check-jsonschema default caching for remote schemas allows for cache confusion

Impact

The default cache strategy uses the basename of a remote schema as the name of the file in the cache, e.g. https://example.org/schema.json will be stored as schema.json. This naming allows for conflicts. If an attacker can get a user to run check-jsonschema against a malicious schema URL, e.g., https://example.evil.org/schema.json, they can insert their own schema into the cache and it will be picked up and used instead of the appropriate schema.

Such a cache confusion attack could be used to allow data to pass validation which should have been rejected.

Patches

A patch is in progress but has not yet been released.

Workarounds

• Users can use --no-cache to disable caching.
• Users can use --cache-filename to select filenames for use in the cache, or to ensure that other usages do not overwrite the cached schema. (Note: this flag is being deprecated as part of the remediation effort.)
• Users can explicitly download the schema before use as a local file, as in curl -LOs https://example.org/schema.json; check-jsonschema --schemafile ./schema.json