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:

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.