This document provides an enhanced design overview of the Simple Realtime Server (SRS), an open-source live streaming server, specifically tailored for threat modeling and security analysis. Building upon the initial design, this version offers more refined details and clarifies critical aspects for security assessment. It outlines the system's architecture, components, data flow, and external interfaces with a stronger focus on security implications. This document is intended to be a robust foundation for identifying potential security vulnerabilities and developing comprehensive mitigation strategies. The design is based on the understanding of SRS as described in the project's GitHub repository: https://github.com/ossrs/srs.
SRS is a versatile, high-performance, open-source real-time streaming server engineered for simplicity, robustness, and efficiency in building live streaming applications. It supports a wide array of protocols, including RTMP, WebRTC, HLS, and HTTP-FLV, catering to diverse streaming needs.
Core Functionalities:
- Ingest (Publishing): Accepts live audio and video streams from encoders and publishers using protocols like RTMP and WebRTC. This is the entry point for content into the SRS ecosystem.
- Processing & Management: Manages and processes incoming streams. This includes core functionalities like session management, stream routing, and optional features such as transcoding, recording, and stream forwarding.
- Delivery (Playback): Distributes live streams to viewers and players via protocols like HTTP-FLV, HLS, and WebRTC. This is the content distribution mechanism to end-users.
- Control Plane (Management & Monitoring): Provides HTTP-based APIs and interfaces for server management, configuration, monitoring, and operational control.
Target Audience:
This document is primarily for security engineers, cybersecurity analysts, developers, and operations teams involved in threat modeling, security architecture review, and security testing of systems that incorporate SRS.
The SRS architecture is structured into distinct layers, each responsible for specific functionalities. This layered approach aids in understanding the system's complexity and identifying potential security boundaries.
- Network Interface Layer: Handles all network-level communication, protocol-specific parsing, and connection management for various streaming protocols and management interfaces.
- Core Logic Layer: Implements the central server functionalities, including session management, stream routing, configuration management, and scheduling of internal tasks.
- Application Service Layer: Provides value-added services and features built upon the core logic, such as transcoding, recording, forwarding, and API handling.
- Data Storage Layer: Manages persistent storage for configuration data, recorded streams, and potentially other operational data.
The following diagram provides a refined visualization of the SRS architecture, emphasizing the data flow and component interactions:
graph LR
subgraph "External Entities"
P["'Publishers (Encoders)'"]
V["'Viewers (Players)'"]
M["'Management Clients (HTTP API)'"]
ES["'External Servers (Forwarding)'"]
ST["'STUN/TURN Servers'"]
end
subgraph "SRS Server"
subgraph "Network Interface Layer"
NI1["'RTMP Handler'"]
NI2["'WebRTC Handler'"]
NI3["'HTTP Handler'"]
end
subgraph "Core Logic Layer"
CL1["'Session Manager'"]
CL2["'Stream Router'"]
CL3["'Config Manager'"]
CL4["'Scheduler'"]
end
subgraph "Application Service Layer"
AS1["'Transcoder (FFmpeg)'"]
AS2["'Recorder'"]
AS3["'Forwarder Logic'"]
AS4["'HTTP API Handler'"]
AS5["'HLS Muxer'"]
AS6["'HTTP-FLV Muxer'"]
end
subgraph "Data Storage Layer"
DS1["'Configuration Files'"]
DS2["'Recorded Files (Disk)'"]
end
end
P --> NI1 & NI2
V --> NI3 & NI2
M --> NI3
ES <-- AS3
ST <-- NI2
NI1 --> CL1
NI2 --> CL1 & ST
NI3 --> CL1 & AS4
CL1 --> CL2
CL1 --> CL3
CL1 --> AS1 & AS2 & AS3 & AS5 & AS6
CL2 --> NI1 & NI2 & NI3 & AS1 & AS2 & AS3 & AS5 & AS6
CL3 --> CL1 & CL2 & NI1 & NI2 & NI3 & AS1 & AS2 & AS3 & AS4 & AS5 & AS6 & DS1
CL4 --> CL1 & CL2 & NI1 & NI2 & NI3 & AS1 & AS2 & AS3 & AS4 & AS5 & AS6
AS1 --> CL2
AS2 --> DS2
AS3 --> ES
AS4 --> CL1 & CL3
AS5 --> NI3 & DS2
AS6 --> NI3
CL3 --> DS1
Data Flow - Scenarios:
-
RTMP Publish & HLS Playback:
- Publish: Publisher (P) -> RTMP Handler (NI1) -> Session Manager (CL1) -> Stream Router (CL2).
- Process: Stream Router (CL2) -> HLS Muxer (AS5) -> HTTP Handler (NI3) -> Data Storage Layer (DS2 - for segments).
