Summary

datamodel-code-generator's built-in HTTP fetcher (http.get_body) issues an httpx.GET against any URL passed to --url (or reached via a redirect chain) with no allow-list, no deny-list, no IP/host validation, and follow_redirects=True. Loopback addresses, RFC1918 ranges, link-local (169.254.169.254 cloud metadata), unique-local IPv6 and any other network-accessible target are all reachable. The JSON/YAML response body is parsed as a schema and reflected into the generated .py source, exfiltrating the response to anyone with access to that file (commonly committed to a repository).

Details

Sink: src/datamodel_code_generator/http.py, get_body (lines 31–61, at tag 0.60.1 / commit a321547e):

def get_body(url, headers=None, ignore_tls=False,
             query_parameters=None, timeout=DEFAULT_HTTP_TIMEOUT) -> str:
    httpx = _get_httpx()
    try:
        response = httpx.get(
            url,
            headers=headers,
            verify=not ignore_tls,
            follow_redirects=True,          # (A)
            params=query_parameters,
            timeout=timeout,
        )
    except Exception as e:
        ...
    if response.status_code >= 400:
        ...
    content_type = response.headers.get("content-type", "").lower()
    if "text/html" in content_type:
        raise SchemaFetchError(...)         # (B) — only filter
    return response.text                    # (C) → embedded in generated.py

• (A) follows redirects unconditionally — a public URL → 302 → internal address chain works.
• (B) the only filter is rejecting text/html. Non-HTML internal endpoints (JSON APIs, cloud metadata, admin services) pass through.
• (C) the response body becomes the schema; its title, description, properties, etc. land in the generated .py as class attributes and Field(description=...) strings.

get_body is called by parser/base.py:1326 (_get_text_from_url), which is reached from CLI argument --url <URL>. (The $ref path is a separate advisory — see GHSA-D.)

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

PoC

A self-contained one-file PoC available here:
https://gist.github.com/thegr1ffyn/18de777d6c800a3b47715425e3f3e8f5

Impact

Who is impacted. Anyone who runs datamodel-codegen with a --url they didn't fully verify, or who runs it inside a network with reachable internal services. Realistic scenarios:

1. Trojan documentation / README. A blog post or README example reads datamodel-codegen --url https://schemas.example.com/user.json -o user.py. The attacker controls example.com, redirects to http://169.254.169.254/latest/meta-data/iam/security-credentials/<role>, and the IAM credentials end up as a docstring in user.py.
2. Internal port scan / disclosure. Iterating --url http://127.0.0.1:<port>/health probes localhost services; non-HTML, non-error responses confirm a service and leak its body into the generated file.
3. CI poisoning. A PR adds a Makefile rule that calls datamodel-codegen --url $(SCHEMA_URL); the CI runner reaches every internal service in its VPC and the response lands in PR artifacts.

Suggested fix. Resolve the URL host, reject loopback / private / link-local / multicast / reserved IPs by default, disable redirects by default (follow_redirects=False), re-validate after each redirect if the user opts into following them, and add an --allow-private-network flag for opt-in legitimate use.

Maintainer resolution

This report was fixed together with GHSA-954p-556p-r752 by the private security PR koxudaxi/datamodel-code-generator-GHSA-rfr2-mq9m-x2qx#1, 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.

The patch hardens the shared HTTP fetcher used by both direct CLI --url fetching and remote JSON Schema/OpenAPI $ref resolution:

• validates HTTP(S) URLs before fetching;
• blocks localhost, loopback, private, link-local, reserved, and other non-public network targets by default;
• disables automatic redirect following and validates each redirect target before requesting it;
• adds --allow-private-network / allow_private_network=True as an explicit opt-in 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)

References

• GHSA-rfr2-mq9m-x2qx
• koxudaxi/datamodel-code-generator@5fdba4a
• https://github.com/koxudaxi/datamodel-code-generator/releases/tag/0.61.0