fastapi-code-generator: Import-time RCE via response status-code key -> unescaped @app.get(..., responses={...}) dict

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

9.3 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N · score source

EPSS

• CVE-2026-63469: 0.31% estimated exploitation activity in the next 30 days; percentile 24.36 (FIRST EPSS, 2026-11-10). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary fastapi-code-generator (koxudaxi) emits an operation's extra responses into the route decorator's responses={...} dict, using each response's status-code key as a single-quoted string literal without escaping the '. A ' in a status-code key closes the dict-key string and, via concatenation, injects a live expression that the decorator argument evaluates when main.py is imported (the ASGI server imports it at startup) -- code execution, no request. The status-code key is a pure data field. Verified on fastapi-code-generator 0.8.0 / Python 3 / FastAPI.

Details ```python @app.get( '/u', response_model=None, responses={ '404' + (import("os").system('…') or "") + 'x': {'model': Thing} }, ) def get_thing() -> None: pass ``` The responses={...} dict is emitted only when the operation has a second (non-primary) response, so the malicious key is any additional response status. The generator post-processes with black/isort, so a syntax-breaking payload aborts generation…

Published by the source

9 Nov 2026, 19:46 UTC

Last modified by the source

9 Nov 2026, 20:00 UTC

Source last checked

10 Nov 2026, 06:35 UTC

1. Revision 2 · amended · 10 Nov 2026

   Amended from osv record GHSA-6xmv-cjr5-w3q8

2. Revision 1 · created · 9 Nov 2026

   Published from ghsa record GHSA-6xmv-cjr5-w3q8

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# Patch Next advisory brief: CVE-2026-63469

Advisory: fastapi-code-generator: Import-time RCE via response status-code key -> unescaped @app.get(..., responses={...}) dict
IDs: CVE-2026-63469, GHSA-6xmv-cjr5-w3q8
Record: https://patchnext.com/advisories/cve-2026-63469 (revision 2, source checked 10 Nov 2026, 06:35 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Critical (CVSS 9.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: fastapi-code-generator 0.8.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI fastapi-code-generator: affected < 0.8.1; fixed in 0.8.1

## Suggested inspection (read-only)
- `python -m pip show fastapi-code-generator`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on fastapi-code-generator.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-6xmv-cjr5-w3q8 (GitHub-reviewed advisory GHSA-6xmv-cjr5-w3q8 lists affected ranges: PyPI fastapi-code-generator >=0 <0.8.1)
- Verified: https://osv.dev/vulnerability/GHSA-6xmv-cjr5-w3q8 (OSV record GHSA-6xmv-cjr5-w3q8 lists affected ranges: PyPI fastapi-code-generator >=0 <0.8.1 (fixed))

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