CVE-2025-66511

MEDIUMNVD 6.56.5—
EchelonGraph scoreMEDIUM confidence

This medium-severity CVE scores 6.5 under NVD CVSS v3. EPSS exploit probability: 0.3%, top 79% of all CVEs by exploit prediction. GitHub Security Advisory data not yet ingested — confidence will rise once GHSA publishes (typical lag: hours to days for open-source ecosystem CVEs; never for infrastructure-only CVEs).

Triggered by: NVD CVSS baseline
Sources: epss, nvd
6.5EG
EchelonGraph verdictMonitorLow exploitation likelihood right now — keep watching.
  • Lower severity and no public exploit yet
CISA-KEV: Not listedEPSS PROB: 0%CVSS: 6.5Exploit: None knownExposed: 0

No vendor fix yet — apply a workaround or compensating control (WAF / firewall / segmentation) and watch for a patch.

Nextcloud Calendar is a calendar app for Nextcloud. Prior to 6.0.3, the Calendar app generates participant tokens for meeting proposals using a hash function, allowing an attacker to compute valid participant tokens, which allowed them to request details and submit dates in meeting proposals. The tokens are not purely random generated. This vulnerability is fixed in 6.0.3.

CVSS v3
6.5
EG Score
6.5(medium)
EG Risk
29(Track)
EG Risk 29/100SSVC: Track

EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).

How it’s computed
Severity65% × 45%
Exploitation0% × 40%
Automatability0% × 15%
Action: Routine — remediate on your standard cadence.
EPSS PROB
0%
EPSS %ILE
22%
KEV
Not listed

Published

December 5, 2025

Last Modified

June 17, 2026

Advisory Details (4)

Auto-updated Aug 8, 2026
Patch available. Sources: github_commit, github_pr, github.
generic

HackerOne

https://hackerone.com/reports/3385434
github Patch Available

Calendar app used predictable proposal participant tokens · Advisory · nextcloud/security-advisories · GitHub

https://github.com/nextcloud/security-advisories/security/advisories/GHSA-whm3-vv55-gf27
github_pr

fix: proposal links

Fix merged in nextcloud/calendar PR #7659 on 2025-11-04 — awaiting tagged release

https://github.com/nextcloud/calendar/pull/7659
github_commit

commit 8de14ae87f32 (nextcloud/calendar)

Fix landed in nextcloud/calendar commit 8de14ae87f32 — awaiting tagged release

https://github.com/nextcloud/calendar/commit/8de14ae87f321f5f09280d9895a27d54d24f33fb

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Data Freshness Timeline

(refreshed 6× in last 7d / 28× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

Showing the most recent 100 of 153 total refreshes for this CVE.

  1. 2026-09-19 15:42 UTCEPSS rescore
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  40. 2026-07-23 03:01 UTCEG score recompute▲ 1.70
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Frequently asked(5)

What is CVE-2025-66511?
CVE-2025-66511 is a medium vulnerability published on December 5, 2025. Nextcloud Calendar is a calendar app for Nextcloud. Prior to 6.0.3, the Calendar app generates participant tokens for meeting proposals using a hash function, allowing an attacker to compute valid participant tokens, which allowed them to request details and submit dates in meeting proposals. The…
When was CVE-2025-66511 disclosed?
CVE-2025-66511 was first published in the National Vulnerability Database on December 5, 2025, with the most recent update on June 17, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2025-66511 actively exploited?
CVE-2025-66511 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 0% probability of exploitation in the next 30 days, which ranks it in the top 77.7% of all scored CVEs.
What is the CVSS score of CVE-2025-66511?
CVE-2025-66511 has a CVSS v3 base score of 6.5 (NVD).
How do I remediate CVE-2025-66511?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2025-66511, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

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