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Threat Model Analysis for mtdowling/cron-expression

Threat: Resource Exhaustion via Pathological Expression (CPU)

  • Description: An attacker crafts a highly complex and computationally expensive cron expression designed to consume excessive CPU cycles. They submit this expression through any input field where cron expressions are accepted. The attacker's goal is to cause a denial of service by overloading the server. They might use combinations of many wildcards, ranges, and steps in unusual and nested ways, specifically targeting the parsing and calculation logic.
  • Impact: Application becomes unresponsive or crashes. Legitimate users are unable to access the service. Other processes on the same server may be affected. Potential for complete system unavailability.
  • Affected Component:
    • CronExpression::factory() (or the constructor) - Initial parsing of the expression.
    • CronExpression::isDue() - Checking if the expression is due at a given time.
    • CronExpression::getNextRunDate() - Calculating the next execution time.
    • CronExpression::getPreviousRunDate() - Calculating the previous execution time.
    • Internal iteration logic within these functions, particularly when handling complex ranges, steps, and combinations.
  • Risk Severity: Critical
  • Mitigation Strategies:
    • Input Validation (Strict): Limit expression length (e.g., 255 characters). Restrict allowed characters (whitelist approach: 0-9,*,/-). Disallow nested or overly complex combinations. Reject expressions that fail initial parsing.
    • Timeouts: Wrap calls to getNextRunDate(), getPreviousRunDate(), and isDue() in timeouts (e.g., 1 second). Terminate calculations that exceed the limit.
    • Rate Limiting: Limit the frequency of cron expression submissions per user/IP address.
    • Resource Monitoring: Track CPU usage of the process handling cron expressions. Alert on anomalies.
    • Sandboxing: Isolate cron expression processing in a separate process with limited resources.

Threat: Resource Exhaustion via Pathological Expression (Memory)

  • Description: Similar to the CPU exhaustion threat, but the attacker crafts an expression that, while potentially not CPU-intensive initially, leads to excessive memory allocation during calculation of many future or past run dates. The attacker might try to generate a huge number of dates.
  • Impact: Application runs out of memory and crashes. Similar denial-of-service impact as CPU exhaustion, potentially affecting the entire system.
  • Affected Component:
    • CronExpression::getMultipleRunDates() - Specifically designed to get multiple run dates. This is the most vulnerable function to this attack.
    • Internal data structures used to store calculated dates within getNextRunDate(), getPreviousRunDate(), and especially getMultipleRunDates().
  • Risk Severity: Critical
  • Mitigation Strategies:
    • Limit getMultipleRunDates(): Strictly limit the count parameter of getMultipleRunDates(). Do not allow users to request an arbitrarily large number of dates. A small, fixed maximum (e.g., 10, 20) is crucial.
    • Input Validation (as above): Same as for CPU exhaustion.
    • Timeouts (as above): Same as for CPU exhaustion.
    • Memory Monitoring: Monitor memory usage, especially when getMultipleRunDates() is used.
    • Sandboxing (as above): Same as for CPU exhaustion.

Threat: Unexpectedly Frequent Execution

  • Description: A user (not necessarily malicious) enters a cron expression that they misunderstand, resulting in the scheduled task running much more frequently than intended. For example, they might use * * * * * (every minute) when they meant once a day.
  • Impact:
    • Overload of downstream systems or APIs.
    • Excessive logging.
    • Unintended data modifications.
    • Potential cost overruns if the task triggers paid services.
  • Affected Component:
    • CronExpression::factory() (or constructor) - Parsing the user-provided expression.
    • CronExpression::isDue() - Used by the scheduler to determine when to run the task.
  • Risk Severity: High
  • Mitigation Strategies:
    • UI/UX: Cron Expression Builder: Provide a visual builder instead of a free-text field.
    • Preview and Confirmation: Display a human-readable summary (e.g., "Runs every minute") before saving the expression. Require explicit user confirmation.
    • Sanity Checks: Reject or warn about expressions that run too frequently (e.g., more than once per hour). Define application-specific thresholds.
    • Documentation: Clearly explain cron syntax and provide examples.

Threat: Library Vulnerability (CVE)

  • Description: A vulnerability (e.g., a parsing bug leading to RCE) is discovered in the cron-expression library itself. An attacker crafts a malicious expression to exploit this vulnerability.
  • Impact: Depends on the specific vulnerability. Could range from denial of service to remote code execution and complete system compromise.
  • Affected Component: Potentially any part of the library, depending on the vulnerability.
  • Risk Severity: Variable (depends on the CVE), potentially Critical.
  • Mitigation Strategies:
    • Dependency Management: Keep cron-expression updated. Use a tool like Composer or npm.
    • Vulnerability Scanning: Use tools like Snyk or Dependabot to detect known vulnerabilities.
    • Monitor Security Advisories: Stay informed about security updates for the library.