Ldap Injection in Flask with Basic Auth

LDAP Injection occurs when untrusted input is concatenated into LDAP queries without proper sanitization or parameterized handling. In Flask applications that rely on LDAP for authentication—especially when paired with HTTP Basic Auth—this typically happens during the binding or search phase after credentials are received. Basic Auth sends credentials in base64-encoded form (not encrypted), and although transport security is handled separately by TLS, the server-side processing of these credentials can still be vulnerable.

Consider a Flask route that receives a username and password via Basic Auth headers. The application may construct an LDAP filter by directly interpolating the provided username into the search filter, for example:

import ldap
from flask import request, abort

def authenticate_ldap():
    auth = request.authorization
    if not auth or not auth.username or not auth.password:
        abort(401)
    
    conn = ldap.initialize('ldap://ldap.example.com')
    conn.simple_bind_s('uid=' + auth.username + ',ou=users,dc=example,dc=com', auth.password)
    return {'status': 'ok'}

In this pattern, auth.username is appended directly into the distinguished name (DN) or filter. An attacker could supply a username such as admin)(uid=*), which alters the DN structure or filter logic, potentially bypassing authentication or extracting unauthorized entries. This mirrors classic injection patterns seen in SQL and command injection, but tailored to LDAP query syntax.

The risk is compounded when the application uses the username to build search filters rather than DNs. For example:

search_filter = '(uid=' + auth.username + ')'

An input like admin)(objectClass=*) could cause the filter to evaluate incorrectly, returning multiple entries or exposing other user records. Because Basic Auth supplies credentials with every request, automated tools can easily probe for these injection points, making this a high-visibility attack surface.

These examples align with the broader OWASP API Top 10 category for security misconfiguration and improper authentication handling. The vulnerability exists not in Basic Auth itself, but in how the application uses user-supplied data to form LDAP operations. Proper validation, escaping, and use of parameterized APIs are essential to prevent LDAP Injection in this context.

To remediate LDAP Injection when using Basic Auth in Flask, avoid string concatenation entirely when constructing DNs and filters. Instead, use parameterized LDAP APIs or strict input validation. Below are concrete, safe patterns.

1. Use parameterized LDAP search with bind

Rather than building a DN from user input, authenticate by binding with a fixed template and a parameterized filter:

import ldap
from flask import request, abort

def authenticate_ldap_safe():
    auth = request.authorization
    if not auth or not auth.username or not auth.password:
        abort(401)
    
    # Validate username: allow only alphanumeric and a few safe characters
    if not re.match(r'^[a-zA-Z0-9._-]+$', auth.username):
        abort(400, 'Invalid username format')
    
    conn = ldap.initialize('ldap://ldap.example.com')
    # Use simple_bind_s with a parameterized approach if supported by the library
    # Some libraries accept a template; here we illustrate cautious assembly:
    base_dn = 'ou=users,dc=example,dc=com'
    search_filter = '(uid={})'.format(ldap.filter.escape_filter_chars(auth.username))
    
    try:
        conn.simple_bind_s(base_dn, auth.password)
    except ldap.INVALID_CREDENTIALS:
        abort(401)
    return {'status': 'ok'}

The key function ldap.filter.escape_filter_chars escapes special characters such as *, (, ), and \\ in the username, neutralizing injection attempts in filter construction. If your LDAP library supports binding directly with a filter (instead of a DN), prefer that approach.

2. Strict input validation and length limits

Enforce strict allowlists on usernames and reject overly long or malformed values before they reach LDAP:

import re
from flask import Flask, request, jsonify

app = Flask(__name__)

@app.route('/login', methods=['POST'])
def login():
    auth = request.authorization
    if not auth:
        return jsonify({'error': 'missing credentials'}), 401
    
    # Allowlist: letters, digits, underscore, hyphen, dot
    if not re.fullmatch(r'[a-zA-Z0-9_.-]{3,64}', auth.username):
        return jsonify({'error': 'invalid username'}), 400
    
    # Proceed with safe LDAP operations
    return jsonify({'status': 'authenticated'})

This pattern ensures that only expected characters are accepted, reducing the attack surface dramatically. It also prevents abuse through excessively long inputs that could trigger edge-case behavior in LDAP libraries.

For production deployments, combine these practices with TLS for transport encryption and consider using more robust identity protocols (such as OAuth2 or SAML) where feasible. The middleBrick CLI can be used to scan your Flask endpoints and validate that no LDAP injection vectors remain by running: middlebrick scan <your-api-url>. The GitHub Action and MCP Server integrations allow continuous validation within development workflows, helping catch regressions before deployment.