PYSEC-2026-435: 
# LDAP Empty Password Authentication Bypass

## Affected Component

 LDAP authentication endpoint:
- `backend/open_webui/routers/auths.py` (lines 468-477, user bind with empty password)
- `backend/open_webui/models/auths.py` (lines 58-60, `LdapForm` model)

## Affected Versions

Current main branch (commit `6fdd19bf1`) and likely all versions with LDAP authentication support.

## Description

 The LDAP authentication endpoint does not validate that the submitted password is non-empty before performing a Simple Bind against the LDAP server. Per RFC 4513 Section 5.1.2, a Simple Bind with a valid DN and an empty password constitutes an "unauthenticated simple authentication" — many LDAP servers (including OpenLDAP in default configuration and some Active Directory setups) return success (resultCode 0) for this operation.

The `LdapForm` Pydantic model accepts `password: str` with no minimum length constraint, so an empty string passes validation. The subsequent `Connection.bind()` call succeeds on vulnerable LDAP servers, and the application issues a full session token for the target user.

```python
# models/auths.py:58-60 — no min_length on password
class LdapForm(BaseModel):
    user: str
    password: str

# auths.py:469-477 — empty password reaches LDAP bind
connection_user = Connection(
    server,
    user_dn,
    form_data.password,    # can be ""
    auto_bind='NONE',
    authentication='SIMPLE',
)
if not await asyncio.to_thread(connection_user.bind):
    raise HTTPException(400, 'Authentication failed.')

# If bind succeeds (which it does with empty password on many servers),
# execution continues and a full session token is issued
```

## CVSS 3.1 Breakdown

| Metric | Value | Rationale |
|--------|-------|-----------|
| Attack Vector | Network (N) | Exploited remotely via the LDAP login endpoint |
| Attack Complexity | Low (L) | Single request with an empty password field |
| Privileges Required | None (N) | No prior authentication needed |
| User Interaction | None (N) | No victim interaction required |
| Scope | Unchanged (U) | Impact within the application's authentication boundary |
| Confidentiality | High (H) | Full access to victim's account data — chats, files, API keys, settings |
| Integrity | High (H) | Can modify victim's data, settings, send messages as victim |
| Availability | None (N) | No direct denial of service |

## Attack Scenario

1. LDAP authentication is enabled on the Open WebUI instance.
2. The underlying LDAP server accepts unauthenticated simple binds (OpenLDAP default, some AD configs).
3. Attacker sends:
   ```
   POST /api/v1/auths/ldap
   {"user": "admin_username", "password": ""}
   ```
4. The app DN bind succeeds normally (line 366), finds the target user via LDAP search.
5. The user bind (line 469-477) sends a Simple Bind with the target's DN and an empty password.
6. The LDAP server returns success for the unauthenticated bind.
7. `authenticate_user_by_email` (line 507) issues a full session token for the target user.
8. Attacker has complete access to the victim's account.

## Impact

- Complete authentication bypass — any LDAP user account can be taken over without knowing the password
- Includes admin accounts if they authenticate via LDAP
- No rate limiting on the LDAP endpoint (unlike the password signin endpoint)
- Zero interaction required from the victim
 
## Preconditions

- LDAP must be enabled (`ENABLE_LDAP=True`, disabled by default)
 - The LDAP server must accept unauthenticated simple binds with empty passwords (OpenLDAP default behavior, configurable on AD)
- Attacker must know a valid LDAP username
CVSS v3.1: 9.1