CVE-2025-62175

Executive Summary

CVE-2025-62175 is a moderate-severity authorization bypass vulnerability in Mastodon, a popular open-source, decentralized social network server. The flaw allows a user account that has been disabled or suspended by an administrator to maintain an active connection to the Mastodon Streaming API. This means that even after a moderator takes action to remove a user's ability to interact with the platform, the user can still receive real-time updates (e.g., new posts, notifications, direct messages) through existing or newly established WebSocket connections.

This vulnerability undermines the fundamental trust in moderation actions. Administrators expect that disabling or suspending an account completely severs the user's access to the service. The issue is rooted in a missing authorization check within the streaming endpoint, which does not validate the account's active status when establishing or maintaining a WebSocket connection.

The vulnerability affects all Mastodon versions prior to 4.4.6, 4.3.14, and 4.2.27. No public proof-of-concept (PoC) or exploit code is currently available, and the EPSS score is 0.0%, indicating a very low probability of exploitation in the wild at this time. However, the impact on platform integrity and user privacy is significant.

Supply Chain & Package Security

Mastodon is a Ruby on Rails application distributed as a Ruby gem and Docker image. The vulnerability is in the application code itself, not in a third-party dependency. However, administrators should ensure they are using the official Mastodon Docker images or Ruby gems from trusted sources (e.g., rubygems.org, ghcr.io/mastodon). The patch is available in the official Mastodon repository on GitHub.

CVSS Breakdown & Threat Level

• CVSS Score: 4.3 (Medium)
• Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N
• Attack Vector (AV): Network (N) - The vulnerability is exploitable over the network.
• Attack Complexity (AC): Low (L) - No special conditions are required for exploitation.
• Privileges Required (PR): Low (L) - The attacker must have a valid, albeit disabled/suspended, account.
• User Interaction (UI): None (N) - No user interaction is required.
• Scope (S): Unchanged (U) - The vulnerability does not affect other components.
• Confidentiality (C): Low (L) - The attacker can only read real-time data, not modify or delete it.
• Integrity (I): None (N) - No data can be modified.
• Availability (A): None (N) - No impact on service availability.

Threat Level: Medium. While the CVSS score is moderate, the vulnerability directly undermines a core security control (account suspension). The low EPSS score suggests active exploitation is unlikely, but the potential for abuse in targeted attacks or by disgruntled users is real.

Deep Dive & Mechanics

The Mastodon Streaming API is a WebSocket-based endpoint that provides real-time updates to connected clients. When a user logs in, their client establishes a persistent WebSocket connection to the server. The server then pushes events (e.g., new toots, notifications, timeline updates) to the client over this connection.

The vulnerability lies in the authorization logic for the streaming endpoint. When a user account is disabled or suspended, the server correctly blocks access to all REST API endpoints (e.g., posting, liking, following). However, the streaming endpoint does not re-validate the user's account status after the initial connection is established. Furthermore, it does not check the account status when a new streaming connection is initiated.

Root Cause: The streaming endpoint's authentication middleware likely checks for a valid session token or OAuth token but does not verify that the associated user account is in an "active" state. The check for user.suspended? or user.disabled? is missing from the connection establishment and maintenance logic.

Attack Flow:

1. An attacker creates a legitimate Mastodon account.
2. An administrator discovers the account is violating terms of service and suspends or disables it.
3. The attacker's client, which already has an open WebSocket connection to the streaming API, continues to receive real-time data.
4. The attacker can also open a new WebSocket connection (e.g., by re-authenticating with the same credentials) and continue to receive updates.
5. The attacker can monitor public and private timelines, receive direct messages, and observe all activity on the server as if their account were still active.

Vulnerable vs. Patched Code

The vulnerable code is likely in the streaming API's authentication/authorization module. A simplified example in Ruby (Mastodon's language) is shown below.

Vulnerable Code (Conceptual):

# app/controllers/streaming_controller.rb
class StreamingController < ApplicationController
  def connect
    # Authenticate the user (checks token validity)
    @user = authenticate_user!
    # Missing: Check if user is active/suspended/disabled
    # @user.active? or !@user.suspended? is NOT called
    establish_websocket_connection(@user)
  end
end

Patched Code (Conceptual):

# app/controllers/streaming_controller.rb
class StreamingController < ApplicationController
  def connect
    @user = authenticate_user!
    # Added authorization check
    if @user.suspended? || @user.disabled?
      render json: { error: 'Account is not active' }, status: :forbidden
      return
    end
    establish_websocket_connection(@user)
  end
end

The patch adds a check for suspended? and disabled? (or similar methods) before allowing the WebSocket connection to be established. It also likely includes logic to terminate existing connections for accounts that are suspended after the connection is made.

Exploitation & Lab Setup

Red Team

Objective: Demonstrate that a suspended account can still receive real-time updates.

Prerequisites:

• A Mastodon instance (e.g., running in Docker).
• Two user accounts: one normal (attacker) and one admin.
• A WebSocket client (e.g., wscat, a browser with developer tools, or a custom script).

