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.

# 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