CVE-2021-29471 in Synapse
Summary
by MITRE • 05/11/2021
Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is an ecosystem for open federated Instant Messaging and VoIP. In Synapse before version 1.33.2 "Push rules" can specify conditions under which they will match, including `event_match`, which matches event content against a pattern including wildcards. Certain patterns can cause very poor performance in the matching engine, leading to a denial-of-service when processing moderate length events. The issue is patched in version 1.33.2. A potential workaround might be to prevent users from making custom push rules, by blocking such requests at a reverse-proxy.
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Analysis
by VulDB Data Team • 05/15/2021
The vulnerability CVE-2021-29471 affects Synapse, a Matrix reference homeserver implemented in Python and distributed as the pypi package matrix-synapse. Matrix represents an open federated ecosystem for instant messaging and voice over IP communications that enables decentralized messaging across interconnected servers. This particular vulnerability resides within the push notification system of Synapse, specifically within the push rules engine that determines when notifications should be triggered based on event conditions. The Matrix protocol's architecture allows users to define custom push rules that specify under what circumstances they should receive notifications, creating a flexible but potentially dangerous mechanism for event matching.
The technical flaw manifests in the implementation of event matching conditions, particularly the `event_match` functionality that allows pattern matching against event content with wildcard support. When certain complex patterns are constructed, the matching engine exhibits extremely poor performance characteristics that scale exponentially with event content length. This performance degradation occurs during the processing of moderate-length events, where the regular expression engine becomes overwhelmed by backtracking operations that can consume excessive CPU cycles and memory resources. The vulnerability represents a classic denial-of-service condition where legitimate system operations become computationally expensive enough to render the service effectively unavailable to users.
The operational impact of this vulnerability extends beyond simple service disruption to potentially affect entire Matrix server deployments. Attackers could craft malicious push rules containing specially constructed patterns that would cause the server to spend excessive resources processing each event, leading to resource exhaustion and service unavailability. This issue affects the core functionality of Matrix homeservers, which rely on efficient event processing to maintain real-time communication capabilities. The vulnerability affects all versions prior to 1.33.2, meaning that any deployment running an older version remains susceptible to this attack vector. The performance characteristics make this particularly dangerous in production environments where multiple users may be simultaneously affected by a single malicious push rule.
Mitigation strategies for CVE-2021-29471 include upgrading to Synapse version 1.33.2 or later, which implements proper bounds checking and performance optimizations for the pattern matching engine. Organizations can also implement defensive measures at the network level by blocking user-generated push rule creation through reverse proxy configurations, effectively preventing users from creating custom rules that could trigger the vulnerability. The fix addresses the underlying issue by implementing proper resource limits and performance constraints within the push rule matching engine, preventing the exponential backtracking behavior that caused the denial-of-service condition. This vulnerability aligns with CWE-400, which covers excessive resource consumption, and relates to ATT&CK technique T1499.004 for network denial-of-service attacks. The issue demonstrates the importance of input validation and performance bounds checking in security-critical systems where user-supplied patterns can be leveraged for resource exhaustion attacks. Organizations should implement monitoring for unusual CPU and memory usage patterns that might indicate exploitation attempts, while also ensuring that all Matrix deployments remain current with security patches to protect against similar vulnerabilities in the federated messaging ecosystem.