Ldap Injection in Flask with Hmac Signatures

LDAP Injection is an injection attack where untrusted input is concatenated into LDAP queries without proper sanitization or parameterization. In a Flask application that uses HMAC signatures to authenticate or bind credentials to an LDAP directory, the security model often assumes that only signed requests can perform directory operations. However, if the application embests user-controlled data into the LDAP filter or DN before signature verification—or uses the signature only to authorize the request without validating the content—the attacker can manipulate the LDAP query itself.

Consider a Flask route that receives a username and constructs an LDAP filter such as (uid={username}). If the username is directly interpolated, an attacker can supply admin)(uid=*)(objectClass=*) to change the query semantics. Even when the request includes a valid HMAC signature, the signature typically covers the raw payload or selected headers, not the semantic safety of the LDAP filter. Therefore, the server may accept the request as authenticated and proceed with a query that returns unintended entries, enabling IDOR or data exposure. This becomes especially dangerous when combined with BOLA/IDOR checks that are bypassed via LDAP injection, allowing attackers to traverse relationships or access other users’ entries. The presence of HMAC signatures does not mitigate injection; it only authenticates the request, so developers must treat LDAP parameters as untrusted regardless of signature validity.

During an LLM/AI Security scan, middleBrick tests for system prompt leakage and output exposure. If an LDAP injection leaks directory contents that contain sensitive data, those results can be reflected in LLM responses, increasing PII or API key exposure risk. The scanner also checks for unsafe consumption patterns, where unchecked LDAP responses are processed without schema validation, which can amplify injection impact. Because the scan is unauthenticated, it probes the public attack surface to confirm whether crafted inputs reach the LDAP layer without triggering validation or escaping.

Defend against LDAP Injection in Flask by treating all user input as untrusted, using parameterized LDAP queries (or an ORM), and validating HMAC scope to include the relevant parameters. Do not rely on the signature alone to guarantee query safety. Below are concrete, realistic examples using the hmac standard library to sign and verify a JSON payload that includes user input, followed by safe LDAP query construction.

Example 1: Signing and verifying a JSON payload in Flask

import json
import hmac
import hashlib
from flask import Flask, request, jsonify

app = Flask(__name__)
SECRET_KEY = b'super-secret-key-kept-out-of-source'

def generate_hmac(payload: dict) -> str:
    body = json.dumps(payload, sort_keys=True).encode()
    return hmac.new(SECRET_KEY, body, hashlib.sha256).hexdigest()

@app.route('/search', methods=['POST'])
def search_users():
    data = request.get_json(force=True)
    received = request.headers.get('X-Request-Signature')
    if not received:
        return jsonify(error='missing signature'), 400
    expected = generate_hmac(data)
    if not hmac.compare_digest(expected, received):
        return jsonify(error='invalid signature'), 401

    username = data.get('username', '')
    if not isinstance(username, str) or not username.isalnum():
        return jsonify(error='invalid username'), 400

    # Safe: use parameterized search with a library that supports it
    # e.g., ldap3.Server, ldap3.Connection.search with filter parameters
    # filter_str = '(&(objectClass=person)(uid={}))'.format(escape_ldap_filter(username))
    return jsonify(status='ok')

Example 2: Using escape patterns and strict validation

import re
from flask import Flask, request, jsonify

app = Flask(__name__)

def escape_ldap_filter(value: str) -> str:
    # Basic escaping for illustrative purposes; prefer library functions
    return value.replace('\\', '\\\\').replace('*', '\\2a').replace('(', '\\28').replace(')', '\\29')

@app.route('/bind', methods=['POST'])
def bind_user():
    username = request.form.get('username', '')
    password = request.form.get('password', '')
    # Validate: allow only safe characters for uid
    if not re.fullmatch(r'[a-zA-Z0-9_.-]{1,64}', username):
        return jsonify(error='invalid username format'), 400

    # Construct filter safely; do not concatenate raw input
    safe_username = escape_ldap_filter(username)
    # filter_expression = '(&(objectClass=inetOrgPerson)(uid={}))'.format(safe_username)
    # Use a bind or search call with the filter expression via an LDAP library
    return jsonify(status='ok')

Checklist for Flask + HMAC + LDAP

• Include all user-influenced data in the HMAC scope (e.g., JSON body fields used in LDAP queries).
• Validate input against strict allow-lists before using in LDAP filters or DNs.
• Use library functions for escaping or, better, parameterized APIs that avoid string assembly entirely.
• Log rejected requests for audit but do not expose internal LDAP structure in responses.
• Combine with other checks: ensure BOLA/IDOR and property authorization are enforced server-side, independent of client-supplied identifiers.