- Playback: Viewer (V) -> HTTP Handler (NI3) -> HLS Muxer (AS5) -> Stream Router (CL2). HTTP Handler serves HLS manifest and segments from AS5/DS2.
-
WebRTC Publish & WebRTC Playback:
- Publish: Publisher (P) -> WebRTC Handler (NI2) -> Session Manager (CL1) -> Stream Router (CL2) -> STUN/TURN Servers (ST) (for NAT traversal).
- Playback: Viewer (V) -> WebRTC Handler (NI2) -> Session Manager (CL1) -> Stream Router (CL2) -> STUN/TURN Servers (ST) (for NAT traversal). WebRTC Handler manages SDP negotiation and media flow.
-
Management API Request (e.g., Get Server Status):
- Management Client (M) -> HTTP Handler (NI3) -> HTTP API Handler (AS4) -> Core Logic Layer (CL1, CL3, etc.) -> HTTP API Handler (AS4) -> HTTP Handler (NI3) -> Management Client (M).
This section provides detailed descriptions of each component, with a heightened focus on security considerations and potential vulnerabilities.
4.1. Network Interface Layer Components:
-
RTMP Handler (NI1):
- Protocol: Real-Time Messaging Protocol (RTMP). Primarily used for publishing.
- Functionality: Manages RTMP connections, handshakes, message parsing (AMF encoding/decoding), and session initiation for publishers.
- Security Considerations:
- RTMP Protocol Weaknesses: Inherent vulnerabilities in the RTMP protocol itself, such as lack of mandatory encryption and potential for command injection if parsing is flawed.
- Buffer Overflows: Potential for buffer overflows in AMF parsing or handling of large RTMP messages.
- Authentication Bypass: If authentication is implemented, weaknesses in the authentication mechanism could lead to unauthorized publishing.
- Denial of Service (DoS): Susceptible to DoS attacks by flooding with connection requests or malformed RTMP packets.
-
WebRTC Handler (NI2):
- Protocol: Web Real-Time Communication (WebRTC). Used for both publishing and playback.
- Functionality: Handles WebRTC signaling (SDP negotiation), ICE candidate exchange, DTLS handshake for encryption, and SRTP for media transport. Interacts with STUN/TURN servers for NAT traversal.
- Security Considerations:
- WebRTC Complexity: The complexity of the WebRTC stack introduces a larger attack surface.
- SDP Vulnerabilities: Potential vulnerabilities in SDP parsing and handling, leading to injection attacks or misconfiguration.
- ICE/STUN/TURN Security: Reliance on STUN/TURN servers introduces dependencies and potential vulnerabilities if these external services are compromised or misconfigured.
- DTLS/SRTP Implementation Flaws: Vulnerabilities in the implementation of DTLS and SRTP libraries used by SRS.
- Man-in-the-Middle (MITM) Attacks: Although DTLS/SRTP provides encryption, misconfigurations or downgrade attacks could weaken security.
-
HTTP Handler (NI3):
- Protocol: Hypertext Transfer Protocol (HTTP). Used for HTTP-FLV, HLS, HTTP API, and potentially static file serving.
- Functionality: Handles HTTP requests, including parsing headers, routing requests to appropriate handlers (API, HLS, FLV), and serving responses.
- Security Considerations:
- Standard Web Server Vulnerabilities: Susceptible to common web server vulnerabilities like path traversal, HTTP header injection, cross-site scripting (XSS) if serving dynamic content or API responses without proper sanitization.
- API Security: HTTP API endpoints require robust authentication and authorization. Vulnerabilities in API design or implementation can lead to unauthorized access and control.
- DoS Attacks: HTTP handlers can be targeted by DoS attacks through excessive requests or slowloris attacks.
- HLS/FLV Specific Issues: Potential vulnerabilities related to HLS manifest generation or FLV streaming if not implemented securely.
4.2. Core Logic Layer Components:
-
Session Manager (CL1):
- Functionality: Manages all active sessions (publishers, viewers, API clients). Handles session creation, authentication, authorization, session lifecycle management, and resource allocation per session.
- Security Considerations:
- Authentication & Authorization Flaws: Weak or missing authentication and authorization mechanisms are critical vulnerabilities.
- Session Management Weaknesses: Session fixation, session hijacking, insecure session storage, and lack of session timeout mechanisms.
- Privilege Escalation: Vulnerabilities that allow users to gain unauthorized privileges.
- Resource Exhaustion: If session management is not robust, attackers could exhaust server resources by creating excessive sessions.
-
Stream Router (CL2):
- Functionality: Routes media streams based on stream names, application logic, and configuration. Directs streams to transcoding, recording, forwarding, and delivery handlers. Manages stream metadata and state transitions.
- Security Considerations:
- Stream Hijacking: Vulnerabilities in routing logic could allow attackers to redirect or intercept streams.
