PT-2026-34525 · Go · Github.Com/Jkroepke/Openvpn-Auth-Oauth2

Published

2026-04-22

·

Updated

2026-04-22

CVSS v3.1

10

Critical

VectorAV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:N

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:
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):
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.

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.

Fix

Improper Authentication

Weakness Enumeration

CWE-287

Related Identifiers

GHSA-246W-JGMQ-88FG

Affected Products

Github.Com/Jkroepke/Openvpn-Auth-Oauth2