EUVD-2026-33704Severity by source
AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N
Primary rating from GitHub Advisory · only source for this CVE.
CVSS VectorGitHub Advisory
CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:N/I:L/A:N
Lifecycle Timeline
3DescriptionGitHub Advisory
Nextcloud is an open source content collaboration platform. From version 6.1.0 to before version 8.2.2, an attacker can craft links that would redirect users to another website, when the victim uses the attackers link to log in via user OIDC. This issue has been patched in version 8.2.2.
AnalysisAI
Open redirect in Nextcloud's user_oidc plugin (versions 6.1.0 through 8.2.1) allows attackers to craft OIDC login links that silently redirect victims to arbitrary external websites upon successful authentication. The root cause is insufficient redirect URL sanitization in LoginController.php - a single regex stripping the protocol and domain could be bypassed using protocol-relative URLs (e.g., //attacker.com), which browsers resolve as full external URLs. No public exploit has been identified at time of analysis, and this CVE is not listed in CISA KEV; however, the phishing potential via trusted login flows warrants prompt patching in user-facing Nextcloud deployments.
Technical ContextAI
The vulnerability resides in the Nextcloud user_oidc plugin's LoginController.php, which handles the post-authentication redirect after OpenID Connect login completion. The pre-patch code sanitized the redirectUrl parameter using a single preg_replace() call - '/^https?:\/\/[^\/]+/' - intended to strip the protocol and domain, leaving only the path. This regex fails to handle protocol-relative URLs beginning with '//' (e.g., '//attacker.com/steal'), which do not match the pattern and are passed through unmodified; browsers interpret these as absolute URLs inheriting the current protocol, routing the victim to an attacker-controlled site. The fix in PR #1273 adds a secondary regex check requiring the sanitized result to start with exactly one forward slash followed by a non-slash character ('/[^/]'), closing the bypass. CWE-601 (URL Redirection to Untrusted Site / Open Redirect) precisely describes this root cause: improper allowlist-based validation of redirect targets. The CPE provided (cpe:2.3:a:nextcloud:security-advisories:*:*:*:*:*:*:*:*) references the advisory repository rather than the specific plugin package; the actual affected artifact is the user_oidc Nextcloud app.
RemediationAI
Vendor-released patch: user_oidc version 8.2.2, which enforces strict post-sanitization validation ensuring redirect URLs resolve to same-origin paths only. Administrators should upgrade the user_oidc plugin to 8.2.2 or later using the Nextcloud app store or by applying the fix from PR #1273 (https://github.com/nextcloud/user_oidc/pull/1273); the advisory is at https://github.com/nextcloud/security-advisories/security/advisories/GHSA-8wjr-5cg8-4w73. If an immediate upgrade is not feasible, the most effective compensating control is to disable the user_oidc plugin entirely - this eliminates the attack surface at the cost of blocking all OIDC-based authentication, requiring users to authenticate via other configured providers. No partial configuration-level workaround (e.g., allowlisting redirect targets) has been documented by the vendor for the pre-patch versions.
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