- Unauthorized Stream Access: Lack of proper access control in stream routing could lead to unauthorized viewing or publishing of streams.
- Injection Attacks (Stream Name): If stream names are not properly validated, injection attacks might be possible.
-
Config Manager (CL3):
- Functionality: Loads, parses, and manages server configuration from files (e.g.,
srs.conf). Provides configuration parameters to all other components. - Security Considerations:
- Configuration File Vulnerabilities: Insecure default configurations, misconfigurations, and vulnerabilities in configuration file parsing.
- Sensitive Data Exposure: Storing sensitive information (passwords, API keys) in configuration files in plain text.
- Unauthorized Configuration Changes: Lack of access control to configuration files or API endpoints that modify configuration.
- Functionality: Loads, parses, and manages server configuration from files (e.g.,
-
Scheduler (CL4):
- Functionality: Manages background tasks, scheduled jobs, and timers within the server.
- Security Considerations:
- Unintended Scheduled Tasks: Vulnerabilities that could allow attackers to inject or modify scheduled tasks to execute malicious code or cause disruption.
- Resource Exhaustion (Scheduled Tasks): Poorly designed scheduled tasks could consume excessive resources and lead to DoS.
4.3. Application Service Layer Components:
-
Transcoder (AS1) (FFmpeg):
- Functionality: Transcodes media streams to different formats, resolutions, and bitrates using FFmpeg as an external process.
- Security Considerations:
- FFmpeg Vulnerabilities: FFmpeg itself is a complex software and may have known or zero-day vulnerabilities.
- Command Injection: If stream metadata or configuration parameters are not properly sanitized before being passed as arguments to FFmpeg, command injection vulnerabilities are possible.
- Resource Exhaustion (Transcoding Load): Excessive transcoding requests can overload the server and lead to DoS.
-
Recorder (AS2):
- Functionality: Records live streams to disk in various formats (e.g., FLV, MP4).
- Security Considerations:
- Storage Location Security: Insecure storage locations for recorded files could lead to unauthorized access or data breaches.
- Access Control to Recordings: Lack of proper access control to recorded files.
- Denial of Service (Disk Filling): Attackers could fill up disk space by initiating excessive recording sessions.
-
Forwarder Logic (AS3):
- Functionality: Forwards streams to other SRS servers or external streaming services.
- Security Considerations:
- Insecure Forwarding Destinations: Forwarding streams to untrusted or compromised external servers.
- Man-in-the-Middle (MITM) Attacks (Forwarding): If forwarding is not done over secure channels (e.g., RTMP without TLS), streams could be intercepted.
- Amplification Attacks: Forwarding could be misused for amplification attacks if not properly controlled.
-
HTTP API Handler (AS4):
- Functionality: Implements the HTTP API for server management, monitoring, and control.
- Security Considerations:
- API Authentication & Authorization: Weak or missing authentication and authorization for API endpoints.
- Input Validation (API Requests): Lack of input validation for API requests can lead to injection attacks (e.g., SQL injection if interacting with a database, command injection).
- API Rate Limiting: Absence of rate limiting can lead to API abuse and DoS attacks.
- Information Disclosure (API Responses): API responses might inadvertently disclose sensitive information.
-
HLS Muxer (AS5):
- Functionality: Segments live streams into HLS format, generates HLS manifests (.m3u8) and media segments (.ts).
- Security Considerations:
- HLS Manifest Manipulation: Vulnerabilities that could allow attackers to manipulate HLS manifests, potentially injecting malicious content or redirecting viewers.
- Access Control to HLS Content: Lack of access control to HLS manifests and segments.
-
HTTP-FLV Muxer (AS6):
- Functionality: Packages live streams into HTTP-FLV format for delivery.
- Security Considerations:
- FLV Streaming Vulnerabilities: Potential vulnerabilities related to FLV streaming if not implemented securely.
- Access Control to FLV Streams: Lack of access control to HTTP-FLV streams.
4.4. Data Storage Layer Components:
-
Configuration Files (DS1):
- Functionality: Stores server configuration parameters in files.
- Security Considerations:
- File System Permissions: Insecure file system permissions on configuration files.
- Plain Text Secrets: Storing sensitive secrets in plain text within configuration files.
- Backup and Recovery: Lack of secure backup and recovery mechanisms for configuration files.
-
Recorded Files (Disk) (DS2):
- Functionality: Stores recorded media streams on disk.
- Security Considerations:
- File System Permissions: Insecure file system permissions on recorded files.
- Data Retention Policies: Lack of proper data retention policies for recorded content, potentially leading to data breaches or compliance issues.
- Encryption at Rest: Recorded files are typically not encrypted at rest, posing a risk if storage is compromised.
This section re-examines external interfaces with a focus on security implications for each interaction point.
