openvpn-auth-oauth2 returns FUNC_SUCCESS on client-deny, allowing unauthenticated VPN access

# Summary When `openvpn-auth-oauth2` is deployed in the **experimental plugin mode** (shared library loaded by OpenVPN via the `plugin` directive), clients that do not support WebAuth/SSO (e.g., the `openvpn` CLI on Linux) are incorrectly admitted to the VPN despite being denied by the authentication logic. **The default management-interface mode is not affected** because it does not use the OpenVPN plugin return-code mechanism. # Impact **Authentication bypass — any VPN client that does not advertise WebAuth/SSO support (`IV_SSO=webauth`) is granted full network access without completing OIDC authentication.** This affects only deployments running the **experimental plugin mode** in versions 1.26.3 through 1.27.2. The default and recommended deployment via the management interface is **not affected**. An unauthenticated attacker can connect to the OpenVPN server using any standard OpenVPN client that does not support webauth (e.g., the Linux `openvpn` CLI). The plugin correctly issues a `client-deny` command via the management interface, but returns `OPENVPN_PLUGIN_FUNC_SUCCESS` (status=0) to OpenVPN. Because the `auth_control_file` content is only consulted when the plugin returns `FUNC_DEFERRED`, OpenVPN interprets status=0 as "authentication passed" and admits the client — granting full access to the internal network behind the VPN. ## Root Cause In `lib/openvpn-auth-oauth2/openvpn/handle.go`, the `ClientAuthDeny` branch of `handleAuthUserPassVerify` wrote `"0"` (deny) to the `auth_control_file` but returned `OPENVPN_PLUGIN_FUNC_SUCCESS`. OpenVPN only reads the `auth_control_file` when the plugin returns `FUNC_DEFERRED`; a synchronous `FUNC_SUCCESS` return is treated as immediate approval regardless of file contents. **Before fix:** ```go case management.ClientAuthDeny: // ... writes "0" to auth_control_file ... if err := openVPNClient.WriteToAuthFile("0"); err != nil { // only returned ERROR on write failure return c.OpenVPNPluginFuncError } return c.OpenVPNPluginFuncSuccess // ← BUG: OpenVPN sees this as "auth passed" ``` **After fix (commit [`36f69a6`](https://github.com/jkroepke/openvpn-auth-oauth2/commit/36f69a6c67c1054da7cbfa04ced3f0555127c8f2)):** ```go case management.ClientAuthDeny: // ... writes "0" to auth_control_file ... if err := openVPNClient.WriteToAuthFile("0"); err != nil { logger.ErrorContext(p.ctx, "write to auth file", slog.Any("err", err)) } return c.OpenVPNPluginFuncError // ← FIX: OpenVPN now correctly rejects the client ``` # Patches This vulnerability is fixed in **v1.27.3**. Users of the experimental plugin mode should upgrade immediately. - **Fix commit:** [`36f69a6`](https://github.com/jkroepke/openvpn-auth-oauth2/commit/36f69a6c67c1054da7cbfa04ced3f0555127c8f2) - **Fix PR:** [#829](https://github.com/jkroepke/openvpn-auth-oauth2/pull/829) # Workarounds - **Switch to standalone management client mode** (the default, non-plugin deployment). This mode is not affected by the vulnerability because authentication decisions are communicated entirely through the management interface protocol, not through the plugin return code. - **Restrict VPN access at the network level** to only clients known to support WebAuth/SSO (e.g., OpenVPN Connect 3+), although this is difficult to enforce reliably and is not recommended as a sole mitigation.

Source published

22 Apr 2026 14:28:11 UTC

Source modified

20 May 2026 19:41:27 UTC

Observed here

24 Oct 2026 13:19:41 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N

Publisher rating · CRITICAL