CVE-2026-81518 in BI Connector
Summary
by MITRE • 08/29/2026
When mongosqld is configured with a client certificate authority file, the listener requests a client certificate during the TLS handshake but does not require one, so a client that presents no certificate is still accepted. In deployments that rely on client certificates as the sole means of identifying users, a remote party with network access to the listener can therefore establish a session and read the MongoDB data exposed through the connector.
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Analysis
by VulDB Data Team • 08/29/2026
The vulnerability in question stems from an incomplete implementation of mutual Transport Layer Security authentication within the mongosqld component of MongoDB. When administrators configure the service with a client certificate authority file, the intention is typically to enforce strict identity verification by requiring all connecting clients to present valid certificates signed by that trusted authority. However, the current logic only requests a client certificate during the TLS handshake phase without enforcing its presence as a mandatory condition for session establishment. This creates a critical gap where the server accepts connections from unauthenticated peers who fail or choose not to provide any credentials at all.
From a technical perspective, this flaw represents a failure in access control enforcement mechanisms. While the cryptographic layer is correctly initialized to expect mutual authentication, the application-level logic does validate the absence of a certificate as an error condition that should terminate the connection. Instead, it proceeds with establishing the session using default or anonymous permissions. This behavior contradicts standard security practices for services where client certificates are designated as the primary method of user identification and authorization. The result is a situation where network access alone is sufficient to bypass what was intended to be a robust authentication barrier.
The operational impact of this vulnerability is severe, particularly in environments that rely exclusively on certificate-based identity mapping without supplementary password or role-based controls. A remote attacker with basic network connectivity to the mongosqld listener can exploit this flaw by initiating a TLS connection and simply omitting any client certificate during the handshake process. Once connected, the attacker gains access to the MongoDB data exposed through the connector under whatever default permissions are assigned to unauthenticated users. This effectively nullifies the security posture provided by the certificate authority configuration, leading to potential unauthorized data exfiltration or manipulation depending on the privileges granted to anonymous sessions.
This issue aligns with CWE-287 Improper Authentication and CWE-306 Missing Authentication for Critical Function within standard vulnerability classification frameworks. In terms of adversary behavior, it facilitates initial access techniques described in MITRE ATT&CK under methods that involve exploiting misconfigured authentication protocols or bypassing mechanisms intended to restrict entry points. The flaw allows an attacker to move laterally into the data layer without needing valid credentials, significantly lowering the barrier for exploitation compared to brute-force attacks against password-based systems.
Mitigation strategies must focus on ensuring strict enforcement of mutual TLS requirements at both the configuration and code levels. Administrators should verify that their deployment configurations explicitly mandate client certificate validation rather than merely requesting it. This often involves setting specific flags or parameters in the mongosqld startup options to enforce non-anonymous access. Additionally, organizations should implement network-level controls such as firewalls or intrusion detection systems to monitor for connections lacking proper TLS client certificates if immediate patching is not feasible. Regular audits of authentication configurations are essential to ensure that security policies intended to restrict access based on identity are correctly implemented and do not contain logical gaps allowing anonymous entry.