Loading YAML safely

What it is

YAML documents can carry tags (!!python/object:…) that tell a loader to construct language-specific objects. PyYAML's documentation warns that yaml.load with the full loader can execute arbitrary code from untrusted input and provides safe_load, which constructs only plain data types. The YAML 1.2 specification defines the core schema that a safe loader implements.

Why it matters

Configuration and data files are often user-supplied (uploads, repositories, CI definitions). A loader that instantiates objects turns a text file into remote code execution.

How to apply

• Use yaml.safe_load (or the library's equivalent safe mode) everywhere; forbid the full loader in code review and with a linter rule.
• Validate the loaded structure with a schema; YAML's implicit typing (on, 1e3, null) produces surprising types otherwise.
• Prefer JSON or TOML for machine-written data; use YAML where the ecosystem requires it.
• Limit document size and nesting depth for uploads.

Pitfalls

Aliases and anchors can expand exponentially ("billion laughs") in naive loaders; safe loaders in current versions guard against it, but size limits still apply. Different libraries default to YAML 1.1 semantics (yes → true); pin behaviour explicitly.