HighCVE-2026-55099

    icalendar: Algorithmic Complexity in Equality (CVE-2026-55099)

    Published Last modified Sources: OSV, CVE.org, Hersteller

    What is affected, what to do

    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.

    The available sources name no fixed version. Check the vendor guidance in the references below.

    Affected
    • Collective Icalendar gte 7.1.0, < 7.1.3..*

    Not sure whether and how you are affected? Ask a question about this vulnerability

    Metrics

    7.5

    Priority

    XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

    Source: Vendor (CNA), CVSS v3

    7.5

    CVSS v3

    Severity High on the scale from 0 to 10.

    Source: OSV, CVE.org, Hersteller

    0.6 %

    EPSS score

    Low — model estimates < 1% exploitation likelihood.

    Source: FIRST EPSS, percentile 49.9

    Show all metrics
    Severity
    High
    Priority (0–325)

    The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first?

    Read in glossary
    82.07 Methodology
    Exploit maturity

    An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack.

    Read in glossary
    no public PoC known
    XIEM Effective CVSS

    XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command.

    Read in glossary
    7.5 Methodology
    CVSS v3

    CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions.

    Read in glossary
    7.5
    EPSS score

    The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“.

    Read in glossary
    0.6 %
    Low — model estimates < 1% exploitation likelihood.
    EPSS percentile

    The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking.

    Read in glossary
    49.9
    High — this CVE ranks above the median of all CVEs scored today (rank ≥ 36%).
    KEV status

    The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching.

    Read in glossary
    —
    Published
    CWE

    CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect.

    Read in glossary
    CWE-400, CWE-407

    Detection rules

    No NEOSEC detection rule is available for this vulnerability yet.

    Classification under NIS2 and CRA

    Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

    NIS2

    NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

    Read in glossary
    High technical severity: assess and remediate the vulnerability in vulnerability handling under Art. 21(2)(e) NIS2 and document the decision.
    CRA

    The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

    Read in glossary
    From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.
    ISO/IEC 27001

    ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

    Read in glossary
    Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

    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.

    Source: OSV (Open Source Vulnerabilities), NVD (NIST), CVE.org (MITRE), cvelistv5, GitHub Security Advisories

    Affected products

    The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

    Affected products
    Affected versionsFixed in
    collectiveicalendar
    • ≥ 7.1.0, < 7.1.3
    • —
    pypiicalendar
    • 7.1.0
    • 7.1.1
    • 7.1.2
    • —
    • —
    • —

    Weakness classes (CWE)

    • CWE-400Uncontrolled Resource Consumption

      Class

      The product does not properly control the allocation and maintenance of a limited resource.

      Entry at MITRE CWE-400

    • CWE-407Inefficient Algorithmic Complexity

      Class

      An algorithm in a product has an inefficient worst-case computational complexity that may be detrimental to system performance and can be triggered by an attacker, typically using crafted manipulations that ensure that the worst case is being reached.

      Entry at MITRE CWE-407

    References & sources

    • github.comcollective/icalendar/security/advisories/GHSA-cv84-9p8j-fj68x_refsource_CONFIRM
    • github.comcollective/icalendar/commit/b6b2608ae3af6de40695b4e40f71847485a…x_refsource_MISC
    • github.comcollective/icalendar/commit/cad40cd112c93fd142ec12cc5b37445a849…x_refsource_MISC
    • github.comcollective/icalendar/releases/tag/v7.1.3x_refsource_MISC
    • openwall.comlists/oss-security/2026/06/23/8
    • github.comcollective/icalendarpackage
    • pypi.orgproject/icalendarpackage
    • GitHub Security Advisoriesadvisories/GHSA-cv84-9p8j-fj68advisory
    • NVD (NIST)vuln/detail/CVE-2026-55099advisory

    Changes to this CVE

    Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

    1. Record modifiedGitHub

      Show 8 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
      —
      https://github.com/collective/icalendar/commit/b6b2608ae3af6de40695b4e40f71847485aa0b49
      Referenceadded
      —
      https://github.com/collective/icalendar/commit/cad40cd112c93fd142ec12cc5b37445a849b8a79
      Referenceadded
      —
      https://github.com/collective/icalendar/releases/tag/v7.1.3
      Referenceadded
      —
      https://github.com/collective/icalendar/security/advisories/GHSA-cv84-9p8j-fj68
      Referenceremoved
      https://github.com/collective/icalendar/commit/b6b2608ae3af6de40695b4e40f71847485aa0b49
      —
      Referenceremoved
      https://github.com/collective/icalendar/commit/cad40cd112c93fd142ec12cc5b37445a849b8a79
      —
      Referenceremoved
      https://github.com/collective/icalendar/releases/tag/v7.1.3
      —
      Referenceremoved
      https://github.com/collective/icalendar/security/advisories/GHSA-cv84-9p8j-fj68
      —

      Source identifier: security-advisories@github.com

    2. Record modifiedCISA ADP

      CISA assessment (SSVC): exploitation: proof of concept, automatable: yes

      Show 2 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
      —
      https://github.com/collective/icalendar/security/advisories/GHSA-cv84-9p8j-fj68
      SSVC assessmentadded
      —
      {"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"}

      Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

    3. Record modifiedCVE Program

      Show 2 original changes
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
      —
      http://www.openwall.com/lists/oss-security/2026/06/23/8
      Referenceremoved
      http://www.openwall.com/lists/oss-security/2026/06/23/8
      —

      Source identifier: af854a3a-2127-422b-91ae-364da2661108

    4. Record modifiedCVE Program

      Show the original change
      Individual changes of this event as published by NVD
      TypeActionBeforeAfter
      Referenceadded
      —
      http://www.openwall.com/lists/oss-security/2026/06/23/8

      Source identifier: af854a3a-2127-422b-91ae-364da2661108

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