T09 · Insecure Skill Coding Practices
- Location
scripts/umask.py:1- Finding
Display Operation Silently Sets the Process Umask to 000
- Content
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Vulnerability Details
File Location:
scripts/umask.py, lines 1–3
Vulnerability Type: Unintended security-sensitive state modification
Risk Level: MediumVulnerable Code
python #!/usr/bin/env python3 import os print(oct(os.umask(0)))Technical Analysis
The call
os.umask(0)returns the previous file-creation mask but also changes the current process mask to000. Consequently, the operation documented as displaying the current mask performs an undocumented security-sensitive mutation.When the script is launched as a separate process, this change ordinarily ends when that child process exits and cannot modify the parent shell's umask. However, if the file is imported, evaluated with
exec, or otherwise run inside a long-lived Python process, its top-level code changes that host process's umask to000. Files subsequently created with a requested mode of0666may therefore be readable and writable by other local users, while directories requested with mode0777may be fully accessible to them.The implementation also does not parse the documented optional mask argument and lacks a
__main__guard, causing the mutation at import time.Attack Path
- A long-lived Python application imports or evaluates
scripts/umask.pyin its own process. - The top-level call to
os.umask(0)replaces the application's existing restrictive mask with000. - The application later creates files or directories without explicitly removing group or world permissions.
- Those objects receive permissions permitted by their requested creation modes, potentially up to
0666for files and0777for directories. - Another local user reads, modifies, or traverses the exposed objects where operating-system and filesystem controls permit it.
This path does not apply in the same way when the utility is executed solely as a short-lived subprocess, because a child process cannot change its parent's umask.
Impact Assessment
The f ...[truncated 572 chars]
- A long-lived Python application imports or evaluates
- Remediation
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Remediation Suggestions
Read the current mask by changing it only temporarily and restoring it immediately:
python def get_umask(): current = os.umask(0) os.umask(current) return currentPrevent import-time side effects and implement the documented command-line behavior explicitly:
python #!/usr/bin/env python3 import argparse import os def get_umask(): current = os.umask(0) os.umask(current) return current def parse_mask(value): try: mask = int(value, 8) except ValueError as exc: raise argparse.ArgumentTypeError("mask must be an octal value") from exc if not 0 <= mask <= 0o777: raise argparse.ArgumentTypeError("mask must be between 000 and 777") return mask def main(): parser = argparse.ArgumentParser() parser.add_argument("mask", nargs="?", type=parse_mask) args = parser.parse_args() if args.mask is None: print(f"{get_umask():03o}") else: os.umask(args.mask) if __name__ == "__main__": main()Additional hardening measures:
- Add tests confirming that display-only execution restores the original mask.
- Add an import test confirming that importing the module has no side effects.
- Validate masks strictly as octal values in the range
000through777. - Document that a standalone child process cannot persistently change its parent shell's umask. If persistent shell modification is required, provide an explicit shell integration mechanism rather than implying that a subprocess can change its caller's state.
- Prefer restrictive defaults such as
077when the utility creates sensitive files or directories.