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threat-modeling.md

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Threat Model Analysis for termux/termux-app

Threat: Command Injection via termux-exec

  • Description: An attacker crafts malicious input to the main application, which is then unsafely passed to the termux-exec utility (or similar functions used to execute commands within Termux). The attacker injects arbitrary shell commands, bypassing intended functionality. For example, if the application takes a filename as input and passes it directly to ls [filename], an attacker could input ; rm -rf /; to execute a destructive command.
    • Impact:
      • Complete compromise of the Termux environment.
      • Potential data loss or modification within Termux.
      • Execution of arbitrary code with the privileges of the Termux user.
      • If the main application has higher privileges and shares data/context with Termux, this could lead to privilege escalation on the entire device.
    • Termux-app Component Affected: termux-exec (and any other functions/methods used for executing shell commands within Termux from the main application). This also includes any custom scripts or utilities within Termux that the main application interacts with.
    • Risk Severity: Critical
    • Mitigation Strategies:
      • Avoid Shell Commands: Prefer using Termux APIs or libraries that directly interact with the desired functionality (e.g., file system APIs instead of ls, network APIs instead of ping).
      • Parameterized Commands: If shell commands are unavoidable, use a safe, parameterized approach. For example, in Java, use ProcessBuilder and pass arguments as a list, never as a single concatenated string. Similar approaches exist in other languages.
      • Strict Input Validation: Implement rigorous input validation, allowing only a very restricted set of characters and patterns. Reject any input that contains shell metacharacters (;, |, &, $, etc.).
      • Whitelisting: If possible, maintain a whitelist of allowed commands and arguments.
      • Escaping (Least Preferred): If absolutely necessary, use robust escaping functions provided by the programming language or a trusted security library. Never attempt to roll your own escaping mechanism.

Threat: Privilege Escalation through Unprotected Intents

  • Description: The main application uses Android Intents to communicate with Termux, but these Intents are not properly protected. A malicious application on the same device can send crafted Intents to the main application, triggering unintended actions within Termux with the privileges of the main application. This bypasses the intended permission model of the main application.
    • Impact:
      • The malicious application gains control over the Termux integration, potentially executing arbitrary commands.
      • If the main application has elevated permissions, this could lead to a full device compromise.
      • Data leakage from the main application to Termux, or vice-versa.
    • Termux-app Component Affected: The Intent-based communication mechanism between the main application and Termux (specifically, any exported Activity, Service, or BroadcastReceiver in the main application that interacts with Termux, and how Termux processes these intents).
    • Risk Severity: High
    • Mitigation Strategies:
      • Explicit Intents: Use explicit Intents (specifying the target component by class name) instead of implicit Intents.
      • Permissions: Require a custom permission for sending Intents to the main application's components that interact with Termux. This permission should be defined with a signature protection level.
      • Intent Validation: Thoroughly validate the contents of all received Intents before processing them. Check the action, data, and extras for unexpected or malicious values.
      • exported=false: Set android:exported="false" in the manifest for any Activity, Service, or BroadcastReceiver that does not need to be accessible from other applications.

Threat: Termux:API Permission Abuse

  • Description: The main application requests excessive permissions for the Termux:API. A malicious Termux script (or a compromised Termux environment, leveraged through the main app's integration) can then use these permissions to access sensitive device features (camera, microphone, location, contacts, etc.) without the user's explicit knowledge or consent. The main application acts as a conduit for this abuse.
    • Impact:
      • Privacy violation (e.g., unauthorized access to camera, microphone, location).
      • Data theft (e.g., contacts, SMS messages).
      • Potential for device compromise.
    • Termux-app Component Affected: Termux:API and the permissions granted to it by the main application, and how the main application uses the Termux:API.
    • Risk Severity: High
    • Mitigation Strategies:
      • Principle of Least Privilege: Request only the absolutely necessary Termux:API permissions.
      • User Consent: Clearly explain to the user why each permission is needed and obtain explicit consent.
      • Runtime Permissions (Android 6.0+): Request dangerous permissions at runtime, not at install time.
      • Auditing: Log and audit the usage of the Termux:API by the main application.