datamodel-code-generator vulnerable to SSRF via JSON-Schema `$ref` to HTTP URL (silent by default)

Summary

JSON-Schema $ref values pointing at HTTP or HTTPS URLs are silently dereferenced by datamodel-code-generator with no IP/host validation, no scheme allow-list, and redirects followed unconditionally. The --allow-remote-refs gate added in 0.56.0 defaults to None, which only emits a deprecation warning and then fetches the URL anyway; only explicit --allow-remote-refs=false blocks the request. The fetched body is parsed as a sub-schema and reflected verbatim into the generated .py source. As a result, any JSON-Schema document the developer feeds to datamodel-codegen — including documents authored by an attacker — can pivot to arbitrary internal addresses and leak the response into the generated code, with no developer cooperation beyond running the tool.

Details

Sink: src/datamodel_code_generator/parser/jsonschema.py, _get_ref_body (lines 4776–4793, at tag 0.60.1 / commit a321547e):

def _get_ref_body(self, resolved_ref: str) -> dict[str, YamlValue]:
    if is_url(resolved_ref):
        if not resolved_ref.startswith("file://") and self.http_local_ref_path is None:
            if self.allow_remote_refs is False:
                raise Error(f"Fetching remote $ref is disabled: {resolved_ref}...")
            if self.allow_remote_refs is None:
                warn_deprecated(                            # (A) warn only
                    "behavior.remote-ref-default",
                    details=f"Reference: {resolved_ref}",
                    stacklevel=2,
                )
        return self._get_ref_body_from_url(resolved_ref)    # (B) fetch fires
    return self._get_ref_body_from_remote(resolved_ref)

• (A) emits a deprecation warning when allow_remote_refs is its default (None); execution falls through to (B).
• (B) routes the URL through _get_text_from_url → get_body, the same fetcher described in other report — no IP validation, redirects followed.

The fetched body is then parsed as a sub-schema and merged into the model graph, so description, title, properties, etc. from the remote document end up in the generated .py source.

Only affects users who installed the [http] extra (pip install 'datamodel-code-generator[http]').

PoC

A self-contained one-file PoC is available here: https://gist.github.com/thegr1ffyn/562a6972d7dc3f2869458ae93fc608c0

Impact

Who is impacted. Anyone running datamodel-codegen on a JSON-Schema or OpenAPI document of uncertain provenance, with the [http] extra installed. Real-world scenarios:

1. Trojaned OpenAPI document. A public REST API publishes openapi.yaml. One $ref points at http://169.254.169.254/latest/meta-data/iam/security-credentials/<role>; running datamodel-codegen against the spec from an EC2 instance leaks the IAM credentials into the generated client.
2. Customer-supplied JSON Schema. A B2B SaaS auto-generates client code from customer-uploaded schemas. The customer adds an HTTP $ref to http://internal-admin:8080/users.json; the response (e.g. JSON user list) ends up in the generated Python the SaaS hands back to the customer.
3. CI on a private network. A PR adds schemas/inbound.json with an HTTP $ref pointed at a service reachable only from the CI cluster's VPC; the CI runner fetches it and the generated .py leaks the response in PR artifacts.

Higher real-world risk than the sibling CLI-flag SSRF (other SSRF in this report bundle) because the schema author chooses the destination — the developer doesn't have to type any URL.

Suggested fix.

1. Flip the default in parser/base.py: allow_remote_refs: bool = False, and remove the silent-fetch-with-warning fallback at parser/jsonschema.py:4786-4791.
2. When fetching is allowed, apply the same IP/host validation proposed for other submitted SSRF report to both the initial $ref URL and every redirect target.
3. Document HTTP $ref in a third-party schema as equivalent to running curl on the developer's host.

Maintainer resolution

This $ref report is fixed by the same shared HTTP fetcher hardening that resolved GHSA-rfr2-mq9m-x2qx. The code landed through the GHSA-rfr2 private security PR koxudaxi/datamodel-code-generator-ghsa-rfr2-mq9m-x2qx#1 and was merged into the public repository as 5fdba4a09f2d7a9996a504975b7ef7d63e3715bb. Follow-up generated-file and coverage fixes were merged in koxudaxi/datamodel-code-generator#3279 and docs were synced in #3280. The patched release is 0.61.0.

No separate net code diff remains in the GHSA-954 private PR because the shared HTTP fetcher patch is already present on main. This advisory remains separate because the affected entry point is remote JSON Schema/OpenAPI $ref resolution rather than direct CLI --url input.

The fix does not flip the --allow-remote-refs compatibility default in this patch. Instead, it mitigates the SSRF issue by blocking localhost, loopback, private, link-local, reserved, and other non-public network targets by default, validating every redirect target before it is fetched, and requiring --allow-private-network / allow_private_network=True for trusted internal schema endpoints. Remote $ref fetching remains controlled by --allow-remote-refs; non-public/internal targets additionally require --allow-private-network.

Submitted by: Hamza Haroon (thegr1ffyn)