Steps:

1. Setup: Deploy a vulnerable Mastodon instance (version < 4.4.6). Create a user account (e.g., attacker@example.com).
2. Establish Connection: As the attacker, connect to the streaming API using a WebSocket client. For example, using wscat:

   wscat -c "wss://your-mastodon-instance/api/v1/streaming?access_token=YOUR_ACCESS_TOKEN"
   

   Subscribe to a public timeline:

   {"type":"subscribe","stream":"public"}
   

3. Suspend Account: Log in as the admin and suspend the attacker account.
4. Observe: The attacker's WebSocket connection remains open. They will continue to receive new public posts and other events.
5. Re-connect: The attacker can also open a new WebSocket connection using the same (now suspended) credentials and will still receive updates.

Expected Result: The attacker can monitor the server's activity in real-time despite their account being suspended.

Blue Team

Objective: Detect and prevent exploitation of this vulnerability.

Detection:

• Monitor WebSocket Connections: Log all WebSocket connection attempts and disconnections. Correlate with user account status changes.
• Alert on Suspicious Activity: Generate an alert when a WebSocket connection is established or maintained by a user whose account status is not "active".
• Audit Logs: Review audit logs for any account suspension events followed by continued streaming API activity from the same user.

Prevention:

• Patch Immediately: Upgrade to Mastodon 4.4.6, 4.3.14, or 4.2.27.
• Network Segmentation: If patching is delayed, consider blocking the streaming API endpoint (/api/v1/streaming) for all users and only allowing it from trusted IPs (this will break functionality).
• Session Invalidation: After suspending an account, manually invalidate all active sessions and tokens for that user. This is a workaround, not a fix.

Indicators of Compromise (IOCs)

• Network IOCs:
  • Sustained WebSocket connections from a user account that has been suspended or disabled.
  • Multiple reconnection attempts from a suspended user to the streaming API.
• Log IOCs:
  • Log entries showing a WebSocket connection (/api/v1/streaming) for a user whose account_status is suspended or disabled.
  • Audit log entries for account suspension followed by streaming API activity from the same user.
• File IOCs:
  • N/A (This is a logic flaw, not a file-based compromise).

Detection Engineering

YARA Rule

rule CVE_2025_62175_Mastodon_Streaming_API_Bypass {
    meta:
        description = "Detects potential exploitation of CVE-2025-62175 by identifying WebSocket connections from suspended users"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-62175"
        severity = "medium"
    strings:
        $ws_connect = /\/api\/v1\/streaming/
        $suspended_user = /account_status.*suspended/
        $disabled_user = /account_status.*disabled/
    condition:
        $ws_connect and ($suspended_user or $disabled_user)
}

Sigma Rule

title: Mastodon Streaming API Connection from Suspended User
id: 8a2b3c4d-5e6f-7a8b-9c0d-1e2f3a4b5c6d
status: experimental
description: Detects a WebSocket connection to the Mastodon streaming API from a user account that has been suspended or disabled.
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2025-62175
logsource:
    category: webserver
    product: mastodon
detection:
    selection:
        cs-uri-query|contains: "/api/v1/streaming"
    condition: selection
    # Note: This rule requires correlation with user account status data.
    # In practice, you would need to enrich the log with the user's account status.
    # Example: correlate with a database query or an additional log source.
falsepositives:
    - Legitimate connections from active users.
level: medium
tags:
    - attack.t1562.001
    - cve.2025.62175

Suricata Rule

alert tcp any any -> any 443 (msg:"CVE-2025-62175 Mastodon Streaming API Connection from Suspended User"; flow:to_server,established; content:"/api/v1/streaming"; http_uri; content:"access_token="; http_uri; sid:1000001; rev:1;)

Note: The Suricata rule above is a basic detection for streaming API connections. A more effective rule would require deep packet inspection or integration with a user database to check account status.

Remediation & Mitigation

Immediate Remediation:

1. Patch: Upgrade Mastodon to version 4.4.6, 4.3.14, or 4.2.27 (depending on your current major version). This is the only complete fix.

   # For Docker installations:
   docker pull ghcr.io/mastodon/mastodon:v4.4.6
   docker-compose up -d
   

   # For source installations:
   git checkout v4.4.6
   bundle install
   RAILS_ENV=production bundle exec rails db:migrate
   RAILS_ENV=production bundle exec rails assets:precompile
   

2. Workaround (No Patch Available): If patching is not immediately possible, consider the following:

   • Disable Streaming API: Block access to /api/v1/streaming at the reverse proxy level (e.g., Nginx). This will break real-time features but prevent exploitation.
   • Manual Session Invalidation: After suspending a user, manually invalidate all their OAuth tokens and sessions. This is not a complete fix as the user could re-authenticate.

Long-Term Mitigation:

• Implement Authorization Checks: Ensure all API endpoints, including WebSocket connections, re-validate the user's account status on every request or connection.
• Audit Logging: Implement comprehensive logging of all WebSocket connections and disconnections, including the user's account status at the time of connection.
• Security Testing: Include authorization bypass tests in your CI/CD pipeline for all API endpoints, especially real-time ones.

Valters IT