How to safely authenticate a user using LDAP?

Question

For context: I am developing a web application where users need to authenticate to view internal documents. I neither need any detailed info on users nor special permission management, two states are sufficient: Either a session belongs to an authenticated user (→ documents can be accessed) or it does not (→ documents cannot be accessed). A user authenticates by providing a username and a password, which I want to check against an LDAP server.

I am using Python 3.10 and the ldap3 Python library.

The code

I am currently using the following code to authenticate a user:

#!/usr/bin/env python3
import ssl

from ldap3 import Tls, Server, Connection
from ldap3.core.exceptions import LDAPBindError, LDAPPasswordIsMandatoryError


def is_valid(username: str, password: str) -> bool:
    tls_configuration = Tls(validate=ssl.CERT_REQUIRED)
    server = Server("ldaps://ldap.example.com", tls=tls_configuration)
    user_dn = f"cn={username},ou=ops,dc=ldap,dc=example,dc=com"

    try:
        with Connection(server, user=user_dn, password=password):
            return True
    except (LDAPBindError, LDAPPasswordIsMandatoryError):
        return False

Demo instance

If you want to run this code, you could try using the FreeIPA's project demo LDAP server.

• Replace CERT_REQUIRED with CERT_NONE because the server only provides a self-signed cert (this obviously is a security flaw, but required to use this particular demo – the server I want to use uses a Let's Encrypt certificate).
• Replace "ldaps://ldap.example.com" with ldaps://ipa.demo1.freeipa.org
• Replace the user_dn with f"uid={username},cn=users,cn=accounts,dc=demo1,dc=freeipa,dc=org"

After doing so, you could try running the following commands:

>>> is_valid("admin", "Secret123")
True
>>> is_valid("admin", "Secret1234")
False
>>> is_valid("admin", "")
False
>>> is_valid("admin", None)
False
>>> is_valid("nonexistent", "Secret123")
False

My question(s)

Does the code above safely determine if a user has provided valid credentials?

Notably, I am concerned about the following particular aspects:

1. Is attempting to bind to the LDAP server enough to verify credentials?
   • The body of the with statement should only be executed if binding was successful and therefore returns True without further ado. Is this safe? Or could it be possible that binding succeeds but the password provided would still be considered wrong and not sufficient to authenticate the user against the web app.
2. Am I opening myself up to injection attacks? If so, how to properly mitigate them?
   • user_dn = f"cn={username},ou=ops,dc=ldap,dc=example,dc=com" uses the untrusted username (that came directly from the web form) to build a string. That basically screams LDAP injection.
3. Is TLS properly configured?
   • The connection should use modern TLS encryption and verify the certificate presented by the server, just like a normal browser would do.

Also, of course, if there is anything else unsafe about my code, I'd be happy to know what it is.

Resources I've already found

I've already searched for answers to the particular aspects. Sadly, I have found nothing definite (i.e. no one definitely saying something I do here is bad or good), but I wanted to provide them as a starting point for a potential answer:

1. Probably yes.
   • “How to bind (authenticate) a user with ldap3 in python3” uses a similar code snippet to bind, and no one explicitly says that that's bad.
   • Auth0 uses this method in their blog post “Using LDAP and Active Directory with C# 101” and they probably know what they're doing.
2. Probably not, so no mitigation is needed.
   • There are a few questions on LDAP injection (like “How to prevent LDAP-injection in ldap3 for python3”) but they always only mention filtering and search, not binding.
   • The OWASP Cheat Sheet on LDAP Injection mentions enabling bind authentication as a way to mitigate LDAP injection when filtering, but say nothing about sanitization needed for the bind DN.
   • I suppose you could even argue that this scenario is not susceptible to injection attacks, because we are indeed processing untrusted input, but only where untrusted input is expected. Anyone can type anything into a login form, but they can also put anything into a request to bind to an LDAP server (without even bothering with the web app). As long as I don't put untrusted input somewhere where trusted input is expected (e.g. using a username in a filter query after binding with an LDAP admin account), I should be safe.
   • However, the ldap3 documentation of the Connection object does mention one should use escape_rdn when binding with an untrusted username. This is at odds with my suppositions, who's right?
3. Probably yes.
   • At least an error was thrown when I tried to use this code with a server that only presented a self-signed certificate, so I suppose I should be safe.

Answer 1

Is attempting to bind to the LDAP server enough to verify credentials?

From the LDAP protocol side, yes, and many systems already rely on this behavior (e.g. pam_ldap for Linux OS-level authentication against an LDAP server). I've never heard of any server where the bind result would be deferred until another operation.

From the ldap3 module side I'd be more worried, as in my experience initializing a Connection did not attempt to connect – much less bind – to the server until I explicitly called .bind() (or unless I specified auto_bind=True), but if your example works then I assume that using a with block does this correctly.

In old code (which holds a persistent connection, no 'with') I've used this, but it may be outdated:

conn = ldap3.Connection(server, raise_exceptions=True)
conn.bind()

(For some apps I use Apache as a reverse proxy and its mod_auth_ldap handles LDAP authentication for me, especially when "is authenticated" is sufficient.)

Am I opening myself up to injection attacks? If so, how to properly mitigate them?

Well, kind of, but not in a way that would be easily exploitable. The bind DN is not a free-form query – it's only a weird-looking "user name" field and it must exactly match an existing entry; you can't put wildcards in it.

(It's in the LDAP server's best interests to be strict about what the "bind" operation accepts, because it's literally the user-facing operation for logging into an LDAP server before anything else is done – it's not just a "password check" function.)

For example, if you have some users at OU=Ops and some at OU=Superops,OU=Ops, then someone could specify Foo,OU=Superops as their username resulting in UID=Foo,OU=Superops,OU=Ops, as the DN – but they'd still have to provide the correct password for that account anyway; they cannot trick the server into using one account's privileges while checking another account's password.

However, it's easy to avoid injection regardless. DN component values can be escaped using:

• ldap3: ldap3.utils.dn.escape_rdn(string)
• python-ldap: ldap.dn.escape_dn_chars(string)

That being said, I dislike "DN template" approach for a completely different reason – its rather limited usefulness; it only works when all of your accounts are under the same OU (flat hierarchy) and only when they're named after the uid attribute.

That may be the case for a purpose-built LDAP directory, but on a typical Microsoft Active Directory server (or, I believe, on some FreeIPA servers as well) the user account entries are named after their full name (the cn attribute) and can be scattered across many OUs. A two-step approach is more common:

1. Bind using your app's service credentials, then search the directory for any "user" entries that have the username in their uid attribute, or similar, and verify that you found exactly one entry;
2. Unbind (optional?), then bind again with the user's found DN and the provided password.

When searching, you do have to worry about LDAP filter injection attacks a bit more, as a username like foo)(uid=* might give undesirable results. (But requiring the results to match exactly 1 entry – not "at least 1" – helps with mitigating this as well.)

Filter values can be escaped using:

• ldap3: ldap3.utils.conv.escape_filter_chars(string)
• python-ldap: ldap.filter.escape_filter_chars(string)

(python-ldap also has a convenient wrapper ldap.filter.filter_format around this, but it's basically just the_filter % tuple(map(escape_filter_chars, args)).)

The escaping rules for filter values are different from those for RDN values, so you need to use the correct one for the specific context. But at least unlike SQL, they are exactly the same everywhere, so the functions that come with your LDAP client module will work with any server.

Is TLS properly configured?

ldap3/core/tls.py looks good to me – it uses ssl.create_default_context() when supported, loads the system default CA certificates, so no extra configuration should be needed. Although it does implement custom hostname checking instead of relying on the ssl module's check_hostname so that's a bit weird. (Perhaps the LDAP-over-TLS spec defines wildcard matching rules that are slightly incompatible with the usual HTTP-over-TLS ones.)

---

An alternative approach instead of manually escaping DN templates:

dn = build_dn({"CN": f"{last}, {first} ({username})"},
              {"OU": "Faculty of Foo and Bar (XYZF)"},
              {"OU": "Staff"},
              ad.BASE_DN)

def build_dn(*args):
    components = []
    for rdn in args:
        if isinstance(rdn, dict):
            rdn = [(a, ldap.dn.escape_dn_chars(v))
                   for a, v in rdn.items()]
            rdn.sort()
            rdn = "+".join(["%s=%s" % av for av in rdn])
            components.append(rdn)
        elif isinstance(rdn, str):
            components.append(rdn)
        else:
            raise ValueError("Unacceptable RDN type for %r" % (rdn,))
    return ",".join(components)