ownCloud Improper Authentication Vulnerability

[ Why it matters ]Priority signals

• Exploited in the wild: on CISA's Known Exploited Vulnerabilities list since 2026-08-27
• EPSS: 43% chance of exploitation in the next 30 days (higher than 99% of CVEs)
• Critical severity: CVSS 9.8

[ Description ]Official description

An issue was discovered in ownCloud owncloud/core before 10.13.1. An attacker can access, modify, or delete any file without authentication if the username of a victim is known, and the victim has no signing-key configured. This occurs because pre-signed URLs can be accepted even when no signing-key is configured for the owner of the files. The earliest affected version is 10.6.0.

CISA: ownCloud contains an improper authentication vulnerability that allows an attacker to access, modify, or delete any file without authentication if the username of a victim is known, and the victim has no signing-key configured.

[ Fixes ]Fixes and patches

CISA required action (US federal deadline 30 Aug 2026): Apply mitigations in accordance with vendor instructions, ensuring compliance with CISA’s BOD 26-04 Prioritizing Security Updates Based on Risk (see URL in Notes) guidance and CISA’s “Forensics Triage Requirements” (see URL in Notes). Follow applicable BOD 26-04 guidance for cloud services or discontinue use of the product if mitigations are unavailable. Stakeholders are responsible for evaluating each asset's internet exposure and ensuring adherence to BOD 26-04 patching guidelines.

No fix or interim patch is linked from our sources yet. Check the vendor references below and limit exposure of affected systems until a fix is available.

[ References ]References

• owncloud.com/security-advisories/webdav-api-authentication-bypass-using-pre-signed-urlsVendor Advisory
• owncloud.org/securityProduct
• hunt.io/blog/chinese-speaking-operator-philippine-nuclear-naval-contractorThird Party Advisory
• www.cisa.gov/known-exploited-vulnerabilities-catalogUS Government Resource