CVE-2023-38686 in matrix-sydent
Summary
by MITRE • 08/04/2023
Sydent is an identity server for the Matrix communications protocol. Prior to version 2.5.6, if configured to send emails using TLS, Sydent does not verify SMTP servers' certificates. This makes Sydent's emails vulnerable to interception via a man-in-the-middle (MITM) attack. Attackers with privileged access to the network can intercept room invitations and address confirmation emails. This is patched in Sydent 2.5.6. When patching, make sure that Sydent trusts the certificate of the server it is connecting to. This should happen automatically when using properly issued certificates. Those who use self-signed certificates should make sure to copy their Certification Authority certificate, or their self signed certificate if using only one, to the trust store of your operating system. As a workaround, one can ensure Sydent's emails fail to send by setting the configured SMTP server to a loopback or non-routable address under one's control which does not have a listening SMTP server.
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Analysis
by VulDB Data Team • 08/23/2023
The vulnerability identified as CVE-2023-38686 affects Sydent, an identity server component within the Matrix communications protocol ecosystem. This flaw represents a critical security oversight in the email verification process that impacts the integrity and confidentiality of user communications. The vulnerability specifically manifests when Sydent is configured to utilize Transport Layer Security for email transmission, creating a dangerous gap in the cryptographic protection mechanism that should safeguard sensitive user data including room invitations and address confirmation messages. The issue stems from inadequate certificate validation procedures that leave the system susceptible to man-in-the-middle attacks, where malicious actors positioned within the network traffic can intercept and potentially modify the email communications intended for users.
The technical implementation flaw resides in Sydent's failure to properly validate the SSL/TLS certificates presented by SMTP servers during the email transmission process. This vulnerability directly maps to CWE-295, which addresses improper certificate validation, and aligns with ATT&CK technique T1566.001 for the use of spearphishing attachments and T1566.002 for spearphishing via email. The absence of certificate verification creates an attack surface where adversaries can establish malicious SMTP servers that present fake certificates, allowing them to seamlessly intercept email communications without detection. This weakness particularly impacts the authentication and authorization processes within Matrix, as the compromised email system undermines the trust relationships between users and the identity server, potentially enabling further attacks such as account takeover or credential harvesting.
The operational impact of this vulnerability extends beyond simple email interception to encompass broader security implications for Matrix network participants. Attackers with network-level access can exploit this weakness to monitor and manipulate room invitation communications, potentially gaining unauthorized access to private chat rooms or intercepting sensitive information exchanged through the Matrix protocol. The vulnerability affects the fundamental security posture of Matrix implementations that rely on Sydent for user identity management, as compromised email verification processes undermine the entire trust model of the communication system. Organizations and users who depend on Matrix for secure communications face significant risks including data leakage, privacy violations, and potential compromise of their communication infrastructure.
Mitigation strategies for CVE-2023-38686 require immediate implementation of proper certificate validation procedures within Sydent configurations. The recommended approach involves ensuring that Sydent instances trust the certificate authorities that issued valid certificates for their configured SMTP servers, which should occur automatically when using certificates from recognized certificate authorities. For deployments utilizing self-signed certificates, administrators must manually configure the system trust store to include the appropriate certificate authority certificates or the specific self-signed certificates. This process aligns with security best practices outlined in NIST SP 800-57 for cryptographic key management and certificate validation. The patch release of Sydent 2.5.6 addresses this vulnerability by implementing proper certificate validation mechanisms, but organizations should verify their certificate configurations to ensure the fix operates correctly. As a temporary workaround, administrators can redirect email transmission to non-routable addresses that lack active SMTP servers, effectively preventing email delivery while maintaining system functionality, though this approach significantly impacts the usability of the identity verification features.