Nextcloud: Authentication Bypass in ID4me handling via Missing JWT Signature Verification in User OIDC

published 2026-06-01 5 mo ago modified 2026-07-22 3 mo ago also EUVD-2026-33675

cvss: 8.1; version 3.1, CNA
cisa kev: Not listed; not in CISA's catalog
epss: 0.4%; EPSS 0.4% chance of exploitation in 30 days, percentile 35%
public exploits: 0; none known

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.

Patch / fix

fixed in 1 links 1

nextcloud user_oidc 3.1.0, 5.1.0, 6.4.0

github.com/nextcloud/user_oidc#1285 pull request

CISA SSVC

Exploitation: none
Automatable: no
Technical impact: total
Assessed: 5 mo ago (2026-06-02)

Exploit prediction

0.41% chance of exploitation in the next 30 days, higher than 35% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by	version	score	vector
CNA	3.1	8.1	CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N

Affected products

nextcloud security-advisories

Classification

Weakness

CWE-287 Improper Authentication

References

Sources

CVE List CVE-2026-45156
NVD CVE-2026-45156

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

CPEs and fixed versions use Anchore's correction of NVD's data: Added CPE configurations because not yet analyzed by NVD.

Summary