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attack-surface.md

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Attack Surface Analysis for mtdowling/cron-expression

Attack Surface: Denial of Service (DoS) via Malicious Cron Expressions

  • Description: An attacker provides a crafted cron expression designed to consume excessive system resources (CPU, memory) during parsing or calculation of the next execution time, leading to a denial of service.
  • How cron-expression Contributes: The library's parsing and calculation logic is the direct target of this attack. The complexity of the cron syntax allows for potentially resource-intensive expressions. This is the core vulnerability of using a cron parsing library.
  • Example:
    • 0-59,0-59,0-59,0-59,0-59 * * * * (Excessive comma-separated values)
    • 1-59/9999999 * * * * (Attempting large iteration counts, though the library should handle this)
    • */1,*/1,*/1,*/1,*/1 * * * * (Many divisions)
  • Impact: Application unavailability, service disruption, potential for complete system unresponsiveness if resource limits are not in place.
  • Risk Severity: High (Potentially Critical if no resource limits are enforced)
  • Mitigation Strategies:
    • Input Validation: Implement strict whitelist-based validation of cron expressions. Reject any input that doesn't conform to a predefined, limited set of allowed characters and patterns. Do not rely solely on the library's built-in validation.
    • Length Limitation: Enforce a maximum length for the cron expression string (e.g., 255 characters, or even shorter).
    • Complexity Limitation: Limit the number of components (commas, hyphens, slashes) within the expression. Be very restrictive.
    • Resource Limits (Crucial): Implement timeouts and resource quotas (CPU time, memory usage) for the cron expression parsing and calculation process. Terminate any operation that exceeds these limits. This is the most important mitigation, and it must be implemented at the application level.
    • Fuzz Testing: Regularly perform fuzz testing with a wide range of valid and invalid inputs to identify potential parsing vulnerabilities.

Attack Surface: Privilege Escalation (Indirect, but Requires cron-expression Vulnerability)

  • Description: A vulnerability within the cron-expression parsing logic (e.g., a bug that allows injection of unintended values), combined with the scheduled task running with elevated privileges, could allow an attacker to indirectly execute arbitrary code with those privileges. This requires a flaw in the library itself that allows for manipulation beyond just DoS.
  • How cron-expression Contributes: The library's parsing vulnerability is the necessary initial entry point. Without a flaw in how the library handles input, this escalation is not possible via the library. The escalation happens because of the context (high-privilege task) in which the parsed expression is used, but a library bug is required.
  • Example: An attacker exploits a hypothetical parsing vulnerability in cron-expression to inject a malicious command into the scheduled task's arguments (if the application uses the parsed expression to construct commands). The task runs as root, giving the attacker root access. This relies on a bug in the library that allows more than just DoS.
  • Impact: Complete system compromise, data breaches, arbitrary code execution with elevated privileges.
  • Risk Severity: Critical
  • Mitigation Strategies:
    • Principle of Least Privilege: Run scheduled tasks with the absolute minimum necessary privileges. Never run tasks as root or administrator unless absolutely unavoidable. This is the primary mitigation.
    • Input Sanitization (Beyond Parsing): Even after the cron expression is parsed by cron-expression, thoroughly sanitize any data derived from it before using that data to construct commands or interact with the system. This is crucial to prevent injection attacks if the library has a parsing flaw.
    • Secure Configuration: Ensure that the system's cron daemon itself is configured securely and that access to cron configuration files is restricted.
    • Assume Library Vulnerability: Because this is critical, operate under the assumption that the library might have an undiscovered parsing vulnerability. The other mitigations are designed to limit the impact of such a vulnerability.