CVE-2026-45156: Nextcloud: Authentication Bypass in ID4me handling via Missing JWT Signature Verification in User OIDC

Severity: high (CVSS 8.1 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.4% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-06-01 · updated 2026-07-22
Weakness: CWE-287

Description

Nextcloud is an open source content collaboration platform. From versions 0.3.0 to before 3.1.0, 5.0.0 to before 5.1.0, and 6.0.0 to before 6.4.0, a missing signature verification in User OIDC allowed a malicious ID4me authority to identify as any user. This issue has been patched in versions 3.1.0, 4.1.0, 5.1.0, 6.4.0 and 8.3.0.

Affected products

nextcloud security-advisories: from 0.3.0, < 3.1.0; from 5.0.0, < 5.1.0; from 6.0.0, < 6.4.0

References

https://github.com/nextcloud/security-advisories/security/advisories/GHSA-qqgv-fqwp-mjpp
https://github.com/nextcloud/user_oidc/pull/1285
https://hackerone.com/reports/3489490

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