Lemur: LDAP Authentication Globally Disables TLS Certificate Verification When LDAP_USE_TLS Is Enabled

Description

Overview

When LDAP TLS is enabled (LDAP_USE_TLS = True), Lemur's LDAP authentication module unconditionally disables TLS certificate verification at the global ldap module level. This allows a man-in-the-middle attacker positioned between Lemur and the LDAP server to intercept all authentication credentials.

Vulnerable Code

Location: lemur/auth/ldap.py, _bind() method, line ~172

if self.ldap_use_tls:
    ldap.set_option(ldap.OPT_X_TLS_REQUIRE_CERT, ldap.OPT_X_TLS_NEVER)

Key issues:

1. ldap.set_option() is a global call (as opposed to self.ldap_client.set_option()), meaning it disables TLS verification for the entire Python process, not just this connection
2. OPT_X_TLS_NEVER means no certificate validation is performed whatsoever — self-signed, expired, wrong hostname, and revoked certificates are all silently accepted
3. There is no configuration option to override this behavior — TLS verification is always disabled when TLS is enabled

Impact

A network-positioned attacker (man-in-the-middle) between Lemur and the LDAP server can:

• Intercept all LDAP credentials (usernames and plaintext passwords) for every user who authenticates
• Modify LDAP responses to inject arbitrary group memberships, granting admin access
• Compromise the entire PKI infrastructure managed by Lemur, since authentication controls access to certificates and private keys

This is particularly severe because Lemur is a certificate management system — the tool designed to manage TLS security is itself vulnerable to a TLS attack.

Steps to Reproduce

1. Deploy Lemur with LDAP TLS enabled:

   LDAP_AUTH = True
   LDAP_USE_TLS = True
   LDAP_BIND_URI = "ldaps://dc.corp.example.com"
   

2. Intercept the LDAP connection using a TLS proxy (e.g., mitmproxy or stunnel):

   # Generate a self-signed certificate
   openssl req -x509 -newkey rsa:2048 -keyout mitm.key -out mitm.crt -days 1 -nodes -subj "/CN=mitm"
   
   # Proxy LDAP traffic
   stunnel -d 0.0.0.0:636 -r real-ldap-server:636 -p mitm.pem
   

3. Point Lemur's LDAP_BIND_URI at the proxy (or perform ARP spoofing/DNS hijacking)

4. Observe that Lemur connects without any certificate verification error

5. All credentials are visible in the proxy's TLS session

Remediation

Remove the global TLS verification bypass and default to strict verification:

if self.ldap_use_tls:
    # Use instance-level option, not global
    self.ldap_client.set_option(ldap.OPT_X_TLS_REQUIRE_CERT, ldap.OPT_X_TLS_DEMAND)
    self.ldap_client.set_option(ldap.OPT_PROTOCOL_VERSION, 3)
    if self.ldap_cacert_file:
        self.ldap_client.set_option(ldap.OPT_X_TLS_CACERTFILE, self.ldap_cacert_file)

If backward compatibility is needed, make it configurable with a secure default:

tls_require_cert = current_app.config.get("LDAP_TLS_REQUIRE_CERT", ldap.OPT_X_TLS_DEMAND)
self.ldap_client.set_option(ldap.OPT_X_TLS_REQUIRE_CERT, tls_require_cert)

Resources

• CWE-295: https://cwe.mitre.org/data/definitions/295.html
• python-ldap TLS documentation: https://www.python-ldap.org/en/python-ldap-3.4.0/reference/ldap.html#tls-options