CVE-2026-55099icalendar: Algorithmic Complexity in Equality
- 7.5HighCVSS base score
- 0.63%Exploitation in 30 daysEPSS · top 49% of all CVEs
- NoKnown exploitednot in CISA's KEV catalog
- YesPublic exploitsa reference or SSVC says so
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.
Public exploit
An exploit or proof of concept is public: anyone can try it against an unpatched system. A reference of the record is tagged as an exploit. CISA's SSVC rates its exploitation: a proof of concept is public.
Show the exploits1
References tagged exploit1 reference
- https://github.com/collective/icalendar/security/advisories/GHSA-cv84-9p8j-fj68 (opens in a new tab)x_refsource_CONFIRMexploit
Timeline
ID reserved
By GitHub_M
- 2 months later
· 2 months later
Published
- 1 day later
· 1 day later
Record updated
Changes after publication (a new score, a public exploit, EPSS crossing 1, 10 or 50 %, leaving KEV) are kept since November 2026; earlier ones are not known.
Scores
| Scored by | CVSS | Score | Vector |
|---|---|---|---|
| GitHub_MCNA (assigner) | 3.1 | High7.5 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H |
| NVDNVD · Deferred | 3.1 | High7.5 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H |
- EPSS (exploitation in 30 days)
- 0.63% · top 49% of all CVEs (14 Nov 2026)
- CISA SSVC
- Exploitation: poc · Automatable: yes · Technical impact: partial
The list uses the assigner's score when there is one, then CISA's, then NVD's: High 7.5.
Affected products
- affected >= 7.1.0, < 7.1.3
Affected packages
From reviewed advisories of the GitHub Advisory Database.
- PyPIicalendarGHSA-cv84-9p8j-fj68 (opens in a new tab)
References
References5
- https://github.com/collective/icalendar/security/advisories/GHSA-cv84-9p8j-fj68 (opens in a new tab)x_refsource_CONFIRMexploit
- https://github.com/collective/icalendar/commit/b6b2608ae3af6de40695b4e40f71847485aa0b49 (opens in a new tab)x_refsource_MISC
- https://github.com/collective/icalendar/commit/cad40cd112c93fd142ec12cc5b37445a849b8a79 (opens in a new tab)x_refsource_MISC
- https://github.com/collective/icalendar/releases/tag/v7.1.3 (opens in a new tab)x_refsource_MISC
- http://www.openwall.com/lists/oss-security/2026/06/23/8 (opens in a new tab)
Sources
The same CVE in the primary databases and aggregators, to check against.
- CVE.org record (opens in a new tab)
- CVE List JSON (GitHub) (opens in a new tab)
- NVD (opens in a new tab)
- FIRST EPSS (opens in a new tab)
- EUVD EUVD-2026-65743 (opens in a new tab)
- OSV.dev (opens in a new tab)
- GitHub Advisory GHSA-cv84-9p8j-fj68 (opens in a new tab)
- CIRCL Vulnerability-Lookup (opens in a new tab)
Badge for a README or a ticket
[](https://cveledger.app/cve/CVE-2026-55099)Record last updated .