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`: ```python from pydantic import BaseModel from pydantic_settings import ( BaseSettings, SettingsConfigDict, NestedSecretsSettingsSource, ) class Db(BaseModel): passwd: str | None = None class Settings(BaseSettings): model_config = SettingsConfigDict( secrets_dir='secrets', secrets_nested_subdir=True, secrets_dir_max_size=100, # outside/passwd is 512 bytes ) db: Db = Db() @classmethod def settings_customise_sources( cls, settings_cls, init_settings, env_settings, dotenv_settings, file_secret_settings ): return (NestedSecretsSettingsSource(file_secret_settings),) ``` On affected versions, `Settings().db.passwd` is populated with the 512-byte out-of-tree file and **no** `SettingsError` is raised, even though the file exceeds `secrets_dir_max_size`. ### Impact Applications that opt into `NestedSecretsSettingsSource` with `secrets_nested_subdir=True` and load secrets from a directory whose entries can be influenced by an attacker or a lower-privileged component (for example, a writable or shared secrets mount, or a secrets directory partially populated from untrusted input) are affected. The impact is: * **Confidentiality:** files outside the configured `secrets_dir` can be read into settings values (local file read). * **Integrity / availability of the safeguard:** the advertised `secrets_dir_max_size` cap can be bypassed, and cyclic symlinks can inflate resource usage during loading. The vulnerability requires the ability to place a symbolic link inside the configured secrets directory; it is not remotely reachable on its own. Applications that do not use `NestedSecretsSettingsSource`, or that point `secrets_dir` at a directory fully under the application's control, are not affected. ### Mitigation Upgrade to **pydantic-settings 2.14.2**, which: * walks the secrets directory explicitly and only descends into directories whose resolved path stays within `secrets_dir`, so symlinked directories pointing outside are never followed; * uses a single, cycle-safe iterator for both the size check and the loader, so the size accounting and the loaded set are always consistent and each real directory is visited at most once; * skips any file whose resolved path escapes `secrets_dir`, as defense in depth. If upgrading is not immediately possible, ensure the configured `secrets_dir` is fully owned and controlled by the application (no writable or attacker-influenced entries), or avoid `secrets_nested_subdir=True`.

Quoted source text, attributed separately from HOL analysis.