CVE-2019-5591 in FortiOS
Summary
by MITRE
A Default Configuration vulnerability in FortiOS may allow an unauthenticated attacker on the same subnet to intercept sensitive information by impersonating the LDAP server.
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Analysis
by VulDB Data Team • 08/15/2026
This vulnerability exists within Fortinet FortiOS operating systems where default configurations fail to properly validate LDAP server certificates during authentication processes. The flaw specifically affects the LDAP client implementation that handles directory service authentication requests. When a FortiOS device is configured to use LDAP for user authentication, the system defaults to accepting any certificate presented by an LDAP server without proper validation. This creates a man-in-the-middle attack vector where an unauthenticated attacker on the same network subnet can impersonate a legitimate LDAP server and intercept or manipulate authentication credentials.
The technical implementation of this vulnerability stems from insufficient certificate validation mechanisms within the LDAP client component of FortiOS. According to CWE-295, this represents a failure in certificate validation that allows for unauthorized certificate acceptance. The vulnerability operates at the network layer where LDAP communication typically occurs over TCP port 389 or 636 for secure connections. Attackers exploiting this weakness can leverage the default trust model to present forged certificates that appear legitimate to the FortiOS device, thereby enabling credential interception and potential unauthorized access to network resources.
The operational impact of CVE-2019-5591 extends beyond simple information interception as it fundamentally undermines the authentication security model of affected FortiOS devices. When an attacker successfully impersonates an LDAP server, they can capture usernames and passwords transmitted during the authentication process, potentially gaining access to sensitive network resources, administrative accounts, and privileged user credentials. This vulnerability is particularly dangerous because it requires no authentication to exploit and can be executed from within the same subnet, making it accessible to attackers with minimal network access. The attack vector aligns with ATT&CK technique T1550.001, which describes legitimate credentials used for lateral movement and privilege escalation.
Mitigation strategies for this vulnerability require immediate configuration changes to enforce certificate validation and implement proper LDAP server authentication. Network administrators should disable default LDAP configurations and implement explicit certificate validation policies that require certificate verification against trusted certificate authorities. The recommended approach includes configuring the FortiOS device to use certificate pinning or explicit certificate trust stores for LDAP servers. Additionally, network segmentation and firewall rules should be implemented to restrict LDAP traffic to trusted network segments only. According to NIST SP 800-53 controls, this vulnerability requires implementation of access control and system configuration management practices to prevent unauthorized certificate acceptance. Regular security audits and certificate management procedures should be established to ensure ongoing compliance with secure configuration standards.