CVE-2023-32323 in matrix-synapse
Summary
by MITRE • 05/26/2023
Synapse is an open-source Matrix homeserver written and maintained by the Matrix.org Foundation. A malicious user on a Synapse homeserver X with permission to create certain state events can disable outbound federation from X to an arbitrary homeserver Y. Synapse instances with federation disabled are not affected. In versions of Synapse up to and including 1.73, Synapse did not limit the size of `invite_room_state`, meaning that it was possible to create an arbitrarily large invite event. Synapse 1.74 refuses to create oversized `invite_room_state` fields. Server operators should upgrade to Synapse 1.74 or newer urgently.
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Analysis
by VulDB Data Team • 06/17/2023
The vulnerability described in CVE-2023-32323 affects Synapse, an open-source Matrix homeserver implementation maintained by the Matrix.org Foundation. This security flaw represents a significant concern for federated messaging environments where trust and communication integrity are paramount. The vulnerability enables a malicious actor with specific permissions to manipulate federation settings between Matrix homeservers, potentially disrupting communication channels within the broader Matrix network. The issue stems from inadequate input validation mechanisms within the Synapse server implementation, specifically concerning the handling of invite room state events that govern how invitations are processed and transmitted across federated networks.
The technical exploitation of this vulnerability occurs through the manipulation of the `invite_room_state` parameter within Matrix invite events. Prior to version 1.74, Synapse failed to enforce size limitations on this parameter, allowing malicious users to construct invite events with arbitrarily large `invite_room_state` fields. This design flaw creates a potential denial of service condition where the server becomes vulnerable to resource exhaustion attacks through the creation of oversized state events. The vulnerability directly relates to CWE-126, which addresses buffer over-read conditions, and CWE-770, which covers allocation of resources without limits or throttling. Attackers with permission to create certain state events can leverage this weakness to disable outbound federation to arbitrary homeservers, effectively creating communication blackouts that could isolate entire network segments.
The operational impact of this vulnerability extends beyond simple service disruption to encompass broader security implications for federated communication networks. When outbound federation is disabled, affected homeservers cannot communicate with other Matrix instances, breaking the fundamental distributed nature of the Matrix protocol. This disruption affects not only the immediate server but can cascade through the federation network, impacting users across multiple connected homeservers. The vulnerability is particularly dangerous because it requires minimal privileges to exploit - only the ability to create specific state events, which may be available to users within certain rooms or with particular roles. This low barrier to exploitation makes the vulnerability particularly concerning for large-scale deployments where user permissions are more complex and potentially less strictly controlled.
Server operators face urgent remediation requirements due to the severity of this vulnerability and its potential for widespread network disruption. The Matrix.org Foundation has addressed this issue in Synapse version 1.74 by implementing proper size limits for the `invite_room_state` field, preventing the creation of oversized invite events. This fix represents a critical security enhancement that aligns with best practices for resource management and input validation. Organizations running Synapse instances should prioritize upgrading to version 1.74 or later immediately, as the vulnerability exists across all versions up to and including 1.73. The mitigation strategy involves not only the software upgrade but also careful monitoring of federation connections and network traffic patterns to detect any potential exploitation attempts. Additionally, administrators should review user permissions and access controls to minimize the attack surface, as the vulnerability requires specific permissions to exploit, making proper access control a crucial secondary defense mechanism. This vulnerability demonstrates the importance of implementing proper resource limits and validation checks in federated systems, where a single compromised node can potentially affect the entire network infrastructure.