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
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