CVE-2020-26257 in Synapse
Summary
by MITRE • 12/10/2020
Matrix is an ecosystem for open federated Instant Messaging and VoIP. Synapse is a reference "homeserver" implementation of Matrix. A malicious or poorly-implemented homeserver can inject malformed events into a room by specifying a different room id in the path of a `/send_join`, `/send_leave`, `/invite` or `/exchange_third_party_invite` request. This can lead to a denial of service in which future events will not be correctly sent to other servers over federation. This affects any server which accepts federation requests from untrusted servers. The Matrix Synapse reference implementation before version 1.23.1 the implementation is vulnerable to this injection attack. Issue is fixed in version 1.23.1. As a workaround homeserver administrators could limit access to the federation API to trusted servers (for example via `federation_domain_whitelist`).
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Analysis
by VulDB Data Team • 12/15/2020
The vulnerability described in CVE-2020-26257 represents a critical security flaw within the Matrix federated messaging ecosystem, specifically affecting the Synapse reference homeserver implementation. This vulnerability stems from inadequate input validation mechanisms that fail to properly verify the consistency between room identifiers specified in API request paths and those contained within the request payloads. The flaw exists in the handling of federation requests that involve room membership operations including send_join, send_leave, invite, and exchange_third_party_invite endpoints. When a malicious or compromised homeserver attempts to exploit this weakness, it can inject malformed events into rooms by manipulating the room_id parameter in the HTTP path while maintaining a different room_id in the request body, creating a discrepancy that the vulnerable system fails to detect and reject.
The technical exploitation of this vulnerability occurs through a specific pattern of federation request manipulation where an attacker can specify one room identifier in the API endpoint path while providing a different room identifier in the request content. This inconsistency allows the malicious server to inject events that appear to belong to one room but are actually being processed as belonging to another room within the federated network. The implementation flaw resides in the lack of proper cross-validation between path parameters and request body content, which constitutes a classic input validation vulnerability. According to CWE classification, this vulnerability maps to CWE-20: Improper Input Validation, as it involves the failure to properly validate and sanitize inputs received from external sources. The vulnerability also aligns with ATT&CK technique T1210: Exploitation of Remote Services, as it leverages federation APIs to execute malicious operations against remote systems.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromise the integrity and consistency of federated messaging across the Matrix network. When successfully exploited, the malformed events can cause cascading failures throughout the federation system, where subsequent events become misrouted or fail to propagate correctly to other servers. This creates a denial of service condition that affects not just the targeted room but potentially impacts the broader federation by corrupting the event graph structure that Matrix relies upon for maintaining consistent message ordering and room state across all participating servers. The vulnerability affects any server that accepts federation requests from untrusted sources, making it particularly dangerous in open federated environments where trust relationships are not strictly enforced.
The fix implemented in Synapse version 1.23.1 addresses this vulnerability through enhanced input validation mechanisms that enforce consistency between path parameters and request body content for all federation endpoints. The patch ensures that the room_id specified in the HTTP path matches exactly with the room_id contained within the request payload, preventing the injection attack vector. As a temporary mitigation measure, system administrators can implement the federation_domain_whitelist configuration option to restrict federation access to only trusted servers, effectively creating a perimeter defense mechanism. This workaround aligns with security best practices for controlling network access and limiting attack surface, though it represents a less comprehensive solution compared to the proper code-level fix. The vulnerability demonstrates the importance of proper input validation in federated systems where multiple independent entities participate in a shared network infrastructure, and highlights the need for robust consistency checks in API implementations that handle cross-server communications. Organizations using Matrix-based messaging systems should prioritize updating to version 1.23.1 or later to ensure protection against this specific injection attack vector that could lead to widespread service disruption across federated networks.