[joaocmendonca]

Fix ValueError('Unknown calendar type') when parsing Windows Task XML with ScheduleByWeek triggers that omit the WeeksInterval element. This occurs in real-world exported/UTF-16 task XML files

The fix:

• Detect ScheduleByWeek directly (not gated on WeeksInterval presence)
• Default weeks_between_triggers to 1 when WeeksInterval is missing
• Safely extract DaysOfWeek children (handle missing elements gracefully)

Summary

Fix ValueError when parsing Windows Task XML with ScheduleByWeek but no WeeksInterval.

Issue

Parsing task XML files like windows/system32/tasks/at26 (UTF-16, exported) raised:
ValueError: Unknown calendar type
This occurred in CalendarTrigger with ScheduleByWeek that lacks WeeksInterval element (which is not mandatory as per specs, see https://github.com/MicrosoftDocs/win32/blob/docs/desktop-src/TaskSchd/taskschedulerschema-weeksinterval-weeklyscheduletype-element.md).

Fix

• Changed condition from elif weeks_between_triggers := ... to elif trigger.find("ScheduleByWeek/") is not None
• Default weeks_between_triggers to 1 when element is missing
• Safely extract DaysOfWeek children

Testing

Validated with standalone test that reproduces the exact scenario from the issue.
The full test suite can be run: pytest tests/plugins/os/windows/test_tasks.py -q

HTH, Best, Joao

[joaocmendonca]

I should have added that this is a fix for #1505. Thx.

[Schamper]

Do you have a sample task that could be added as a unit test? Or otherwise can craft one?

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[joaocmendonca]

Do you have a sample task that could be added as a unit test? Or otherwise can craft one?

Here you are @Schamper!

I am unable to attach this as a file, so you have it as text.
This was extracted and redacted from a Windows 7 Ultimate system.
Note the inconsistent presence of backslashes on the Command field.

<?xml version="1.0" encoding="UTF-16"?>
<Task version="1.0" xmlns="http://schemas.microsoft.com/windows/2004/02/mit/task">
  <RegistrationInfo />
  <Triggers>
    <CalendarTrigger>
      <StartBoundary>2017-11-11T11:11:00</StartBoundary>
      <ScheduleByWeek>
        <DaysOfWeek>
          <Sunday />
          <Monday />
          <Tuesday />
          <Wednesday />
          <Thursday />
          <Friday />
          <Saturday />
        </DaysOfWeek>
      </ScheduleByWeek>
    </CalendarTrigger>
  </Triggers>
  <Principals>
    <Principal id="Author">
      <UserId>@AtServiceAccount</UserId>
      <LogonType>InteractiveTokenOrPassword</LogonType>
      <RunLevel>HighestAvailable</RunLevel>
    </Principal>
  </Principals>
  <Actions Context="Author">
    <Exec>
      <Command>C:\AAAAA\\AaAaaAaaa\AaAaaAaaa.exe</Command>
      <Arguments>/m</Arguments>
    </Exec>
  </Actions>
</Task>

[joaocmendonca]

@DissectBot agree

[Schamper]

Could you add this file as a test file and expand the existing unit tests?