CVE-2023-42453 in Synapse
Summary
by MITRE • 09/27/2023
Synapse is an open-source Matrix homeserver written and maintained by the Matrix.org Foundation. Users were able to forge read receipts for any event (if they knew the room ID and event ID). Note that the users were not able to view the events, but simply mark it as read. This could be confusing as clients will show the event as read by the user, even if they are not in the room. This issue has been patched in version 1.93.0. Users are advised to upgrade. There are no known workarounds for this issue.
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Analysis
by VulDB Data Team • 04/23/2025
The vulnerability identified as CVE-2023-42453 affects Synapse, the open-source Matrix homeserver implementation maintained by the Matrix.org Foundation. This security flaw represents a significant issue in the message synchronization and read receipt functionality of the Matrix protocol ecosystem. The vulnerability allows authenticated users to manipulate read receipt data for events within rooms they do not belong to, creating a false impression that they have consumed content they never actually viewed. This represents a fundamental breach in the integrity of the messaging system's read status tracking mechanism.
The technical nature of this vulnerability stems from insufficient authorization checks within the read receipt processing logic of the Synapse homeserver. When users submit read receipt data, the system fails to properly validate whether the requesting user has legitimate access to the specific room and event combination. This allows malicious actors to forge read receipts by simply knowing the room ID and event ID, bypassing normal access controls that should prevent such manipulation. The vulnerability operates at the application layer and specifically targets the event processing and state management components of the Matrix protocol implementation. According to CWE classification, this corresponds to CWE-284: Improper Access Control, which describes insufficient access control mechanisms that allow unauthorized users to access resources or perform actions they should not be permitted to execute.
The operational impact of this vulnerability extends beyond simple data manipulation, as it creates confusion and misinformation within the Matrix communication ecosystem. Clients displaying read receipts will show false positive read status indicators, potentially misleading users about the actual consumption of messages by other participants. This could be particularly problematic in professional or sensitive communication environments where accurate read status tracking is essential for proper workflow management and accountability. The vulnerability affects the fundamental trust model of the messaging system, as users cannot rely on read receipt information to determine actual message consumption. From an attacker perspective, this represents a low-effort but high-impact vector for creating misleading communication patterns that could be exploited for social engineering or information warfare purposes.
Organizations and users running Synapse homeservers are strongly advised to upgrade immediately to version 1.93.0 or later, as this release contains the necessary patches to address the authorization flaw. The vulnerability does not have any known workarounds that can be implemented without modifying the core system behavior, making the upgrade the only viable mitigation strategy. This issue highlights the critical importance of proper access control implementation in distributed messaging systems, particularly those that rely on federated architectures where trust relationships between different server instances must be carefully managed. The Matrix protocol's design relies heavily on the integrity of read receipt data for proper user experience, making this vulnerability particularly concerning for environments where accurate message tracking is essential. Security practitioners should monitor for similar access control vulnerabilities in other federated messaging systems and ensure proper input validation and authorization checks are implemented across all components handling user state information. The ATT&CK framework would categorize this vulnerability under T1078: Valid Accounts and T1566: Phishing, as it could enable attackers to manipulate user state information and potentially create misleading communication patterns that could be used in social engineering attacks.