pydantic-settings: NestedSecretsSettingsSource follows symlinks outside secrets_dir, enabling local file read and bypassing secrets_dir_max_size

Summary

NestedSecretsSettingsSource reads secret values from files in a configured secrets_dir. When secrets_nested_subdir=True, a directory entry inside secrets_dir that is a symbolic link pointing outside secrets_dir is followed, so files outside the configured directory are read into settings values. The same code path bypasses the documented secrets_dir_max_size protection. An attacker or lower-privileged component able to influence entries in the configured secrets directory (for example, a writable or shared secrets mount) can turn this into an unintended local file read into settings and can defeat the advertised loading-size cap. This report does not claim network reachability by itself.

Details

NestedSecretsSettingsSource performed two passes over secrets_dir using two different, inconsistent directory-traversal implementations:

• The size check in validate_secrets_path() used Path.glob('**/*'), which does not descend into a symbolically-linked directory.
• The loader in load_secrets() used glob.iglob(f'{path}/**/*', recursive=True) followed by read_text(), which does follow symlinked directories and reads through the link target.

Because the two passes disagreed on symlinks, a symlinked directory inside secrets_dir whose target lives elsewhere was invisible to the size accounting (counted as 0 bytes) while still being fully read by the loader. This produces two distinct problems:

1. Out-of-tree read (CWE-22 / CWE-59). A symlinked directory (or file) inside secrets_dir that resolves outside it is followed, and the external file's contents are loaded into the corresponding settings field.
2. secrets_dir_max_size bypass (CWE-400). The size check never sees the out-of-tree content, so the documented size cap is neither respected nor able to reject the oversized external file. A related amplification exists for cyclic in-tree symlinks, which glob.iglob(recursive=True) re-traverses, inflating the size accounting and the number of loaded secrets.

Reproduction

In a clean Linux container, with a secrets_dir containing a symlink secrets/db -> /path/outside and an outside/passwd file of 512 bytes, while secrets_dir_max_size=100: