Case study: Python RCE vulnerability in Celery

Research into Python object traversal found that recursively resolving user-influenced dotted attribute paths with `getattr()` can expose private data, bypass access controls, or enable remote code execution when dangerous methods are reached and invoked. Using custom rules in Snyk Code’s static analysis engine, the researcher identified this pattern in Celery’s `exception_to_python` function, where JSON-backed exception data could control a module name, attribute path, and argument. By supplying values that resolved to `os.system` and a command string, the researcher demonstrated stored command injection, tracked as CVE-2021-23727, which could allow attackers with backend access to execute commands and potentially compromise systems or move laterally through a network. Celery fixed the issue in version 5.2.2 by validating target modules and resolved attribute types before invoking functions, underscoring the need to treat deserialized or remotely stored data as untrusted and validate it before object resolution or method calls.

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