3', 'timestamp': '2026-08-26T15:05:19.449076Z'} -
CVE Modified by af854a3a-2127-422b-91ae-364da2661108
Aug. 26, 2026
Action Type Old Value New Value Added Reference http://www.openwall.com/lists/oss-security/2026/06/23/8 Removed Reference http://www.openwall.com/lists/oss-security/2026/06/23/8 -
CVE Modified by af854a3a-2127-422b-91ae-364da2661108
Aug. 25, 2026
Action Type Old Value New Value Added Reference http://www.openwall.com/lists/oss-security/2026/06/23/8 -
New CVE Received by [email protected]
Aug. 25, 2026
Action Type Old Value New Value Added Affected [{'vendor': 'collective', 'product': 'icalendar', 'versions': [{'status': 'affected', 'version': '>= 7.1.0, < 7.1.3'}]}] Added Description icalendar is an RFC 5545 compatible parser and generator of iCalendar files for Python. From 7.1.0 until 7.1.3, the Component equality method in src/icalendar/cal/component.py compares nested subcomponents with two membership loops, and each membership test invokes the same method on child components, causing O(2^n) work relative to nesting depth. Component.from_ical accepts arbitrarily nested BEGIN:VEVENT blocks without a depth limit, so an attacker can submit a sub-kilobyte .ics file containing equal nested subtrees and trigger the cost when an application performs equality, inequality, membership, deduplication, test-assertion, round-trip, or normalization comparisons. Parsing alone does not trigger the issue, and comparisons that differ early short-circuit, but a few hundred bytes can pin a CPU core for minutes or indefinitely, causing denial of service in calendar sync or import endpoints, invite processing, and other comparison paths. This issue is fixed in version 7.1.3. Added CVSS V3.1 AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H Added CWE CWE-407 Added CWE CWE-400 Added Reference https://github.com/collective/icalendar/commit/b6b2608ae3af6de40695b4e40f71847485aa0b49 Added Reference https://github.com/collective/icalendar/commit/cad40cd112c93fd142ec12cc5b37445a849b8a79 Added Reference https://github.com/collective/icalendar/releases/tag/v7.1.3 Added Reference https://github.com/collective/icalendar/security/advisories/GHSA-cv84-9p8j-fj68