CVE-2025-30355

Executive Summary

CVE-2025-30355 is a critical vulnerability in Synapse, the open-source Matrix homeserver implementation. A malicious server can craft specially designed events that, when received by a vulnerable Synapse instance (versions up to and including 1.127.0), cause the server to cease federating with other Matrix servers. This effectively isolates the affected homeserver from the Matrix network, disrupting communication and collaboration. The vulnerability has been confirmed as exploited in the wild. The fix is included in Synapse v1.127.1, and no workarounds are available. The CVSS score is 7.1 (High), with a vector emphasizing network-based, low-complexity exploitation without authentication.

Supply Chain & Package Security

Synapse is a Python-based project distributed via PyPI. The vulnerability affects the core federation handling logic. Organizations using Synapse should immediately verify their installed version and update to v1.127.1 or later. Since this is a server-side vulnerability, client-side packages are not directly affected, but the impact on the server's ability to federate can disrupt all users of the affected homeserver.

CVSS Breakdown & Threat Level

• CVSS Score: 7.1 (High)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
• Attack Vector (AV): Network – The attack is executed over the network.
• Attack Complexity (AC): Low – No special conditions are required for exploitation.
• Privileges Required (PR): None – No authentication is needed.
• User Interaction (UI): None – No user action is required.
• Scope (S): Unchanged – The vulnerability does not affect other components.
• Confidentiality (C): None – No data is exposed.
• Integrity (I): None – No data is modified.
• Availability (A): High – The primary impact is denial of service (federation disruption).

Threat Level: High. The vulnerability is actively exploited, has a low attack complexity, and requires no authentication. The impact on availability is significant, as it can completely isolate a homeserver from the Matrix network.

Deep Dive & Mechanics

The vulnerability lies in Synapse's event processing logic for federation. When a malicious server sends a crafted event, Synapse fails to properly validate or handle the event's structure. This causes an internal error that corrupts the server's federation state, preventing it from sending or receiving events from other servers. The exact mechanism involves improper handling of event fields or signatures, leading to a state where the server's federation client or server components become non-functional.

The attack is executed by a malicious Matrix server that is federated with the target. The malicious server sends a specially crafted event (e.g., a room event or a server-to-server transaction) that triggers the vulnerability. Once processed, the target server's federation module enters a broken state, and it can no longer participate in the Matrix federation.

Vulnerable vs. Patched Code

The fix in Synapse v1.127.1 addresses the improper validation of incoming federation events. While the exact code change is not publicly detailed, the patch likely involves adding checks for event structure, signatures, or specific fields that were previously assumed to be safe.

Vulnerable Code (Conceptual):

# In federation event handler (pre-1.127.0)
async def on_receive_event(event):
    # No validation of event structure
    process_event(event)

Patched Code (Conceptual):

# In federation event handler (1.127.1+)
async def on_receive_event(event):
    if not validate_event_structure(event):
        raise InvalidEventError("Malformed event")
    process_event(event)

The patch ensures that events with unexpected or malicious structures are rejected before they can corrupt the federation state.

Exploitation & Lab Setup

Red Team (Adversarial Simulation)

Objective: Simulate the exploitation of CVE-2025-30355 to disrupt federation.

Lab Setup:

1. Target: A Synapse server running version 1.127.0 or earlier.
2. Attacker: A separate Matrix server (or a script simulating one) that is federated with the target.
3. Tooling: Custom Python script using the Matrix federation API (e.g., matrix-federation-sdk or raw HTTP requests).

Exploitation Steps:

1. Establish federation between the attacker and target servers.
2. Craft a malicious event that triggers the vulnerability. The exact payload is not publicly available, but it likely involves sending an event with a malformed content field, an invalid signatures object, or a non-standard type.
3. Send the event to the target server via the federation endpoint (e.g., PUT /_matrix/federation/v1/send/{txn_id}).
4. Verify that the target server stops responding to federation requests (e.g., GET /_matrix/federation/v1/version returns errors or timeouts).

Example Payload (Conceptual):

{
  "origin": "malicious.server",
  "origin_server_ts": 1234567890,
  "type": "m.room.message",
  "content": {
    "body": "trigger",
    "msgtype": "m.text"
  },
  "room_id": "!room:target.server",
  "sender": "@attacker:malicious.server",
  "signatures": {
    "malicious.server": {
      "ed25519:key": "INVALID_SIGNATURE"
    }
  }
}

Blue Team (Defensive Analysis)

Objective: Detect and mitigate exploitation attempts.

Detection Steps:

1. Monitor federation logs for unusual events or errors.
2. Look for events with malformed signatures or unexpected fields.
3. Implement network monitoring for anomalous federation traffic.

Mitigation:

• Immediately update to Synapse v1.127.1.
• If patching is not possible, consider temporarily disabling federation (e.g., by blocking port 8448 or modifying the Synapse configuration).
• Monitor for signs of exploitation (see IOCs).

Indicators of Compromise (IOCs)

• Network IOCs:
  • Unusual federation traffic from a specific server.
  • Events with invalid signatures or malformed JSON structures.
  • Repeated connection attempts from a server that is not in the allowed federation list.
• Log IOCs:
  • Errors in Synapse logs related to event processing (e.g., Failed to process event, Invalid event structure).
  • Sudden loss of federation capability (e.g., Unable to send events to remote server).
• System IOCs:
  • Synapse process consuming high CPU or memory after receiving a malicious event.
  • Federation endpoints returning 500 errors or timing out.

Detection Engineering

YARA Rule

rule CVE_2025_30355_Synapse_Federation_Event {
    meta:
        description = "Detects malicious federation events targeting CVE-2025-30355"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-30355"
    strings:
        $json_start = "{"
        $event_type = "m.room.message"
        $malformed_sig = "INVALID_SIGNATURE"
    condition:
        $json_start at 0 and $event_type and $malformed_sig
}

Sigma Rule

title: CVE-2025-30355 Synapse Federation Disruption
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects exploitation attempts against CVE-2025-30355 in Synapse logs
author: Valters IT
date: 2025-04-10
logsource:
    category: application
    product: synapse
detection:
    selection:
        EventID: "Failed to process event"
        Message: "*Invalid event structure*"
    condition: selection
falsepositives:
    - Legitimate malformed events from misconfigured servers
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-30355 Synapse Federation Exploit"; flow:to_server; content:"/_matrix/federation/v1/send/"; http_uri; content:"INVALID_SIGNATURE"; http_client_body; classtype:attempted-dos; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Patch: Upgrade Synapse to version 1.127.1 or later. This is the only known fix.
2. Verify: After patching, verify that federation is working correctly by checking the server's ability to send and receive events from other servers.

Alternative Mitigations (if patching is delayed):

• Disable Federation: Temporarily disable federation by setting federation_domain_whitelist to an empty list or blocking port 8448 at the firewall. This will isolate the server but prevent exploitation.
• Monitor: Implement the detection rules above to identify potential exploitation attempts.
• Restrict Federation: Use federation_domain_whitelist to only allow trusted servers.

Long-Term Recommendations:

• Keep Synapse updated to the latest version.
• Regularly review federation logs for anomalies.
• Implement network segmentation for critical infrastructure.

Valters IT
DevSecOps & Cyber Threat Intelligence Expert