What’s Vulnerable › Vulnerabilities

CVE-2023-39959: Existence of calendars and address books can be checked by unauthenticated users

• Severity: medium (CVSS 5.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.6% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2023-08-10 · updated 2026-06-17
• Weakness: CWE-284

Description

Nextcloud Server provides data storage for Nextcloud, an open source cloud platform. Starting in version 25.0.0 and prior to versions 25.0.9, 26.0.4, and 27.0.1, unauthenticated users could send a DAV request which reveals whether a calendar or an address book with the given identifier exists for the victim. Nextcloud Server versions 25.0.9, 26.0.4, and 27.0.1 and Nextcloud Enterprise Server versions 25.0.9, 26.0.4, and 27.0.1 contain a patch for this issue. No known workarounds are available.

Affected products

• Nextcloud Server: from 25.0.0 before 25.0.9; from 26.0.0 before 26.0.4; 27.0.0 — fixed in 25.0.9, 26.0.4

How to fix

Upgrade to a fixed version: Nextcloud Server 25.0.9, 26.0.4. Check the vendor advisory for mitigations.

References

• https://github.com/nextcloud/security-advisories/security/advisories/GHSA-g97r-8ffm-hfpj
• https://github.com/nextcloud/server/pull/38747
• https://hackerone.com/reports/1832126
• https://github.com/nextcloud/security-advisories/security/advisories/GHSA-g97r-8ffm-hfpj
• https://github.com/nextcloud/server/pull/38747
• https://hackerone.com/reports/1832126

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