CVE-2023-45129 in Synapse
Summary
by MITRE • 10/25/2023
Synapse is an open-source Matrix homeserver written and maintained by the Matrix.org Foundation. Prior to version 1.94.0, a malicious server ACL event can impact performance temporarily or permanently leading to a persistent denial of service. Homeservers running on a closed federation (which presumably do not need to use server ACLs) are not affected. Server administrators are advised to upgrade to Synapse 1.94.0 or later. As a workaround, rooms with malicious server ACL events can be purged and blocked using the admin API.
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Analysis
by VulDB Data Team • 10/25/2023
The vulnerability CVE-2023-45129 represents a significant denial of service risk within the Synapse Matrix homeserver implementation that affects versions prior to 1.94.0. This issue specifically targets the handling of server access control list events which are fundamental components in maintaining secure federated communication networks. The Matrix protocol relies heavily on server ACLs to define which servers can participate in specific rooms and maintain network integrity, making this vulnerability particularly dangerous in federated environments where multiple servers interact. The flaw manifests when malicious actors craft specially crafted server ACL events that cause the homeserver to process these entries in a manner that consumes excessive computational resources or creates persistent state conditions that cannot be easily resolved.
From a technical perspective, this vulnerability operates at the protocol level where server ACL events are processed and validated within the homeserver's event handling pipeline. The malicious server ACL events can trigger inefficient processing loops or resource allocation patterns that either temporarily consume excessive CPU cycles or create permanent performance degradation states. This behavior aligns with CWE-400 which categorizes resource exhaustion vulnerabilities, and the issue demonstrates how improper input validation and event processing can lead to denial of service conditions. The attack vector leverages the legitimate functionality of server ACLs while exploiting implementation weaknesses in how Synapse handles potentially malicious or malformed ACL events, creating a scenario where normal network operations become impossible.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromise entire federation networks that rely on the affected Synapse instances. When a server becomes vulnerable, it can affect not only its own performance but also create cascading effects throughout the federated network as other servers attempt to communicate with the compromised instance. The vulnerability's persistence characteristic means that once exploited, the denial of service condition can remain active even after the initial malicious event is removed, requiring administrator intervention to restore normal operations. This aligns with ATT&CK technique T1499.004 which describes network disruption attacks that can persist and cause ongoing damage to system availability. The affected servers essentially become unusable for their intended purpose of facilitating secure communication, making this a critical issue for organizations relying on Matrix-based communication platforms.
Organizations running Synapse versions prior to 1.94.0 must prioritize immediate upgrade to the patched version to eliminate this vulnerability. The upgrade process should include thorough testing in staging environments to ensure compatibility with existing configurations and room structures. As a temporary mitigation strategy, administrators can utilize the admin API to purge and block rooms containing malicious server ACL events, effectively isolating the problematic content from the rest of the federation. However, this workaround only addresses the symptoms rather than the root cause and requires ongoing manual intervention. The recommended approach follows the principle of least privilege and defensive programming practices, ensuring that all incoming events are properly validated before processing, which directly addresses CWE-252 which emphasizes the importance of proper validation and sanitization of inputs. Server administrators should also implement monitoring solutions to detect unusual resource consumption patterns that might indicate exploitation attempts, and maintain regular backups to facilitate quick recovery in case of persistent denial of service conditions.