CVE-2024-52815

Executive Summary

CVE-2024-52815 is a vulnerability in Synapse, the open-source Matrix homeserver, affecting versions before 1.120.1. The flaw allows a malicious federated server to send a specially crafted invite that disrupts the invited user's /sync functionality, effectively causing a denial-of-service (DoS) condition for that user. The vulnerability stems from improper validation of invites received over federation. The fix in version 1.120.1 rejects such malformed invites and restores sync for affected users. No public proof-of-concept (PoC) or exploit code is currently available.

Supply Chain & Package Security

Synapse is a Python-based project distributed via PyPI. The vulnerability is introduced in the source code and affects all deployments using the affected versions. Organizations should verify their Synapse version and update immediately. The package is typically installed via pip install matrix-synapse or through container images.

CVSS Breakdown & Threat Level

CVSS Score: 0.0 (NVD has not yet assigned a score; this is a placeholder based on available information) Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L (Estimated)

• Attack Vector: Network (AV:N) – The attack is carried out over the Matrix federation protocol.
• Attack Complexity: Low (AC:L) – No special conditions are required.
• Privileges Required: None (PR:N) – The attacker only needs to operate a federated Matrix server.
• User Interaction: None (UI:N) – The attack does not require the victim to perform any action.
• Scope: Unchanged (S:U) – The vulnerability affects only the targeted user's sync.
• Confidentiality Impact: None (C:N) – No data is exposed.
• Integrity Impact: None (I:N) – No data is modified.
• Availability Impact: Low (A:L) – The attack disrupts the /sync endpoint for the targeted user, causing a denial-of-service condition.

Threat Level: Medium (Estimated) – The attack is easy to execute remotely with no authentication, but the impact is limited to a single user's ability to sync.

Deep Dive & Mechanics

The vulnerability lies in the federation invite handling logic in Synapse. When a remote server sends an invite to a local user, Synapse processes the event and stores it. The /sync endpoint then retrieves and processes these invites to present them to the user. A malicious server can craft an invite event that contains malformed or unexpected data (e.g., invalid room state, missing required fields, or deeply nested structures) that causes the sync processing to fail or enter an infinite loop.

The core issue is insufficient validation of the invite event's structure and content before it is stored and later processed during sync. The fix in version 1.120.1 adds validation checks to reject invites that do not conform to the expected schema, preventing them from being stored and thus protecting the sync endpoint.

Vulnerable vs. Patched Code

The exact code change is not publicly detailed, but the logic can be inferred. The vulnerable code likely processes an invite event without checking for required fields or valid structure.

Vulnerable Code (Conceptual):

# In federation handler
async def on_invite(event):
    # No validation of event structure
    await store.store_event(event)
    return

# In sync handler
async def get_sync_data(user):
    invites = await store.get_invites(user)
    for invite in invites:
        # Process invite - may fail if malformed
        process_invite(invite)

Patched Code (Conceptual):

# In federation handler
async def on_invite(event):
    # Validate invite structure
    if not validate_invite(event):
        logger.warning("Rejected invalid invite from %s", event.sender)
        return
    await store.store_event(event)
    return

def validate_invite(event):
    # Check required fields
    if 'room_id' not in event or 'sender' not in event:
        return False
    # Additional schema validation
    return True

Exploitation & Lab Setup

Red Team

Prerequisites:

• A malicious Matrix server (e.g., using a modified Synapse instance or custom implementation).
• Knowledge of the target user's Matrix ID (e.g., @user:targetserver.com).

Exploitation Steps:

1. Set up a malicious Matrix server that can send federation requests.
2. Craft a specially formed invite event. The exact payload is unknown, but it likely involves:
   • Missing or malformed room_id or sender fields.
   • Invalid event type or state key.
   • Excessively nested or large event content.
3. Send the invite to the target user via the federation API (e.g., PUT /_matrix/federation/v2/invite/{roomId}/{eventId}).
4. The target user's Synapse server accepts the invite and stores it.
5. When the target user calls /sync, the malformed invite causes the sync to fail, resulting in a denial-of-service for that user.

Example (Conceptual):

import requests

target_server = "https://targetserver.com:8448"
malicious_event = {
    "room_id": "!invalid:maliciousserver.com",
    "sender": "@attacker:maliciousserver.com",
    "type": "m.room.member",
    "state_key": "@victim:targetserver.com",
    "content": {
        "membership": "invite",
        # Missing required fields or malformed data
    }
}

# Send invite via federation API
response = requests.put(
    f"{target_server}/_matrix/federation/v2/invite/!invalid:maliciousserver.com/$event_id",
    json=malicious_event
)

Blue Team

Detection:

• Monitor for unusual federation invite events with missing or malformed fields.
• Alert on repeated failed sync attempts for a single user.
• Use the detection rules provided in the Detection Engineering section.

Mitigation:

• Immediately upgrade to Synapse 1.120.1 or later.
• If upgrade is not possible, consider blocking federation from untrusted servers temporarily.
• Monitor logs for warnings about invalid invites.

Hunting:

• Search for logs containing "Rejected invalid invite" (after patching) or "Failed to process invite" (before patching).
• Review federation traffic for invites with unusual event structures.

Indicators of Compromise (IOCs)

• Network: Federation requests to /_matrix/federation/v2/invite/ with malformed JSON payloads.
• Logs: Synapse logs showing errors during sync processing for a specific user, or warnings about invalid invites.
• User Impact: A user reports that their Matrix client cannot sync or shows errors.

Detection Engineering

YARA Rule

rule Detect_Malformed_Matrix_Invite {
    meta:
        description = "Detects malformed Matrix invite events in federation traffic"
        author = "Valters IT"
        date = "2024-12-15"
        reference = "CVE-2024-52815"
    strings:
        $invite_endpoint = "/_matrix/federation/v2/invite/"
        $missing_room_id = "\"room_id\":"
        $missing_sender = "\"sender\":"
        $malformed_content = "\"content\":{}"
    condition:
        $invite_endpoint and (
            not ($missing_room_id) or
            not ($missing_sender) or
            $malformed_content
        )
}

Sigma Rule

title: CVE-2024-52815 - Malformed Federation Invite
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects malformed Matrix invite events that may indicate exploitation of CVE-2024-52815
author: Valters IT
date: 2024-12-15
logsource:
    category: application
    product: synapse
detection:
    selection:
        event: "Federation invite received"
        invite_validation: "failed"
    condition: selection
falsepositives:
    - Legitimate malformed invites from misconfigured servers
level: high

Suricata Rule

alert http $EXTERNAL_NET any -> $HOME_NET any (
    msg:"CVE-2024-52815 - Malformed Matrix Federation Invite";
    flow:to_server;
    content:"/_matrix/federation/v2/invite/"; http_uri;
    pcre:"/\"room_id\"\s*:\s*\"[^\"]*\"/";
    pcre:"/\"sender\"\s*:\s*\"[^\"]*\"/";
    pcre:"/\"content\"\s*:\s*\{\s*\}/";
    sid:1000001;
    rev:1;
)

Remediation & Mitigation

Immediate:

• Upgrade Synapse to version 1.120.1 or later. This is the only complete fix.
• If upgrade is delayed, consider restricting federation to trusted servers only using the federation_domain_whitelist configuration option.

Long-term:

• Implement input validation for all federation events.
• Monitor for similar vulnerabilities in other Matrix homeserver implementations.
• Subscribe to Synapse security advisories.

Workaround:

• No effective workaround exists other than upgrading or restricting federation.

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert