CVE-2025-66511Network attack vector

Unauthorized proposal access via deterministic participant tokens

Published Dec 5, 2025 · Updated Dec 5, 2025

Insufficient token randomness in Nextcloud Calendar before 6.0.3 allows remote attackers to read and modify meeting proposals. ProposalService hashes the proposal database ID and participant address with MD5, making valid participant tokens computable. No account or user interaction is required, but exploitation requires derivation of both token inputs; impact is limited to viewing proposal details and submitting dates.

CVSS severity4.8
Medium
EPSS probability0.29%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Insufficient token randomness in Nextcloud Calendar before 6.0.3 allows remote attackers to read and modify meeting proposals. ProposalService hashes the proposal database ID and participant address with MD5, making valid participant tokens computable. No account or user interaction is required, but exploitation requires derivation of both token inputs; impact is limited to viewing proposal details and submitting dates.

The record

CVE
CVE-2025-66511
Published
Dec 5, 2025
Updated
Dec 5, 2025
Vendor
Nextcloud GmbH
Product
Security Advisories
Classifications
CWE-330
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Dec 5, 2025CVE publishedPublication date reported by the CVE source.
  2. Dec 5, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=>= 6.0.0-rc.1, < 6.0.3

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Nextcloud GmbH · Security Advisoriesversion=>= 6.0.0-rc.1, < 6.0.3

Published CVSS scores

6.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
4.8GitHubCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-330Use of Insufficiently Random Values

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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