Ldap Injection in Flask with Bearer Tokens

Ldap Injection occurs when an attacker can influence an LDAP query constructed from uncontrolled input. In Flask applications that use LDAP for authentication or group membership checks, developers often build filter strings by concatenating user-supplied values. When such an application also uses Bearer Tokens for API authentication, the two mechanisms can interact in risky ways.

Consider a Flask route that receives an access token, validates it, and then uses claims from the decoded token (for example, a username or group list) to form an LDAP search filter. If the application directly interpolates these claims into the filter without escaping, an attacker who can influence the token payload (for example, via a compromised identity provider or a leaked token) can inject LDAP metacharacters such as (, ), *, &, or |. This can change the semantics of the LDAP query, potentially bypassing intended access controls or extracting more data than intended.

Another scenario involves a Flask API that accepts both a Bearer Token and additional optional query parameters that are forwarded to LDAP. An attacker might supply a malicious filter value in a query parameter while using a trusted token for authentication. Because the scan tests unauthenticated attack surfaces, middleBrick can detect endpoints where user-controlled input is reflected into LDAP filter construction even when a valid Bearer Token is present.

Real-world LDAP filter metacharacters have well-known injection patterns. For example, a filter like (&(uid=admin)(memberOf=cn=${user_group})) becomes dangerous if ${user_group} is attacker-controlled. Injection can lead to authentication bypass, unauthorized data read, or in some configurations, remote code execution depending on the LDAP server and backend. The OWASP API Security Top 10 and related application security references classify this as an authorization and injection issue, with related patterns appearing in BOLA/IDOR and Property Authorization checks.

Because middleBrick runs 12 security checks in parallel, it evaluates whether endpoints that accept Bearer Tokens also reflect user-controlled data into LDAP contexts without proper sanitization. The scanner does not rely on internal architecture details; it focuses on observable behavior and spec-defined inputs, making it effective at identifying these combinations in black-box testing.

Remediation focuses on strict input validation, canonicalization, and avoiding direct concatenation of any user-influenced data into LDAP filters. When using Bearer Tokens in Flask, treat all decoded claims as untrusted and apply the same defensive practices as you would for any external input.

Example: Unsafe construction

from flask import request, jsonify
import ldap

def unsafe_search():
    token_claims = get_claims_from_bearer(request.headers.get("Authorization"))  # hypothetical
    username = token_claims.get("username")
    # UNSAFE: directly interpolating user-influenced claim
    search_filter = f"(uid={username})"
    conn = ldap.initialize("ldap://ldap.example.com")
    conn.simple_bind_s()
    conn.search_s("dc=example,dc=com", ldap.SCOPE_SUBTREE, search_filter)
    return jsonify({"status": "ok"})

Example: Safe construction with allowlist and escaping

import re
from flask import request, jsonify
import ldap

def safe_search():
    token_claims = get_claims_from_bearer(request.headers.get("Authorization"))
    username = token_claims.get("username")
    # Allowlist: only alphanumeric, underscore, and hyphen
    if not re.match(r"^[A-Za-z0-9_-]+$", username):
        return jsonify({"error": "invalid username"}), 400
    # Use ldap.filter.escape_filter_chars to escape special characters
    safe_username = ldap.filter.escape_filter_chars(username)
    search_filter = f"(uid={safe_username})"
    conn = ldap.initialize("ldap://ldap.example.com")
    conn.simple_bind_s()
    conn.search_s("dc=example,dc=com", ldap.SCOPE_SUBTREE, search_filter)
    return jsonify({"status": "ok"})

Additionally, prefer parameterized filter APIs when available, or build filters using a library that enforces strict escaping. Do not rely on Bearer Token signatures alone to prevent injection; validate and sanitize every piece of data that influences the LDAP query. In a Flask context, you can centralize this logic in request preprocessing or dedicated service modules to ensure consistent protection across all endpoints.

When using the middleBrick CLI (middlebrick scan <url>) or GitHub Action to add API security checks to your CI/CD pipeline, you can detect whether your endpoints expose injection-prone LDAP handling. The scanner reports findings with severity, reproducible steps, and remediation guidance, helping you prioritize fixes without requiring internal implementation details.