CVE-2022-39335 in matrix-synapse
Summary
by MITRE • 05/26/2023
Synapse is an open-source Matrix homeserver written and maintained by the Matrix.org Foundation. The Matrix Federation API allows remote homeservers to request the authorization events in a room. This is necessary so that a homeserver receiving some events can validate that those events are legitimate and permitted in their room. However, in versions of Synapse up to and including 1.68.0, a Synapse homeserver answering a query for authorization events does not sufficiently check that the requesting server should be able to access them. The issue was patched in Synapse 1.69.0. Homeserver administrators are advised to upgrade.
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Analysis
by VulDB Data Team • 04/23/2025
The vulnerability identified as CVE-2022-39335 affects Synapse, an open-source Matrix homeserver developed by the Matrix.org Foundation, which serves as a critical component in the Matrix communication protocol ecosystem. This issue resides within the Matrix Federation API's authorization event handling mechanism, where remote homeservers can request authorization events from a room to validate event legitimacy. The fundamental flaw occurs when a Synapse homeserver processes these authorization event requests without properly validating whether the requesting server has adequate access permissions to retrieve such sensitive information. This represents a significant security gap in the federated messaging system's access control framework, potentially allowing unauthorized entities to gain access to room authorization data that should be restricted to legitimate participants.
The technical implementation of this vulnerability stems from insufficient input validation within the federation API's authorization event retrieval process. When a remote homeserver makes a request for authorization events, the vulnerable Synapse version fails to perform proper authentication checks against the requesting server's credentials and access rights. This flaw allows malicious actors to exploit the federation protocol to obtain authorization events from rooms they should not have access to, effectively bypassing the intended access controls. The vulnerability is categorized under CWE-284 Access Control, specifically addressing improper access control mechanisms within the authorization validation process. From an operational perspective, this weakness creates a path for attackers to potentially reconstruct room access policies and identify legitimate participants, undermining the privacy and security guarantees that the Matrix protocol aims to provide.
The impact of this vulnerability extends beyond simple information disclosure, as it could enable sophisticated attacks targeting the Matrix ecosystem's integrity. An attacker who successfully exploits this flaw could potentially manipulate room access controls, gain insights into room membership structures, or even facilitate further attacks by understanding the authorization event chains that govern room operations. This vulnerability directly relates to ATT&CK technique T1566.002 Credential Access: Phishing for Information, as it allows unauthorized access to authorization data that could be used to understand and potentially compromise the federated network. The issue was particularly concerning because it affected Synapse versions up to and including 1.68.0, which were widely deployed across Matrix networks, potentially exposing numerous federated homeservers to exploitation. The patch implemented in Synapse 1.69.0 addressed the core access control validation issue by strengthening the authentication checks during authorization event requests.
Organizations running Synapse homeservers should prioritize immediate upgrade to version 1.69.0 or later to remediate this vulnerability. System administrators should also implement network-level monitoring to detect unusual federation API activity patterns that might indicate exploitation attempts. The fix involves strengthening the access control validation logic within the federation API, ensuring that all authorization event requests undergo proper authentication and authorization checks before any data is returned. Additionally, administrators should review their federation configurations and implement proper logging of federation API interactions to detect potential abuse. The vulnerability demonstrates the critical importance of access control validation in federated systems where multiple independent servers must trust each other while maintaining proper security boundaries. This incident underscores the need for continuous security auditing of federation protocols and proper validation of all incoming requests in distributed systems.