-
Publishers (Encoders):
- Protocols: RTMP, WebRTC.
- Security Perspective: Publishers are the primary content providers. Compromised publishers or insecure publishing channels can inject malicious content or disrupt service. Strong authentication and authorization are crucial. Input validation of incoming streams is essential to prevent injection attacks.
-
Viewers (Players):
- Protocols: HTTP-FLV, HLS, WebRTC.
- Security Perspective: Viewers consume content. Security concerns are primarily around access control (preventing unauthorized viewing) and protecting viewers from potentially malicious content (though SRS primarily streams live content, not user-generated content in the typical sense).
-
Management Clients (HTTP API):
- Protocol: HTTP (HTTPS recommended).
- Security Perspective: Management clients have administrative control over the SRS server. API security is paramount. Strong authentication, authorization, input validation, and secure communication (HTTPS) are mandatory.
-
External Transcoding Engines (e.g., FFmpeg):
- Protocol: Inter-process communication.
- Security Perspective: External processes introduce security risks. Input to FFmpeg must be carefully sanitized to prevent command injection. Monitor resource consumption of transcoding processes to prevent DoS.
-
External Forwarding Destinations (Other Servers, CDNs):
- Protocols: RTMP, etc.
- Security Perspective: Forwarding to untrusted destinations can expose streams to unauthorized parties. Secure communication channels for forwarding are recommended. Verify the security posture of forwarding destinations.
-
STUN/TURN Servers (for WebRTC):
- Protocols: STUN/TURN.
- Security Perspective: Reliance on external STUN/TURN servers introduces trust dependencies. Ensure communication with STUN/TURN servers is secure and that these services are reliable and trustworthy.
This document is designed to be used for threat modeling. Here's how to leverage it and a categorized list of security considerations to guide the process, potentially using a methodology like STRIDE:
How to Use this Document for Threat Modeling:
- Component-Based Threat Modeling: Use Section 4 (Component Descriptions) to analyze each component individually. For each component, consider the STRIDE categories (Spoofing, Tampering, Repudiation, Information Disclosure, Denial of Service, Elevation of Privilege) and identify potential threats.
- Data Flow Diagram Analysis: Use the Mermaid diagram in Section 3 to trace data flows (e.g., publish, playback, management API). For each data flow, consider potential threats at each stage of the flow.
- External Interface Analysis: Use Section 5 (External Interfaces) to analyze interactions with external entities. Consider threats arising from each external interface and the security measures needed to mitigate them.
- Scenario-Based Threat Modeling: Develop specific use cases or scenarios (e.g., "Unauthorized user attempts to publish a stream," "Attacker tries to take down the server"). Use the design document to analyze these scenarios and identify potential vulnerabilities and attack paths.
Categorized Security Considerations (STRIDE-aligned):
-
Spoofing (Authentication):
- Weak or missing authentication for publishers, viewers, and management API.
- Vulnerabilities in RTMP handshake or WebRTC signaling that could allow spoofing identities.
- Lack of mutual authentication where required.
-
Tampering (Integrity):
- Lack of integrity checks on configuration files.
- Potential for manipulation of HLS manifests or media segments.
- Insecure forwarding channels that could allow stream tampering.
- Command injection vulnerabilities that could allow attackers to modify server behavior.
-
Repudiation (Non-Repudiation):
- Insufficient logging and auditing to track actions and events.
- Lack of mechanisms to prove or disprove actions taken by users or components.
-
Information Disclosure (Confidentiality):
- Exposure of sensitive data in configuration files (plain text secrets).
- Information leakage through API responses or error messages.
- Unauthorized access to recorded streams or configuration files.
- Insecure storage of recorded files (no encryption at rest).
-
Denial of Service (Availability):
- DoS attacks targeting network handlers (RTMP, WebRTC, HTTP).
- Resource exhaustion through excessive transcoding, recording, or session creation.
- Vulnerabilities in scheduled tasks that could lead to resource depletion.
- API abuse due to lack of rate limiting.
- Disk filling attacks through excessive recording.
-
Elevation of Privilege (Authorization):
- Authorization bypass vulnerabilities in session management or API handlers.
- Privilege escalation flaws that allow users to gain administrative access.
- Insecure default configurations that grant excessive privileges.
This improved design document provides a more detailed and security-focused overview of the SRS architecture. By elaborating on component functionalities, data flows, and external interfaces, and by categorizing security considerations, this document aims to be a highly effective tool for conducting thorough threat modeling of SRS deployments. It encourages a systematic approach to security analysis, enabling security professionals to identify, assess, and mitigate potential vulnerabilities, ultimately strengthening the security posture of live streaming systems built with SRS. This document should be considered a living document, to be updated as the SRS project evolves and new security insights emerge.