CVE-2025-49090: Matrix Specification State Vulnerability

CVE-2025-49090 Overview

CVE-2025-49090 affects the Matrix specification before version 1.16, covering room versions earlier than 12 and State Resolution before 2.1. The flaw stems from deficient state resolution logic that allows manipulation of room state in federated Matrix deployments. An attacker with low privileges on the network can exploit weaknesses in how Matrix homeservers resolve conflicting state events. The issue affects the integrity of room membership, power levels, and other authoritative state across federated servers. The Matrix.org Foundation addressed the issue in specification v1.16 and the new State Resolution v2.1 algorithm introduced as part of Project Hydra.

Critical Impact

Successful exploitation enables unauthorized modification of room state, including membership and permission events, undermining trust assumptions in federated Matrix rooms.

Affected Products

• Matrix specification versions prior to 1.16
• Matrix room versions prior to 12
• Matrix State Resolution algorithm versions prior to 2.1

Discovery Timeline

• 2025-10-02 - CVE-2025-49090 published to NVD
• 2026-04-15 - Last updated in NVD database

Technical Details for CVE-2025-49090

Vulnerability Analysis

Matrix is a federated, decentralized communication protocol where multiple homeservers share authoritative state for shared rooms. State resolution is the algorithm responsible for reconciling divergent views of room state events such as m.room.member, m.room.power_levels, and m.room.join_rules. The pre-2.1 algorithm contains logical deficiencies that allow a participant to craft event chains causing the resolution process to converge on attacker-favorable state. This is categorized under [CWE-642] (External Control of Critical State Data). The flaw is not a memory safety issue but a protocol-level design weakness in how authority is established across federated event graphs.

Root Cause

The root cause lies in the state resolution algorithm's handling of conflicting authorization events and forks in the room directed acyclic graph (DAG). An attacker can introduce events that, once resolved against legitimate events, produce an outcome that elevates privileges or rewrites state. The fix in State Resolution v2.1 and room version 12 changes how authority chains are evaluated and how conflicts are tiebroken, removing the manipulation surface. See the Matrix Blog Project Hydra Update for the algorithmic rationale.

Attack Vector

The attack is network-based and requires the adversary to participate in a federated room or operate a malicious homeserver. The attacker injects specially crafted state events into the room DAG and relies on federation to propagate them. Other homeservers running pre-1.16 specification logic then resolve the manipulated state and accept the unauthorized outcome. Attack complexity is high because event chains must be carefully constructed to satisfy partial authorization checks while exploiting resolution edge cases. No user interaction is required on victim homeservers once the malicious events federate.

No public proof-of-concept code is published. For technical details, refer to the Matrix Spec Release v1.16 and the Matrix Blog Security Release.

Detection Methods for CVE-2025-49090

Indicators of Compromise

• Unexpected changes to m.room.power_levels or m.room.member events that do not trace back to legitimate sender authority
• Federation traffic from unfamiliar homeservers producing state forks in shared rooms
• Room state divergence reports between homeserver instances participating in the same room

Detection Strategies

• Audit homeserver logs for anomalous state resolution events and inspect room DAG forks for malformed authorization chains
• Compare resolved room state across multiple trusted homeservers and flag mismatches involving privileged events
• Monitor federation API endpoints such as /send and /state for unusual event volumes from low-reputation servers

Monitoring Recommendations

• Enable verbose audit logging on Synapse, Dendrite, or Conduit homeservers for federation and state resolution events
• Track room version upgrades and alert when rooms remain on versions earlier than 12 after the upgrade window
• Forward homeserver logs to a centralized analytics platform for correlation across federated state changes

How to Mitigate CVE-2025-49090

Immediate Actions Required

• Upgrade all Matrix homeservers to versions implementing Matrix specification 1.16 and State Resolution v2.1
• Create new sensitive rooms using room version 12 or later and upgrade existing rooms via m.room.tombstone events
• Restrict federation with untrusted homeservers until the upgrade is complete

Patch Information

The Matrix.org Foundation released the corrected specification in Matrix Spec Release v1.16. Homeserver implementations including Synapse, Dendrite, and Conduit shipped corresponding updates announced in the Matrix Blog Security Release. Administrators must update both the homeserver software and migrate rooms to room version 12 to benefit from State Resolution v2.1.

Workarounds

• Limit federation to a curated allowlist of trusted homeservers using server ACLs (m.room.server_acl)
• Reduce the blast radius by lowering default power levels and restricting who can send state events in critical rooms
• For sensitive deployments, disable open federation until homeservers and rooms are upgraded

# Example Synapse server ACL event content restricting federation
{
  "allow": ["trusted.example.org", "partner.example.com"],
  "deny": ["*"],
  "allow_ip_literals": false
}