CVE-2026-109357 in LibreNMS
Summary
by MITRE • 10/12/2026
LibreNMS before 26.9.0 contains an LDAP filter injection in ldap-authorization and ad-authorization modes because usernames are inserted unescaped into LDAP search filters and authentication only checks user existence. When REMOTE_USER is not forwarded, unauthenticated attackers can send a wildcard username via HTTP Basic Authorization (PHP_AUTH_USER) to match all users and groups and gain admin access.
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Analysis
by VulDB Data Team • 10/12/2026
The vulnerability identified in LibreNMS versions prior to 26.9.0 represents a critical authentication bypass rooted in improper input validation within the LDAP integration subsystems. Specifically, the flaw exists in both ldap-authorization and ad-authorization modes where user-supplied usernames are inserted directly into Lightweight Directory Access Protocol search filters without proper escaping or sanitization. This architectural oversight allows for LDAP filter injection attacks, enabling attackers to manipulate the structure of the query sent to the directory server rather than merely providing a value that is matched against existing entries. The core technical deficiency lies in the application's logic which prioritizes user existence verification over strict credential validation during certain authentication flows. By failing to restrict special characters or control sequences within the username field, the system inadvertently permits the injection of LDAP metacharacters such as asterisks and parentheses, fundamentally altering the intent of the search operation.
The operational impact of this vulnerability is severe, allowing unauthenticated attackers to escalate privileges from a guest state to full administrative access under specific conditions. When the REMOTE_USER environment variable is not forwarded by the web server or reverse proxy, the application falls back to using credentials provided via HTTP Basic Authorization headers, specifically PHP_AUTH_USER. An attacker can exploit this fallback mechanism by submitting a wildcard username pattern that matches all entries in the directory structure. Because the authentication logic primarily checks for user existence rather than verifying specific password hashes against an injected filter payload, successfully matching any valid entry grants access to the corresponding account. If such an account possesses administrative privileges or if the system defaults to granting elevated permissions upon successful LDAP match verification, the attacker gains complete control over the network monitoring infrastructure. This bypass effectively neutralizes the authentication layer, allowing unauthorized individuals to view sensitive network data, modify configurations, and potentially execute further attacks against connected systems.
From a classification perspective, this vulnerability aligns with CWE-90 for Improper Neutralization of Special Elements used in an SQL Command known as LDAP Injection (CWE-90) within the context of directory services, although technically it is specifically LDAP injection rather than SQL. It also relates to CWE-287 which covers Improper Authentication, particularly regarding the failure to properly validate credentials and reliance on weak authentication mechanisms. In terms of offensive security frameworks, this exploit maps directly to MITRE ATT&CK technique T1078 Valid Accounts, where adversaries use legitimate account credentials or forged identities to gain initial access. The specific method of bypassing authentication through input manipulation also touches upon aspects of T1621 Authentication Mechanism Interception if the HTTP Basic Auth headers are captured and replayed with malicious payloads in a network context.
Mitigation strategies must focus on immediate patching and defensive coding practices. Organizations running LibreNMS should upgrade to version 26.9.0 or later where this filter injection has been remediated through proper input sanitization and escaping of special characters within LDAP queries. For environments unable to update immediately, administrators can enforce stricter web server configurations that ensure REMOTE_USER is always populated from trusted sources rather than relying on client-supplied HTTP headers for authentication decisions. Additionally implementing Web Application Firewall rules to detect and block common LDAP injection patterns such as unescaped asterisks or parentheses in user input fields can provide a temporary layer of defense. Long-term remediation requires adopting secure coding standards that mandate parameterized queries or strict allow-listing of permitted characters for all inputs interacting with directory services, ensuring that user data is treated strictly as data and never as executable query logic.