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Attack Tree Analysis for mtdowling/cron-expression

Objective: Unauthorized Command Execution OR Denial of Service via cron-expression

Attack Tree Visualization

Goal: Unauthorized Command Execution OR Denial of Service via cron-expression ├─── OR ─── │ ├─── 1. Unauthorized Command Execution │ │ ├─── AND ─── │ │ │ ├─── 1.1. Inject Malicious Cron Expression [CRITICAL] │ │ │ │ ├─── OR ─── │ │ │ │ │ ├─── 1.1.1. Unvalidated User Input [HIGH RISK] [CRITICAL] │ ├─── 2. Denial of Service (DoS) │ │ ├─── AND ─── │ │ │ ├─── 2.1. Resource Exhaustion via Frequent Execution │ │ │ │ ├─── 2.1.1. Inject High-Frequency Cron Expression [CRITICAL]

Attack Tree Path: 1. Unauthorized Command Execution

  • 1.1. Inject Malicious Cron Expression [CRITICAL]

    • Description: This is the fundamental step for achieving unauthorized command execution. The attacker needs to somehow get a malicious cron expression into the system where it will be parsed and used by the cron-expression library and the application. This node is critical because preventing the injection of malicious expressions blocks all subsequent command execution attempts via this library.
    • Sub-Vectors: See 1.1.1 below.

  • 1.1.1. Unvalidated User Input [HIGH RISK] [CRITICAL]

    • Description: The application accepts cron expressions directly from untrusted users (e.g., via a web form, API endpoint, etc.) without performing any validation or sanitization. This allows an attacker to craft a malicious cron expression that, when parsed and used by the application, leads to unintended command execution. This is the highest risk and most critical vulnerability.
    • Likelihood: High (if user input is directly used) / Medium (if some basic validation exists, but it's insufficient)
    • Impact: Very High (Potential for full system compromise. The attacker could gain complete control of the server.)
    • Effort: Low (The attacker simply needs to provide a malicious string.)
    • Skill Level: Low (Basic understanding of cron syntax and command injection is sufficient.)
    • Detection Difficulty: Medium (Depends on the level of logging and monitoring. If the application logs the cron expressions being used, it might be easier to detect. However, if the attacker is careful, the malicious expression might not stand out.)
    • Mitigation Strategies:
      • Strict Input Validation: Implement rigorous input validation using a whitelist approach. Only allow a very limited set of characters and patterns known to be safe for cron expressions. Reject any input that doesn't strictly conform to the whitelist.
      • Whitelist Allowed Characters: Specifically, allow only digits (0-9), commas (,), hyphens (-), asterisks (*), forward slashes (/), and potentially specific month/day names (if needed, and validated separately). Do not allow spaces, semicolons, pipes, backticks, or any other characters that could be used for shell command injection.
      • Pattern Validation: Define a regular expression that precisely matches the allowed structure of a cron expression. This regex should be as restrictive as possible.
      • Length Limits: Impose a reasonable maximum length on the cron expression to prevent excessively long inputs that might be used for other attacks (e.g., ReDoS, though that's less of a concern here).
      • Visual Cron Expression Builder: The best approach is to use a visual cron expression builder component in the user interface. This eliminates the need for free-form text input and drastically reduces the attack surface. The user selects options from dropdowns and other UI elements, and the component generates the valid cron expression.
      • Context-Specific Validation: If the application has specific requirements for the cron expressions (e.g., only certain times of day are allowed), enforce these restrictions in the validation logic.
      • Reject "Special" Values: Be extremely cautious about allowing values like @reboot, @yearly, etc., unless absolutely necessary and thoroughly understood. These can have unintended consequences.

Attack Tree Path: 2. Denial of Service (DoS)

  • 2.1. Resource Exhaustion via Frequent Execution

    • Description: The attacker aims to overload the system by causing tasks to be executed too frequently.

  • 2.1.1. Inject High-Frequency Cron Expression [CRITICAL]

    • Description: The attacker provides a cron expression that specifies a very high execution frequency (e.g., every second, every few seconds, or even more frequently if the library and system allow it). This can lead to resource exhaustion (CPU, memory, disk I/O) and make the application or the entire server unresponsive. This node is critical because controlling the execution frequency directly prevents this type of DoS.
    • Likelihood: Medium (If the application doesn't have any frequency limits, the likelihood is high. If there are some basic limits, it's lower.)
    • Impact: Medium to High (Depends on the resources consumed by the scheduled task and the server's capacity. Can range from degraded performance to complete unavailability.)
    • Effort: Low (The attacker just needs to provide a high-frequency cron expression.)
    • Skill Level: Low (Basic understanding of cron syntax is sufficient.)
    • Detection Difficulty: Low (Easily detectable through monitoring resource usage. High CPU utilization, increased task queue lengths, and slow response times are clear indicators.)
    • Mitigation Strategies:
      • Minimum Execution Interval: Enforce a minimum time interval between task executions. For example, allow tasks to run no more frequently than once per minute, or even less frequently depending on the application's needs.
      • Rate Limiting: Implement rate limiting on the execution of scheduled tasks. This can be done at the application level or using system-level tools.
      • Configuration-Based Limits: Allow administrators to configure the maximum execution frequency through a configuration file or administrative interface.
      • Reject Unrealistic Expressions: Reject cron expressions that are clearly unrealistic or overly frequent (e.g., * * * * * * - every second, if supported).
      • Monitoring and Alerting: Set up monitoring to track resource usage and alert administrators if tasks are being executed too frequently or consuming excessive resources.