Python execute phase aborts with error_analysis when the package closes stdout (MCP stdio servers)

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Valutazione

Difficoltà
3/5
Tempo stimato
1-2 giorni
Idoneità per principianti
78/100
Tipo di issue
Bug
Chiarezza
Specificata chiaramente
Stato di attività
Attiva
Stack tecnologico
python
Ambito
security

Direzione di ricerca

Start in sandboxes/dynamicanalysis/analyze-python.py, reading execute_package, execute_module, and do_execute to trace how output failures propagate. Reproduce with the rel-38 analysis image and mcp-server-time 2026.8.18. Done means a package that closes stdout no longer aborts execute with error_analysis, and later modules are still executed.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

Summary

When a Python package closes sys.stdout during the execute phase — every MCP stdio server does, because the protocol owns stdout — analyze-python.py crashes on its own next print(). The phase ends with status error_analysis, and all modules after the current one are never executed.

The handler meant to survive a failing module prints as well, so the exception escapes it (sandboxes/dynamicanalysis/analyze-python.py on main, unchanged since #1153):

def execute_module(module):
    ...
    try:
        do_execute(module)
    except BaseException:
        print('Failed to execute code for module', module)   # raises again

Reproduction

Image gcr.io/ossf-malware-analysis/analysis (rel-38, sha256:1f3104f3f0cfddeaf62f0952d663e129cb5efe38486766e676fb926fa84c4c27), ecosystem pypi, package mcp-server-time 2026.8.18 (the reference MCP server from modelcontextprotocol/servers).

install and import complete. execute ends with "Status": "error_analysis", and its stderr is:

  File "/usr/local/bin/analyze-python.py", line 123, in execute_module
    do_execute(module)
  File "/usr/local/bin/analyze-python.py", line 170, in do_execute
    print('[skipped members]', ' '.join(skipped_names))
ValueError: I/O operation on closed file.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/bin/analyze-python.py", line 338, in <module>
    exit(main())
  File "/usr/local/bin/analyze-python.py", line 332, in main
    phase(package)
  File "/usr/local/bin/analyze-python.py", line 112, in execute_package
    execute_module(module)
  File "/usr/local/bin/analyze-python.py", line 126, in execute_module
    print('Failed to execute code for module', module)
ValueError: I/O operation on closed file.

Expected

The harness keeps its own output channel independent of the package under analysis (for example, it writes to a duplicate of fd 1 taken at start-up, or restores sys.stdout / falls back to sys.__stderr__ when a write fails), so a package that closes stdout is analysed completely instead of aborting the execute phase.

Why it matters

Consumers that treat error_analysis as an incomplete result get no verdict for most MCP stdio servers. A malicious package could also close stdout on purpose to stop the execute phase before later modules run.

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